Advancements in Design and Development of Lab Scale Photobiorectors for Improved Microalgal Feedstock

INTRODUCTION

Algae are a diverse group of photoautotrophs uses light to produce their food via a process of photosynthesis, capturing water and carbon dioxide from the atmosphere (73). During the process, light energy is converted into chemical energy and stored in various carbon forms (72). Their photosynthetic efficiency is 10-50 times higher than that of terrestrial plants and therefore is a promising source of CO₂ sequestration (75). They are abundantly found in all known habitats where light and moisture are available (30). They are considered a bio-factory and produce a wide range of biologically active micro and macro molecules, including carbohydrates, carotenoids, chlorophylls, enzymes, phycobilins, polysaccharides, proteins, vitamins and many other primary and secondary metabolites (63,65), which make them a unique and fascinating group of micro-organisms among other microbes and plants (47). Algae are being cultivated in large scale at a commercial level worldwide for the mass production of noble bioactive compounds for their application in various fields, including green energy (22,19,28). Microalgal species are potential sources for biofuel, bioethanol and biohydrogen and are considered a promising source of third-generation feedstock for biofuel and biodiesel production (46). Due to this, a large number of biological researcher globally have chosen them as noble organisms for their study for different purposes (68, 67, 69, 71, 70).

Microalgae can be used as a potential tool for biofuel production to replace petroleum, but are limited by high production cost and variable efficiency (17). Compared to the first and second, third-generation feedstock has higher yields and high oil content and have advantage of growing in non-arable lands, non-potable water bodies such as wastewater, polluted water (86). Food security concerns have been raised by first-generation biofuels, which are economically produced from edible oils like sugarcane, maize and palm oil as well as vital food crops. As a result, lignocellulosic biomass, municipal wastes, agricultural residues, and non-edible oils have all been used to create second-generation biofuels (15).

“Bioreactor” refers to any apparatus or container used to perform one or more biological reactions in order to transform any material (i.e., a substrate) into a product. Utilizing bioreactors is essential for any production method based on biotechnology, including the transformation of one molecule into another, the generation of biomass or metabolites, or the breakdown of undesirable waste. Airlift devices, packed beds, fluidized beds, bubble columns, stirred tank reactors and PBRs are the main categories of bioreactor setups that are covered (10). PBR uses a light source to cultivate microorganism. These microorganism uses CO₂ and light for photosynthesis. Light source can be natural or artificial. They can be used to cultivate microalgae and macroalgae. Under an artificial environment, the rate of light and other nutrient concentrations can be controlled to accelerate the rate of biomass production. Nutrient-rich domestic wastewater, agricultural wastewater and industrial wastewater can be used to culture algae and other microorganisms. PBRs have advantages like controlled pH, temperature and nutrient conditions, less water evaporation, high rate of biomass production and easy algal harvesting.

PBR can be open like a natural lake or pond system or artificial raceways and closed like tubular PBR, spiral PBR, plate PBR and horizontal PBR, as shown in figure 1. Algal growth in open lakes and pond are the best example of an open system. Algae can grow in mono or mixed culture according to environmental and nutrient conditions. These algae best suited for the study of climate change and water pollution. But due to their dependence on the environment, the productivity is limited, and water evaporation is high. Open ponds have greater CO₂ storage capacity than tubular reactors because of their greater culture volume per square meter. However, after recarbonation, open ponds tend to desorb CO₂ to the atmosphere (86).

This review paper addresses the research gap by enlisting different types of closed PBR of lab scale and their importance in microalgal production. This paper includes a basic introduction of photobiorector and their types, working parameters, criteria for lab-scale aquaculturing, and recent developed innovations in PBR. It also describes difficulties faced in large-scale open PBRs over lab-scale closed PBRs.

MICROALGAE CULTURE SYSTEM

Microalgae culture system can be divided into two classes i.e., open system (natural and artificial) and closed system (PBR). Open systems represent a great variety of combinations, including natural ponds having a natural wind agitation system, while artificial pond/ circular pond having a mixing process ensured by a central stirrer or raceway pond with a pedalled agitation system. On the other hand , closed systems represent a great variety of closed vessels such as PBRs where microalgae can be cultured in a controlled way by managing light, temperature, carbon dioxide and nutrients to facilitate photosynthesis. Different types of microalgae culture systems are described as under.

1. Raceway pond:

Raceways are artificial open systems most widely used due to low construction cost, ease to maintainance, low energy requirement and their ease of scalabity. It can be constructed on a horizontal surface with a few meter heights. Raceways have an advantage over ponds or lake i.e we can control nutrient concentration and can produce a single culture. But, raceway ponds have limitation of controlling temperature, water evaporation, high contamination risk and low microalgal biomass concentration. During summers the evaporation rate is high, and to maintain the media it needs to be refilled continuously. On rainy days, it needs to be protected from extra nutrient addition through rain-water. It implies power consumption as it requires shear force pump, airlifts or paddled wheels (86).

• Tubular PBR:

Tubular PBR has an interconnected parallel tube system made up of plastic or glass tubes arranged horizontally or vertically. It is connected with the central utilities installation with a pump, sensors, nutrients and CO₂. A pump circulates the microalgal culture through the tube, where a central tank collects and recycles it. They are helpful in large-scale production of microalgae(49). As a closed system, they offer better control over growth conditions and lead to high purity. They are used for the cultivation of Chlorella vulgaris and Arthospira platensis for human nutrition and animal feed. They are also used for the production of compounds like astaxanthin, omega-3 fatty acids and phycobillins for food supplements and cosmetics.

• Spiral PBR:

Spiral PBR system has a layout of springs which increase the surface area to volume ratio, ensuring that more cells have access to light as culture circulates. The combination of turbulence and the closed concept allows a clean operation and a high operational availability. It is a three-dimensional structure enabling maximum light efficiency and reducing shade. It does not require a large space to be installed. This technique enables the temperature of the bioreactor to be controlled without external cooling and can be used in different climate zones, which is another critical hurdle on the path to the industrial production of microalgae. The experimental spiral PBR’s key benefit includes (1) a large ratio of culture volume to surface area combined with optimal light penetration depth; (2) simple temperature and contaminant control; (3) efficient fresh air and CO₂ spatial distribution; (4) improved CO₂ transfer through a large interface surface between fresh air and culture liquid medium: and (5) a novel automated flow-through sensor that continuously monitors cell concentration. (3).

• Plate PBR:

Plate PBR uses less expensive material than tubular PBR. Instead of tubular system plate PBR has disposable bag equipped with gas spared at the bottom of plastic bag for aeration and a heat exchanger. The bag can be easily replaced when convent, excessive fouling or contamination being the most common factors requiring a replacement. (82).

• Horizontal PBR:

Horizontal PBR solves the problem of even distribution of incidence light. Tubular system is placed horizontally and hence larger light quantity can be exploited in order to improve photoconversion efficiency. The mixing is done by rotary pump.

WORKING PRINCIPLE OF PBR

For large scale production of algae at commercial scale the strain of algae should be selected on some specific criteria like algal life cycle, growth rate, productivity of the desired products, genetic stability, nutritional requirement and tolerance to shear stress (74).

Key parameters influencing the performance of the PBR, include light, mixing, mass transfer, temperature, pH and capital and operational costs. The lifespan and the costs of cleaning and temperature control should also be emphasized for commercial exploitation. A typical PBR is a three-phase system that includes the culture medium as the liquid phase, the cells as the solid phase and CO₂ enriched air as the gas phase in auto-phototrophic cultivation. As a unique feature, the light in a PBR is a superimposed radiation field that is usually referred to as a fourth phase. (8).

Factors affecting the growth of microalgae in PBR can be divided under design and installation parameter, environmental parameter and operational parameter as described in figure 2 (13). Out of several major factors light source and CO₂ are major influencing factors which directly affect photosynthesis and hence the rate of cell growth and biomass. Other factors like pH and temperature indirectly affects the growth by altering optimum conditions for cellular functions which include photosynthesis (14). Factors affecting the growth of microalgae are mentioned below:

1.  Light:

The growth of microalgae is equally dependent on the type and intensity of the light. Three phases can be distinguished in the relationship between light intensity and the photoautotrophic growth of microalgae: the light limitation phase, the light saturation phase, and the light inhibition phase (as shown in figure 3).Light in the cultivation system must be distributed equally during the saturation period, but this is made difficult by the shading effect. The effect of shade grows in tandem with the concentration of cell and biofilm growth on the reactor vessel’s surface (5). Cells can be regularly mixed to get around this. One investigation showed how light intensity affected the levels of Mg^–, pH, NADPH, and lipids (41). The available light sources are discharge lamps (such mercury, xenon, or fluorescent lamps), light-emitting diodes (LEDs), lasers, and incandescent lamps (tungsten or halogen lamps). Light intensity above the saturation point is dangerous because it damages photosystems and leads to photo-oxidation. We refer to this stage as the inhibition phase.

• CO₂:

It is possible to cultivate microalgae utilising a variety of carbon sources, including carbon dioxide, methanol, acetate, glucose, and other organic substances, in photoautotrophic heterotrophic and mixotrophic environments (90). In photoautotrophic environments, microalgae obtain their carbon supply from inorganic carbons such carbon dioxide or bicarbonates during photosynthesis (61). In the presence or absence of light, microalgae that are heterotrophic directly consume organic carbon (12). The majority of microalgae exhibit 10- 50 fold greater rates of cell growth and CO₂ fixation than terrestrial plants, indicating yet another benefit of directly converting photoautotrophic microalgal development. Thus, it makes more sense to manufacture microalgae-based biofuels through the photoautotrophic development of microalgae from an economic and environmental protection standpoint. Another idea, though, is to use organic carbon sources (such sugars) obtained from biomass to grow microalgae under heterotrophic conditions and then turn them into biofuels. Because the heterotrophic development of microalgae is typically quicker than the autotrophic growth, this could significantly increase the output of biofuels (6). In PBR, the medium needs to be supplemented with more CO₂ in order to support the algae’s rapid development. This extra CO₂ is provided as enhanced CO₂ gas mixture bubbles in enclosed PBRs. A CO₂ concentration of more than 1% (by volume) inhibits the growth of algae. Algae that had acclimated to low partial pressure grew less quickly when exposed to a gas with a high CO₂ partial pressure (40). CO₂ dissolution rate in relation to reactor temperature, pressure, and bubble contact time (77). It is possible to buy CO₂ that has been produced commercially by burning fossil fuels. Through pretreatment and utilisation of waste biomass resources, such as feedstock crops, organic carbon can be generated from raw biomass. One by-product of the sugar industry that has been studied as a substrate for microalgal culture is waste molasses, which is rich in simple sugars. Chlorella protothecoides, with a biomass, oil content, and oil output of 70.9 g/L, 57.6%, and 40.8 g/L, respectively, could efficiently use hydrolysed waste molasses as a carbon source. Furthermore, employing molasses as the feedstock resulted in a 50% decrease in the cost of producing biofuels (92).

• Contaminants:

In nature, algae are typically found in mixed cultures and can be infected by bacteria, fungi, insects, or predatory algal species. However, in order to produce algae on a commercial scale or for scientific purposes, it is necessary to isolate them. Industrialists typically choose extremophilic microalgae for production because they can endure harsh environments that predators cannot. Because close systems, like PBRs, shield the algae from the outside world, they help prevent contamination, whereas open systems are more vulnerable to invasion by predatory organisms.

• Temperature:

The environment has a significant impact on how quickly microalgae develop. Temperature and growth rate exhibit a bell-shaped curve, meaning that an initial rise in temperature promotes a higher growth rate, but further increases result in a drop phase. A study found that increasing the temperature from 10˚C to 25˚C initially increased the pace at which biomass was produced in Scenedesmus sp. LX1, but that increasing the temperature further led to a falling phase (89). Since it is challenging to control this feature in open PBR, locked PBR are generally considerably simpler to work with (57).

• pH:

The measure of the water’s acidity or basicity is called pH. Variations in it may have an impact on the growth and production of the intended product. For the growth of microalgae, the range 7-9 is ideal (2, 23). pH may have an impact on a variety of biological processes in microalgae culture, including ion uptake, cell growth, and metabolism (36). The pH is raised by the CO₂ that is used in photosynthesis. The most readily available form of dissolved inorganic carbon (DIC) is CO₃^2-; beyond 8, CO₂ is the most readily available form; below 7, CO₂ is more prevalent than HCO^3-; and from 7 to 10, HCO^3- is more prevalent than CO₂ (20).

• Nutrients:

Among the macronutrients required for microalgae growth are phosphorus and nitrogen. Trace metals including Fe, Mg, B, Mo, K, Co, Zn, and Mb are necessary for the growth of microalgae. Changes in morphology and physiology can result from excess or insufficiency of certain nutrients, which can also disrupt metabolic pathways (45). The synthesis of proteins and other metabolites depends on nitrogen, a macronutrient. Insufficient nitrogen causes an increase in the synthesis of lipids (42), while algae with high protein content that are utilised as dietary supplements need more nitrogen. These sources of nitrogen include urea and amino acids, or they might be inorganic and come in the form of nitrate, nitrite, and ammonium (43).

• O₂ removal:

The process of photosynthesis produces oxygen in exchange for CO₂ fixation, but too much of it can lead to the creation of free oxygen reactive species, which can harm cells through oxidative stress. The O₂ content in the medium shouldn’t be higher than 400% in relation to the air saturation level. The concentration of O₂ in the medium might increase during the dark phase of photosynthesis. Because of photorespiration or the Wurburg effect, certain algae are unable to tolerate such high concentrations for longer than two to three hours. This issue can be resolved by increasing turbulence or cycling fresh air to remove O₂ (77).

• Salinity:

The variety of habitats found in microalgae is widely recognised. Freshwater, saltwater, subterranean, terrestrial, and buildings can all contain them. High salinity environments, or 1.7 M, are home to marine species. Additionally, it was found that when an additional 0.5 or 1 M NaCl was added to the culture medium at the conclusion of the exponential phase of growth, D. tertiolecta ATCC 30929 showed an increase in lipid content of up to 70% from 60% (with 0.5 M NaCl) (79). Saturated and polyunsaturated fatty acid concentrations (91), as well as the extractability of hydrocarbons (18), can all be impacted by salinity. Additionally, salinity can negatively affect microalgae growth rate and photosynthetic efficiency (25).

• Cleaning:

Microalgae growth rate is impacted by periodic PBR cleaning. The likelihood of contamination and biofilm growth on the wall is decreased. Easier cleaning can be achieved by structural reforms during design, such as fewer bends, larger internal dimensions for easier washing, and smooth walls (85).

1. Designing and installation:

The surface area to volume ratio is a significant determinant of PBR effectiveness. Additionally, a material with high transparency, like glass, should be employed. Although glass is durable, clear, and has a long lifespan, maintaining it is expensive. The substitutes for glass are acrylic PVC, polyvinylchloride (PVC), polyethylene, and Plexiglas. However, oxidation might cause them to lose their transparency over time. An additional factor to take into account is the running cost, which covers the price of cleaning, maintenance to stop leaks, temperature and pH controls, and auxiliary energy for mixing and gas transfer.

SELECTION CRITERIA FOR LAB SCALE AQUACULTURING

In order to employ a sufficient culture volume to enable regular culture sample extractions for measuring a number of crucial parameters during the experiments, the ideal culture compartment size should be sufficiently large enough to be appropriate for a variety of experimental setups. For microalgae production, the reactor should have optimal features such as an even light route and effective gas exchange. Multiple sensors and electrodes should be able to be inserted into the bioreactor in order to log and regulate various cultivation parameters. The reactors design should occupy less bench area and include an easy-to-use, reasonably priced temperature control system. In order to reduce stress in breakable materials like glass generated by contact with tougher materials like metals, the materials should be sufficiently sturdy to withstand heavy use, taking into consideration features like the strength of the threads of ports and openings. Simple, repeatable sample extractions from culture should be possible with this methodology. The ability to autoclave the system is a crucial feature for monoculture tests conducted on a lab scale. A control system for regulating various parameters, including temperature, pH, agitation, CO₂ supply, and optical density (OD), as well as the ability to log parameters like dissolved oxygen (DO) and generated gas, should be incorporated into the PBR system. (76).

Materials used for the various PBR components should be evaluated based on standards like the following (76):

• Autoclavability and heat resistance
• Oxidation and corrosion resistance
• Possible negative impacts on the microalgae cultures
• Weight durability
• Market availability 
• Transparency when required

RECENT DEVELOPED INNOVATIONS

1.  Electrochemical Oxidative PBR

Electrochemical oxidative (EO) PBR is a model formed by combing EO system and bubble column PBR (BPBR) system as described in figure 4. It uses distillery wastewater (DWW) as conductive medium due to its high concentration of ionic substance and reduce its COD (Chemical oxygen demand) and colour. It makes nitrogen and phosphorus available to the biological system to increase its productivity. In the sequential process, the algal biomass generated can replace 6% of the total energy used for EO. Therefore, consecutive EO-BPBR treatment can recover 32% of the energy used for EO. EO-PBR model was developed (26) and used Ti-RuO₂ anodes and then supplied the EO-DWW for culturing of microalgae present in BPB. Anode made up of Ti-PbO₂ , Ti-SbO₅ – SnO_{2 ,} BDD can also be used for EO of DWW (58,78,1.)

•  Airlift PBR  

A tubular PBR was designed which used airlift pump to circulate the culture. This airlift pump helped to amalgamate fluid mechanics principle and study the effect of irradiance, flow velocity and dilution rate on the growth of culture (49). It is illustrated in figure 5. It was observe that during maximum irradiance photoinhibition occur and lead to decrease in photosynthetic activity. Low flow velocity can cause degradation of culture but above the threshold velocity it directly affects bio productivity. The effect of oxygen concentration was studied in both outdoor and indoor conditions. It was observed, the outdoor setup has low productivity due to high illumination and photooxidation. 

•  Microfluidic PBRs

Microfluidic PBRs are recent option for studying microalgae. These chips consist of several culture compartments and connected with a fluidic channel, to allow the run-in of microalgae as well as nutrients. Usually built with polydimethylsiloxane (PDSM) layers, in which single coloies are trapped in an array. The dimension of each layer is around 2-3 cm long, 7-8 cm wide and 3 mm thick as shown in figure 6 (37). It allows the preparation of high- throughput experiments, since several different conditions can be studied at once in a small space e.g. effect of light cycle and light intensity variability in microalgae cultivation. 

Figure 6: The high-throughput microfluidic microalgal PBR array. (A) The platform was composed of four layers- a light blocking layer, a microfluidic light-dark cycle control layer, a microfluidic light, a microfluidic light intensity control layer, and a microalgae culture layer. (B) Enlarged view of a single culture compartment having five single-colony trapping sites. (C) A single- colony trapping site composed of four micropillars. (37).

• Rocking PBR

Traditional PBR design were energy and money consuming and to upgrade these designs they require further more expensive technique. To deal with this and inspired by BICCAPS (bicarbonate-based integrated carbon capture and algae production system) strategy (7,8) a model was developed, (95) which used natural force like gravity for mixing on a sea saw rocker by placing simple plastic bags at the ends of sea saw as PBR chamber as shown in figure 7. It helped to reduce electricity usage and still stay more productive. These plastic bags were covered with transparent cover on top with holes in it to prevent evaporation and oxygen release. Effect of factors like PBR chamber depth, rate of rocking cycle and light intensity was observed on bioproductivity, pH fluctuation and DO accumulation. It was concluded that model was proven to be easy scaling- up, feasible, driven by nature force and less expensive. As a hybrid design of PBR and open pond, it is essentially a horizontal PBR, which combines the benefits of both, such as low manufacturing costs, closed systems, short easy scaling-up and light path. (16).

• Insulated Glazed Photovoltaic Flat PBR

The large scale practical application of the lean management methodologies examined here is practicability, final product market value, system life and commercial feasibility. Combining existing and emerging technologies for enhanced light management a model was developed (53) that can be helpful to increase algal biomass and have low manufacturing cost and having less negative impact on environment e.g. and insulated glazed photovoltaic flat PBR (IGU PBR) (figure 8). Photosynthetically useful PAR is used by algae for its growth while UV and IR wavelength are useful for electricity generation. This type of PBR has advantages like productivity and overall low coat of cooling and energy usage. Table 1 lists some advantages and disadvantages of IGU PBR.

TABLE 1: Techniques used in IGU PBR (53)

Figure 8: Microalgae cultivation flat plate PBR. (a) Image of the PBR in operation at the Algae R&D Centre, Murdoch University, Australia. Left to right: passive evaporative cooling (PEC), infrared reflecting film (IRF) and insulated-glazed photovoltaic (IGP) photobiorectors. (b) Schematic showing construction details of the IGP PBR. (53).

• Hybrid Anaerobic Buffled Reactor PBR

This model was developed by integrating Hybrid Anaerobic Buffled Reactor (HABR) with PBR (PBR) for producing algal biofuel and waste water treatment as shown in figure 9. Wastewater being rich in organic and ionic content is used for generating feedstock with the help of HABR for cultivating biofuel rich microalgae in PBR unit (35). This type of photobiorecror has two layouts i.e HABR layout and PBR layout.

HABR layout has seven chambered anaerobic chamber either insulated or uninsulated where 1^st chamber – Settling chamber, 2^nd– 5^th chamber- Hanging baffled reactor, 6^th& 7^th chamber – Floated filter media chamber. HABR gets inoculated with waste water passing through settling chamber and then the following chambers. Floated filter media chamber gets filled with shredded soft drink lids to avoid clogging during treatment process. Insulation was done by applying 2 inch thick foam layer of polyurathane at room temperature (27- 25⁰ C). PBR set up was constructed using clear soft drink bottles upto a desired height. Conical head of bottle was placed at bottom of the set up so that after cultivating microalgae gravity settling can help in harvesting.

• Quorum Quenching (QQ) Membrane PBR

The technique of Quorum Quenching (QQ) was implemented in Membrane PBR (MPBR) as a strategy to mitigate fouling issues (21). Figure 10 is describing this model. The QQ activity of the beads was assessed by using N-octanoyl homoserine lactone (C8-HSL) as a representative signal molecule (8). With the help of QQ beads, antifouling ability was analysis on membrane surface. It’s affecting factors were transmembrane pressure (TMP), extracellular polymeric substance (EPS), algal species and its bloom morphology. According to the findings, the TMP of the experimental MPBR (MPBR-QQ) was only 448 mbar and 676 mbar, respectively, while the TMP of the control MPBR (MPBR-C) reached 818 mbar and 912 mbar on the operation hours of 35 and 170 indicating 54% and 26% reduction respectively. The TMP reductions at both time points suggest that the experimental MPBR-QQ may offer a viable alternative to conventional membrane systems, especially in applications where lower TMP and longer operational lifetimes are crucial for sustainability and cost-effectiveness. All of these results point to a more effective membrane fouling control performance from the experimental MPBR-QQ, which could result in lower operating costs and a longer membrane lifespan.

• Internet of Things PBR

Integration of Internet of Things (IoT) with different type of PBR designs can help to up-scale the results as compared to manual data. IoT is set-up that helps to connect mechanical or digital machine and other devices and transfer data with or without human to human or human to computer interaction. With the help of AutoCAD numerous PBR designs can be drawn and increase the execution efficiency of design as illustrated in figure 11. It helps to monitor parameters like pH, temperature, light, liquid level and other things whether we are physically present in lab or not. An IoT used water proof sensors carried through Arduino NodeMCU board linked with smartphone application Blynk (81).

• Rotating floating PBR

RFP model was developed by placing the simple PBR in open flowing water attached with a paddle wheel (24).Waves generated by flowing water turn the paddle wheel and thus causing the rotation of PBR.The waves generated can be due to flowing stream, river, tidal waves or open raceway pond.It has advantages like cost efficient, energy saving, controlled temperature, uniform light distribution, simple design, easy maintenance, easy mixing and many more.Many researchers have tried different type of updates with floating PBR to make them economical and productive. Figure 12 shows open raceway pond present in CCS University, Meerut.

1.  Thermosiphon PBR

A novel passive heat control technology for growing photosynthetic microorganisms like algae is the thermosiphon PBR. It works on the basis of thermosiphon circulation, which does not require energy-intensive pumps since temperature variations within the bioreactor create natural convection currents. The culture medium is effectively circulated throughout the bioreactor using this technique, guaranteeing ideal light exposure and temperature control for photosynthesis. Thermal and hydrodynamic performance of the prototype reactor based on the thermosiphon phenomenon and employing computational fluid dynamics (CFD) was examined (11). Photobiorectors develop temperature- induced density shifts as an outcome of light absorption. A radiative transport equation that integrates experimentally measured spectrum irradiance and attenuation characteristics was used to describe the uneven volumetric sensible heating from absorption of light, which is predominantly mediated by microbial cells in the reactor. The buoyancy-driven convection in the thermosiphon PBR (TPBR) has been explained via the boussinesq approximation as well as by empirical and analytical heat transfer coefficients. Figure 14 describes a diagrammatic illustration of TPBR.

Figure 13: Diagrammatic representation of the TBPR’s lighting. The front surface (z=0), to the rear surface(z=L), with A, q_(I), I_z(λ) , I_0(λ) being the irradiated area, incident radiant illumination, radiant illumination at any point z and volumetric sensible heat generated at that point, z within the riser respectively.

1. Miniature Algal Raceway Setup (MARS)

Inspired from raceway pond and open pond we developed a  miniature setup to cultivate algae in small scape at Department of Botany , CCS University, Meerut as described in figure 14. This small, hand-made model displays a raceway pond system, which is a popular aquaculture technique for raising fish or other aquatic creatures. The model, which was made of sturdy plastic trays, depicts the distinctive layout and flow dynamics of a raceway pond on a smaller size. A continuous, moving water system that replicates natural water conditions while guaranteeing ideal oxygenation and waste disposal is frequently provided by the raceway pond. This model has several interconnected trays set up to mimic water flow and the way aquatic life is raised in such systems. Plastic trays provide a lightweight, adaptable structure that is perfect for presentations, teaching, or as a decorative piece. The design provides a straightforward and understandable method of comprehending the fundamentals of raceway pond engineering by emphasising important components such the water entry, flow channels, and overflow systems. It’s the ideal illustration of how aquaculture systems effectively promote environmentally friendly fish farming methods while utilising the least amount of space and resources possible.

ECONOMIC ISSUES AND LIMITATIONS OF LARGE-SCALE PBRS

With their regulated settings that increase biomass productivity, large-scale PBRs (PBRs) offer a viable path towards sustainable microalgae cultivation. However, a number of obstacles prevent them from being economically viable. Depending on the design and materials chosen, the initial capital investment required to build PBRs can be significant, frequently exceeding €0.5 million per hectare (51). Energy-intensive requirements for temperature regulation, lighting, and mixing, particularly in closed systems intended to avoid contamination, significantly increase operational costs (9). Additionally, light attenuation problems limit the scalability of PBRs; as culture depth increases, light penetration decreases, which impacts photosynthetic efficiency (48). Surface area-maximizing designs are therefore required, which unintentionally increase land consumption and related expenses. Microalgal biomass harvesting presents additional financial difficulties because low cell densities in cultures need processing huge amounts of water, which raises the cost of dewatering and drying (60). Operational costs are further increased by the need for strict sterilization procedures and monitoring due to the potential of biofouling organisms and contamination (84). Even while PBR technology is being developed to address these problems—such as creating vertical and thin-layer systems to improve light utilization—economic viability is still a major obstacle to widespread use (56). As a result, resolving these financial constraints is essential to the economic and sustainable use of PBRs in microalgae farming.

IMPORTANCE OF LAB-SCALE PBRS

The development of algal biotechnology and bioengineering is greatly aided by lab-scale PBRs (PBRs), which provide a controlled environment for the investigation and production of microalgae. Researchers can improve growth conditions, examine physiological reactions, and examine the effects of environmental factors including temperature, light intensity, carbon dioxide content, and nutrition availability using these small-scale systems (84). The dependability and reproducibility of experimental results are guaranteed by the contamination-free environment that lab-scale PBRs offer in contrast to open pond systems (83). Design versatility allows for customized experiments to meet particular species and goals. Examples of these are flat-panel, tubular, bubble column, and airlift PBRs (56). They are essential for screening high-value metabolites, such as proteins, lipids, pigments, and antioxidants, under various stress situations, and their use is not restricted to biomass productivity (59). Prior to scaling up, lab-scale PBRs enable early-stage research, enabling parameter fine-tuning and risk mitigation (62). Additionally, they supply vital information for modelling and simulation projects, which aid in the optimization of system design and the prediction of large-scale behaviour (48). According to Wijffels et al. (2013), these reactors are crucial for metabolic engineering research because they allow genetically changed strains to be assessed under specific circumstances. Additionally, they support small-scale, manageable wastewater treatment trials and environmental applications such as CO₂ sequestration (60). The ability to automate lab-scale PBRs using sensors for  pH, dissolved oxygen, light, and nutrition levels improves the precision of experiments (4). In classrooms, lab-scale PBRs are useful resources for teaching and illustrating the fundamentals of photosynthesis, microbiology, and bioprocess engineering. They are affordable and accessible to a variety of labs and institutions because of their small size (39). In general, laboratory- scale PBRs are essential for research, innovation, and knowledge creation, all of which advance commercial-scale application and long-term solutions in microalgal biotechnology.

FUTURE OUTLOOK:

Commercial biofuel production requires the perfect balance of system and process technology improvements, as well as realistic economic viability. It is clear that adding enabling technologies like genetic engineering and nanomaterials might greatly boost the production of biofuel. The latter has a noticeable impact on the productivity of microalgae-based biofuel production. Numerous microalgal species were investigated, and the production of biofuel and harvesting efficiencies were enhanced by the use of nanoparticles. Furthermore, transcriptome analyses were presented because of their function in microalgal species’ metabolite production. Techno-economic and environmental factors are the primary obstacles to the manufacturing of biofuel. Although cultivating algae in an isolated setting may assist with reducing environmental difficulties, pretreatment processes’ energy and cost breakeven values need to be lowered because they are far from optimal. Further research and development, beginning with the cultivation of micro algae and the biofuel production process, can help overcome these obstacles and arrive at a commercially viable, affordable alternative. (54).

CONCLUSION:

The latest developments in PBRs for increasing the biomass output of biofuel derived from microalgae, as well as the challenges and restrictions associated with their usage, were the main topics of this review. It is clear that the use of enabling technologies could greatly boost the productivity of biofuels. Aquaculturing of microalgae in PBRs has been done for long time, but it is still challenging. Issues like high cost, labour-intensive, technology, space, maintenance and much more. PBR configuration also gets affected by environmental factors like light and temperature conditions, pH and others. In order to model cultural systems and forecast their output, the outcomes of meticulously regulated circumstances are crucial. This review paper aims to list different types of PBR dealing separately with the above-mentioned issues, hoping to provide a brief vision to researchers for designing a comprehensive setup of PBR that addresses these issues.

AUTHORS’ CONTRIBUTION:

 All authors have contributed equally to the writing of the original draft and the review and editing of the manuscript. The authors approved the final version of the submitted manuscript.

ACKNOWLEDGMENT:

The authors are thankful to the Head, Botany Department, Chaudhary Charan Singh University, Meerut, India, for providing the research facilities. Authors are thankful to the Govt. of Uttar Pradesh for financial support. Deepti Gupta is also thankful to the C.C.S. University, Meerut for financial assistance as University Fellowship.

REFRENCES:

1. Alvarez-Pugliese, C.E., Moreno-Wiedman, P., Machuca-Martínez, F. & Marriaga- Cabrales, N. (2011). Technol. Distillery wastewater treated by electrochemical oxidation with boron-doped diamond electrodes. J. Adv. Oxid.  14 , 213–219. https://doi.org/10.1515/jaots-2011-0205
2. Berberoglu, H., Pilon, L. & Melis, A. (2008). Radiation characteristics of Chlamydomonas reinhardtii CC125 and its truncated chlorophyll antenna transformants tla1, tlaX and tla1-CW+. Int J Hydrogen Energy. 33(22), 6467–83. https://doi.org/10.1016/j.ijhydene.2008.07.071
3. Briassoulis, D., Panagakis, P., Chionidis, M., Tzenos, D., Lalos, A., Tsinos, C., Berberidis, K., & Jacobsen, A. (2010). An experimental helical-tubular PBR for continuous production of Nannochloropsis sp. Bioresource Technology. 101(17), 6768-6777. https://doi.org/10.1016/j.biortech.2010.03.103
4. Carvalho, A. P., Meireles, L. A., & Malcata, F. X. (2006). Microalgal reactors: a review of enclosed system designs and performances. Biotechnology Progress. 22(6), 1490–1506. https://doi.org/10.1021/bp060065r
5. Chen, C.Y., Saratale, G.D., Lee, C.M., Chen, P.C. & Chang, J.S. (2008). Phototrophic hydrogen production in PBRs coupled with solar-energy-excited optical fibers. Int. J.Hydrog. Energy.  33, 6878–6885. https://doi.org/10.1016/j.ijhydene.2008.09.014
6. Chen, F. (1996). High cell density culture of microalgae in heterotrophic growth. Trends in biotechnology. 14(11), 421-426.
7. Chi, Z., O’Fallon, J.V. & Chen, S. (2011).  Bicarbonate produced from carbon capture for algae culture. Trends Biotechnol. 29(11), 537–541.
8. Chi, Z., Xie, Y., Elloy, F., Zheng, Y., Hu, Y. & Chen, S. (2013). Bicarbonate-based Integrated Carbon Capture and Algae Production System with alkalihalophilic cyanobacterium. Bioresour Technol. 133, 513–521. https://doi.org/10.1016/j.biortech.2013.01.150
9. Chisti, Y. (2007). Biodiesel from microalgae. Biotechnology Advances, 25(3), 294–306. https://doi.org/10.1016/j.biotechadv.2007.02.001
10. Chisti,Y. & Moo-Young, M. (2006). Bioreactor design. Basic Biotechnology. 3, 181-200.
11. Cho, B. A., & Pott, R. W. M. (2019). The development of a thermosiphon PBR and analysis using Computational Fluid Dynamics (CFD). Chemical Engineering Journal. 363, 141-154. https://doi.org/10.1016/j.cej.2019.01.104
12. Chojnacka, K., & Noworyta, A. (2004). Evaluation of Spirulina sp. growth in photoautotrophic, heterotrophic and mixotrophic cultures. Enzyme and microbial technology. 34(5), 461-465. https://doi.org/10.1016/j.enzmictec.2003.12.002
13. Cuaresma, M., Janssen, M., Vílchez, C., & Wijffels, R. H. (2011). Horizontal or vertical PBRs? How to improve microalgae photosynthetic efficiency.  Bioresource technology. 102(8), 5129-5137. https://doi.org/10.1016/j.biortech.2011.01.078
14. Dasgupta, C. N., Gilbert, J. J., Lindblad, P., Heidorn, T., Borgvang, S. A., Skjanes, K., & Das, D. (2010). Recent trends on the development of photobiological processes and PBRs for the improvement of hydrogen production. International Journal of Hydrogen Energy.  35(19), 10218-10238. https://doi.org/10.1016/j.ijhydene.2010.06.029
15. Deviany, D., & Chaerun, S. K. (2024). Limitations of the First‐and Second‐Generation Solid‐Gaseous Biofuels in a Time of Climate Emergency. Solid‐Gaseous Biofuels Production; 229-243.
16. Dogaris. I., Welch, M., Meiser, A., Walmsley, L., & Philippidis, G. (2015). A novel horizontal PBR for high-density cultivation of microalgae. Bioresour Technol. 198, 316–324. https://doi.org/10.1016/j.biortech.2015.09.030
17. Ebhodaghe, S.O.; Imanah, O.E. & Ndibe, (2022). H. Biofuels from microalgae biomass: A review of conversion processes and procedures. Arab. J. Chem; 15. 103591
18. Furuhashi, K., Hasegawa, F., Saga, K., Kudou, S., Okada, S., Kaizu, Y., & Imou, K. (2016). Effects of culture medium salinity on the hydrocarbon extractability, growth and morphology of Botryococcus braunii. Biomass and bioenergy. 91, 83-90. https://doi.org/10.1016/j.biombioe.2016.05.007
19. Gauri ,Gupta D, Doli, Goswami V, Bhardwaj S, Sharma P, Malik D, Sarma K, Sharma H, Kant R. (2024) .Exopolysaccharides from Cyanobacteria: Potential Source, Extraction Process and Application: A Review. Bioscene , 259-280. 06ce0-258-281.10000348.pdf.
20. Goldman, J. C., Dennett, M. R., & Riley, C. B. (1982). Effect of nitrogen‐mediated changes in alkalinity on pH control and CO₂ supply in intensive microalgal cultures. Biotechnology and Bioengineering. 24(3), 619-631.  https://doi.org/10.1002/bit.260240308
21. Güneş, G., & Taşkan, E. (2022). Quorum quenching strategy for biofouling control in membrane PBR. Chemosphere. 288, 132667. https://doi.org/10.1016/j.chemosphere.2021.132667
22. Gupta D, Gauri, Doli, Sarma K, Sharma H, Malik D, Sharma P, Bhardwaj S, Goswami V, Kant R. (2025). Liquid Biofuel from Microalgae: A Review. Plant Archives. 25 (1). https://doi.org/10.51470
23. Ho, S. H., Chen, C. Y., Lee, D. J., & Chang, J. S. (2011). Perspectives on microalgal CO₂-emission mitigation systems—a review. Biotechnology advances. 29(2),189-198. https://doi.org/10.1016/j.biotechadv.2010.11.001
24. Huang, J. J., Bunjamin, G., Teo, E. S., Ng, D. B., & Lee, Y. K. (2016). An enclosed rotating floating PBR (RFP) powered by flowing water for mass cultivation of photosynthetic microalgae. Biotechnology for biofuels. 9, 1-18. https://doi.org/10.1186/s13068-016-0633-8
25. Jacob, A., Kirst, G. O., Wiencke, C., & Lehmann, H. (1991). Physiological responses of the Antarctic green alga Prasiola crispa ssp. antarctica to salinity stress. Journal of Plant Physiology. 139(1), 57-62. https://doi.org/10.1016/S0176-1617(11)80165-3
26. Johnson, I., Krishnan, C., & Kumar, M. (2023). A sequential electrochemical oxidation–algal PBR system for the treatment of distillery wastewater. Journal of Environmental Chemical Engineering. 11(5), 110208. https://doi.org/10.1016/j.jece.2023.110208
27. Kant, R. (2011). Unicellular and colonial cyanobacterial diversity of Tripura. In Proceedings of National Conference on water, energy and biodiversity. Eds. Ghosh, N.C., Bhoaumik, S., Gupta, A.K., Lodh, T., Debbarma, M., Chakraborty, S. Excel Bharat Publishers, N. Delhi. pp. 284-293.
28. Kant, R. (2012a). Natural pigments production by two Strains of Tolypothrix Kützing Scytonemataceae, Blue-Green Algae. Nat. J. Life Scs. 9 (2), 221-223. 10.21608/egyjs.2024.295582.1029
29. Kant, R. (2012b). Distribution pattern of taxa of family Nostocaceae, Nostocales, Cyanoprokaryote in rice-fields of Kailashahar and adjoining area. L.Sc.Bulletin. 9 (2), 395-397.
30. Kant, R., Tiwari, O. N., Tandon, R. & Tiwari, G. L. (2004a.) Morphology and taxonomy of cyanobacteria: I. The genus Aphanothece (Chroococcales). Nat. J. Life Sciences. 1 (1), 1-10.
31. Kant, R., Tiwari, O. N., Tandon, R. & Tiwari, G. L. (2004b). Biodiversity characterization of Indian unicellular and colonial cyanobacteria. Nat. J. Life Sciences.1(2), 293-304
32. Kant, R., Tiwari, O. N., Tandon, R. & Tiwari, G. L. (2006). On the validity of the genus Aphanothece Nägeli, Chroococcales, Cyanoprokaryota. J. Indian Bot. Soc. 85 (1-4), 61-65.
33. Kant, R., Tiwari, O.N., Tandon, R. & Tiwari, G.L. (2006) Cyanobacteria-wonder microbes: hope for 21 st Century. Natl. Acad. Sci. Lett. 29 (11&12), 399-409.
34. Kant. R., Sarma, K., Singh, J., Ziyaul, N., Saini, A. & Kumar, S. (2020b). Seasonal fluctuation in cyanobacterial flora of anthropogenic water reservoir of Kailashahar, Unakoti, Tripura, India. Plant Archives 20 (2), 3467-3474.
35. Khalekuzzaman, M., Alamgir, M., Islam, M. B., & Hasan, M. (2019). A simplistic approach of algal biofuels production from wastewater using a Hybrid Anaerobic Baffled Reactor and PBR (HABR-PBR) System. PLoS One.  14(12), e0225458. https://doi.org/10.1371/journal.pone.0225458
36. Khalil, Z. I., Asker, M. M., El-Sayed, S., & Kobbia, I. A. (2010). Effect of pH on growth and biochemical responses of Dunaliella bardawil and Chlorella ellipsoidea. World Journal of Microbiology and Biotechnology. 26, 1225-1231. https://doi.org/10.1007/s11274-009-0292-z
37. Kim, H.S., Weiss, T.L., Thapa, H.R., Devarenne, T.P., & Han A. (2014). A microfluidic PBR array demonstrating high-throughput screening for microalgal oil production. Lab Chip. 14, 1415. https://doi.org/10.1039/C3LC51396C
38. Kim, Z.H., Park, H., Hong, S.J., Lim, S.M., & Lee, C.G. (2016). Development of a floating PBR with internal partitions for effcient utilization of ocean wave into improved mass transfer and algal culture mixing. Bioprocess Biosyst Eng. 39, 713–723. https://doi.org/10.1007/s00449-016-1552-6
39. Kumar, K., Mishra, S. K., Shrivastav, A., Park, M. S., & Yang, J. W. (2015). Recent trends in the mass cultivation of algae in raceway ponds. Renewable and Sustainable Energy Reviews. 51, 875–885. https://doi.org/10.1016/j.rser.2015.06.033
40. Lee,Y.-K. & H.-S.Tay .(!991). High CO₂ partial pressure depresses productivity and bioenergetics growth yield of Chlorella pyrenoidosa culture. Appl. Phycol.  3, 95-101. https://doi.org/10.1007/BF00003690
41. Lv, J. M., Cheng, L. H., Xu, X. H., Zhang, L., & Chen, H. L. (2010). Enhanced lipid production of Chlorella vulgaris by adjustment of cultivation conditions. Bioresource technology. 101(17), 6797-6804. https://doi.org/10.1016/j.biortech.2010.03.120
42. Mandal, S., & Mallick, N. (2009). Microalga Scenedesmus obliquus as a potential source for biodiesel production. Applied microbiology and biotechnology. 84, 281-291. https://doi.org/10.1007/s00253-009-1935-6
43. Markou, G., Vandamme, D., & Muylaert, K. (2014). Microalgal and cyanobacterial cultivation: The supply of nutrients. Water research. 65, 186-202. https://doi.org/10.1016/j.watres.2014.07.025
44. Masakazu, T., Shimpei, A., Takahiro, Y., Akihiko, K., & Hiroshi, K. (2015). A pilot-scale foating closed culture system for the multicellular cyanobacterium Arthrospira platensis NIES-39. J Appl Phycol.   27,1–12. https://doi.org/10.1007/s10811-014-0484-2
45. Mercer, P. & Armenta, R.E. (2011). Developments in oil extraction from microalgae. European journal of lipid science and technology.  113(5),  539-547.  https://doi.org/10.1002/ejlt.201000455
46. Miao X & Wu Q. (2004). High yield bio oil production from fast pyrolysis by metabolic controlling of Chlorella protothecoides. J Biotechnol;110:85–93.
47. Mofijur, M.; Rasul, M.; Hassan, N & Nabi, M. (2019). Recent Development in the Production of Third Generation Biodiesel from Microalgae. Energy Procedia ; 156,  53–58.
48. Molina Grima, E., Belarbi, E. H., Acién Fernández, F. G., Robles Medina, A., & Chisti, Y. (1999). Recovery of microalgal biomass and metabolites: process options and economics. Biotechnology Advances, 20(7–8), 491–515. https://doi.org/10.1016/S0734-9750(02)00050-2
49. Molina, E., Fernández, J., Acién, F. G., & Chisti, Y. (2001). Tubular PBR design for algal cultures. Journal of biotechnology.  92(2), 113-131. https://doi.org/10.1016/S0168-1656(01)00353-4
50. Naqqiuddin, M.A., Nor, N.M., Omar, H., & Ismail, A. (2014). Development of simple floating PBR design for mass culture of Arthrospira platensis in outdoor conditions: effects of simple mixing variation. J Algal Biomass Util.   5, 46–58
51. Norsker, N. H., Barbosa, M. J., Vermuë, M. H., & Wijffels, R. H. (2011). Microalgal production—a close look at the economics. Biotechnology Advances, 29(1), 24–27. https://doi.org/10.1016/j.biotechadv.2010.08.005
52. Novoveska, L., Zapata, A.K.M., Zabolotney, J.B., Atwood, M.C., & Sundstrom, E.R. (2016). Optimizing microalgae cultivation and wastewater treatment in large-scale offshore PBRs. Algal Res.  18, 86–94. https://doi.org/10.1016/j.algal.2016.05.033
53. Nwoba, E. G., Parlevliet, D. A., Laird, D. W., Alameh, K., & Moheimani, N. R. (2020). Pilot-scale self-cooling microalgal closed PBR for biomass production and electricity generation. Algal Research. 45, 101731. https://doi.org/10.1016/j.algal.2019.101731
54. Olabi, A. G., Alami, A. H., Alasad, S., Aljaghoub, H., Sayed, E. T., Shehata, N., Rezk, H., & Abdelkareem, M. A. (2022). Emerging Technologies for Enhancing Microalgae Biofuel Production: Recent Progress, Barriers, and Limitations. Fermentation;8(11), 649. https://doi.org/10.3390/fermentation8110649
55. Park, H., Jung, D., Lee, J., Kim, P., Cho, Y., Jung, I., Kim, Z.H., Lim, S.M. & Lee, C.G., (2018). Improvement of biomass and fatty acid productivity in ocean cultivation of Tetraselmis sp. using hypersaline medium. Journal of applied phycology. 30, 2725-2735. https://doi.org/10.1007/s10811-018-1388-3
56. Posten, C. (2009). Design principles of photo-bioreactors for cultivation of microalgae. Engineering in Life Sciences, 9(3), 165–177. https://doi.org/10.1002/elsc.200900003
57. Pragya, N., Pandey, K.K.,  & Sahoo, P.  (2013). A review on harvesting, oil extraction and biofuels production technologies from microalgae. Renewable and Sustainable Energy Reviews.  24, 159-171. https://doi.org/10.1016/j.rser.2013.03.034
58. Prasad, R. K., & Srivastava, S.N. (2009). Electrochemical degradation of distillery spent wash using catalytic anode: factorial design of experiments. Chem. Eng. J. 146, 22–29. https://doi.org/10.1016/j.cej.2008.05.008
59. Pulz, O., & Gross, W. (2004). Valuable products from biotechnology of microalgae. Applied Microbiology and Biotechnology. 65(6), 635–648. https://doi.org/10.1007/s00253-004-1647-x
60. Rawat, I., Ranjith Kumar, R., Mutanda, T., & Bux, F. (2013). Dual role of microalgae: Phycoremediation of domestic wastewater and biomass production for sustainable biofuels production. Applied Energy. 88(10), 3411–3424. https://doi.org/10.1016/j.apenergy.2010.11.025
61. Ren, L. J., Ji, X. J., Huang, H., Qu, L., Feng, Y., Tong, Q. Q., & Ouyang, P. K. (2010). Development of a stepwise aeration control strategy for efficient docosahexaenoic acid production by Schizochytrium sp. Applied microbiology and biotechnology.   87, 1649-1656. https://doi.org/10.1007/s00253-010-2639-7.
62. Richmond, A. (2004). Principles for attaining maximal microalgal productivity in PBRs: an overview. Hydrobiologia. 512(1), 33–37. https://doi.org/10.1023/B:HYDR.0000020364.12375.20
63. Sarma, K., Chavak, P., Doli, Sharma, M., Kumar, N. & Kant, R. (2024a). Influence of anaerobically digested dairy waste on growth and bioactive compounds of Spirulina subsalsa (Cyanobacteria) under semi-continuous culture conditions Microbiol. Biotechnol. Lett. 52(2), 114–121.
64. Sarma, K., Doli, Gupta, D., Gauri, Kour, N., Singh, J., Saini, A, Kumar, S., Sharma, M., Kumar, N., Das, D., Das, S. & Kant, R. (2023a). Diversity and morpho-taxonomy of the genus Scytonema: A heterocystous cyanoprokaryote from Tripura, India. Indian Hydrobiology, 22(1), 163–170.
65. Sarma, K., Doli, Kumar, N., Sharma, M., Halder, N. C., Tyagi, A., Gupta, D., Gauri, Malik, V. & Kant, R. (2024b). Biochemical profiling of Chlorophylls, Carotenoids, Proteins and Lipids of Trentepohlia aurea (L) C. Martius, Chlorophyta. Egyptian Journal of Phycology 25 (1), 15-20. 10.21608/egyjs.2024.295582.1029
66. Sarma, K., Kant, R., Kumar, N., Sharma, M., Malik, A., Baliyan, M., Sakshi, Baliyan, M., Doli, Gupta, D. & Gauri (2023b). Biogenic synthesis of green-nanoparticles using ethanolic extract of Nostoc punctiforme (Kützing ex Hariot) Hariot and assessment of antibacterial activity. J. Appl. Biosci. 49 (1-2), 63-67. http://www.taaleem.in/ojs/index.php/joab/article/view/166
67. Sarma, K., Kumar, N., Das, D., Das, S. & Kant. R. (2022b). Diversity and distribution pattern of the genus Calothrix Agardh ex Bornet et Flahault: A Heteropolar Cyanoprokaryote. Plant Archives. 22 (2), 383-389. 10.51470/PLANTARCHIVES.2022.v22.no2.066
68. Sarma, K., Kumar, N., Pandey, A., Das, D. & Kant. R. (2022a). Heterocystous cyanoprokaryotes: Anabaena Bory ex Bornet et Flahault and Trichormus (Ralfs ex Bornet et Flahault) Komárek et Anagnostidis (Nostocaceae, Nostocales) from Tripura, India. Journal of Plant Science & Research. 9 (2), 225-233.
69. Sarma, K., Kumar, S.; Singh, J., Saini, A., Ziyaul, N. & Kant, R. (2020). Exploring Biofuel potential of dominant microalgae of North-East Region of India. Biotech Today. 10 (1), 24 28. 10.5958/2322-0996.2020.00004.6
70. Sarma, K., Tyagi, A., Doli, Gupta, D., Gauri, Kumar, N., Sharma, M., & Kant, R. (2023). Optimization of nutrient media for enhanced production of bioactive compounds in Spirulina fusiformis Voronichin. J. Indian bot. Soc.103 (4), 290-293. https://doi: 10.61289/jibs2023.01.25.1143
71. Sarma, K.; Singh, J.; Ziyaul, N., Doli, Saini, A., Das, D., Bhattacharya, M. & Kant. R. (2021). Diversity and distribution pattern of the genus Oscillatoria Vaucher Ex Gom. (Oscillatoriales, Cyanoprokaryote) in Tripura, India. Plant Archives. 21 (2), 251-258. https://doi.org/10.51470/PLANTARCHIVES.2021.v21.no2.039
72. Satpati, G. G., & Pal, R. (2020). Photosynthesis in algae. Applied algal biotechnology; 49-68.
73. Satpati, G. G., Barman, N., Chakraborty, T., & Pal, R. (2011). Unusual habitat of algae. J. Algal Biomass Utln; 2(4), 50-52.
74. Sierra, E., Acién, F. G., Fernández, J. M., García, J. L., González, C., & Molina, E. (2008). Characterization of a flat plate PBR for the production of microalgae. Chemical Engineering Journal. 138(1-3), 136-147. https://doi.org/10.1016/j.cej.2007.06.004
75. Singh, R. N., & Sharma, S. (2012). Development of suitable PBR for algae production–A review. Renewable and Sustainable Energy Reviews. 16(4), 2347-2353. https://doi.org/10.1016/j.rser.2012.01.026
76. Skjånes, K., Andersen, U., Heidorn, T., & Borgvang, S. A. (2016). Design and construction of a PBR for hydrogen production, including status in the field. Journal of applied phycology. 28, 2205-2223. https://doi.org/10.1007/s10811-016-0789-4
77. Suh, I. S., & Lee, C. G. (2003). PBR engineering: design and performance. Biotechnology and bioprocess engineering.  8(6), 313-321. https://doi.org/10.1007/BF02949274
78. Susree. M., Asaithambi .P., Saravanathamizhan. R., Matheswaran. M. (2013). Studies on various mode of electrochemical reactor operation for the treatment of distillery effluent. J. Environ. Chem. Eng. 1, 552–558. https://doi.org/10.1016/j.jece.2013.06.021
79. Takagi, M., & Yoshida, T. (2006). Effect of salt concentration on intracellular accumulation of lipids and triacylglyceride in marine microalgae Dunaliella cells. Journal of bioscience and bioengineering. 101(3), 223-226. https://doi.org/10.1263/jbb.101.223
80. Tandon, R., Kant, R., Dwivedi, V.K. & Tiwari, G.L. (2014b). Morphological distinction of Cyanodermatium (Hydrococcaceae) and Radaisia (Hyellaceae) of Chroococcales (Cyanoprokaryota). Proc. Nat. Acad. Sci. 84 B (III), 481-486. https://doi.org/10.1007/s40011-014-0305-z
81. Tham, P. E., Ng, Y. J., Vadivelu, N., Lim, H. R., Khoo, K. S., Chew, K. W., & Show, P. L. (2022). Sustainable smart PBR for continuous cultivation of microalgae embedded with Internet of Things. Bioresource Technology. 346, 126558. https://doi.org/10.1016/j.biortech.2021.126558
82. Travieso, L., Hall, D. O., Rao, K. K., Benıtez, F., Sánchez, E., & Borja, R. (2001). A helical tubular PBR producing Spirulina in a semicontinuous mode. International Biodeterioration & Biodegradation. 47(3), 151-155. https://doi.org/10.1016/S0964-8305(01)00043-9
83. Tredici, M. R. (2004). Mass production of microalgae: PBRs. In A. Richmond (Ed.), Handbook of Microalgal Culture: Biotechnology and Applied Phycology . 178–214. Blackwell Science.
84. Ugwu, C. U., Aoyagi, H., & Uchiyama, H. (2008). PBRs for mass cultivation of algae. Bioresource Technology, 99(10), 4021–4028. https://doi.org/10.1016/j.biortech.2007.01.046
85. Wang, B., Lan, C. Q., & Horsman, M. (2012). Closed PBRs for production of microalgal biomasses. Biotechnology advances. 30(4), 904-912. https://doi.org/10.1016/j.biotechadv.2012.01.019
86. Weissman, J. C., Goebel, R. P., & Benemann, J. R. (1988). PBR design: mixing, carbon utilization, and oxygen accumulation. Biotechnology and bioengineering. 31(4), 336-344.  https://doi.org/10.1002/bit.260310409
87. Wijffels, R. H., Kruse, O., & Hellingwerf, K. J. (2013). Potential of industrial biotechnology with cyanobacteria and eukaryotic microalgae. Current Opinion in Biotechnology, 24(3), 405–413. https://doi.org/10.1016/j.copbio.2013.04.004
88. Wiley, P. E. (2013). Microalgae cultivation using offshore membrane enclosures for growing algae (OMEGA). University of California, Merced. 3598410.
89. Xin, L., Hong-Ying, H., & Yu-Ping, Z. (2011). Growth and lipid accumulation properties of a freshwater microalga Scenedesmus sp. under different cultivation temperature. Bioresource technology. 102(3), 3098-3102. https://doi.org/10.1016/j.biortech.2010.10.055
90. Xu, H., Miao, X., & Wu, Q. (2006). High quality biodiesel production from a microalga Chlorella protothecoides by heterotrophic growth in fermenters. Journal of biotechnology. 126(4), 499-507. https://doi.org/10.1016/j.jbiotec.2006.05.002
91. Xu, X. Q., & Beardall, J. (1997). Effect of salinity on fatty acid composition of a green microalga from an antarctic hypersaline lake. Phytochemistry. 45(4), 655-658. https://doi.org/10.1016/S0031-9422(96)00868-0
92. Yan, D., Lu, Y., Chen, Y. F., & Wu, Q. (2011). Waste molasses alone displaces glucose-based medium for microalgal fermentation towards cost-saving biodiesel production. Bioresource technology. 102(11), 6487-6493. https://doi.org/10.1016/j.biortech.2011.03.036
93. Zhu, C., Zhai, X., Wang, J., Han, D., Li, Y., Xi, Y., & Chi, Z. (2018). Large-scale cultivation of Spirulina in a floating horizontal PBR without aeration or an agitation device. Applied microbiology and biotechnology.  102, 8979-8987.  https://doi.org/10.1007/s00253-018-9258-0
94. Zhu, C., Zhai, X., Xi, Y., Wang, J., Kong, F., Zhao, Y., & Chi, Z. (2019). Progress on the development of floating PBR for microalgae cultivation and its application potential. World Journal of Microbiology and Biotechnology. 35, 1-10. https://doi.org/10.1007/s11274-019-2767-x
95. Zhu, H., Zhu, C., Cheng, L., & Chi, Z. (2017). Plastic bag as horizontal PBR on rocking platform driven by water power for culture of alkalihalophilic cyanobacterium. Bioresources and Bioprocessing. 4, 1-10. https://doi.org/10.1186/s40643-017-0176-2