1. Introduction

The increasing demand for agricultural production and food processing has led to the generation of enormous quantities of agro-industrial waste worldwide. Agricultural residues such as rice husk, wheat bran, sugarcane bagasse, corn stover, coconut residues, and fruit and vegetable peels constitute a major proportion of this waste and are produced in millions of tonnes annually [1]. Although these lignocellulosic residues are rich in renewable carbon and essential nutrients, they remain largely underutilized and are commonly disposed of through open burning, landfilling, or uncontrolled dumping [2]. Such disposal practices contribute significantly to greenhouse gas emissions, environmental pollution, soil degradation, and the loss of valuable biomass resources, highlighting the urgent need for sustainable waste management strategies [3].

The concept of the circular bioeconomy has emerged as an effective approach for converting agricultural waste into value-added products while minimizing environmental impacts [4]. Agro-industrial residues are primarily composed of cellulose, hemicellulose, and lignin, making them an abundant and inexpensive feedstock for microbial bioprocesses [5]. However, the rigid and recalcitrant structure of lignocellulosic biomass restricts its direct utilization by microorganisms because lignin limits the accessibility of cellulose and hemicellulose to enzymatic hydrolysis [6]. Therefore, efficient biological conversion of agro-waste requires microorganisms capable of producing extracellular hydrolytic enzymes that decompose complex polymers into fermentable sugars suitable for downstream bioprocessing.

Microbial enzymes have become indispensable biocatalysts in modern biotechnology because of their high catalytic efficiency, substrate specificity, and environmentally friendly characteristics [7]. Among these, cellulases, amylases, and proteases are of particular industrial importance due to their ability to degrade cellulose, starch, and proteins, respectively. These enzymes play a crucial role in biomass saccharification and provide simple carbon sources for microbial metabolism, thereby supporting the production of various industrially valuable metabolites [8]. Their extensive applications in food processing, pharmaceuticals, textiles, paper, detergents, animal feed, and biofuel industries have substantially increased the demand for economical enzyme production using renewable substrates [9].

In addition to industrial enzymes, microbial fermentation has become a sustainable platform for producing commercially important organic acids from renewable biomass [10]. Organic acids such as citric acid and lactic acid possess wide industrial applications as food preservatives, acidulants, pharmaceutical ingredients, cosmetic additives, and precursors for biodegradable polymers. The utilization of agro-industrial waste as a fermentation substrate not only reduces production costs but also enhances biomass valorization by converting inexpensive lignocellulosic residues into high-value biochemicals [11].

Another important advancement in industrial biotechnology is the microbial production of polyhydroxyalkanoates (PHAs), a family of biodegradable intracellular polyesters synthesized by numerous bacterial species under conditions of excess carbon and nutrient limitation [12]. PHAs have attracted increasing attention as sustainable alternatives to petroleum-based plastics because of their biodegradability, biocompatibility, and thermoplastic properties. Their applications extend to food packaging, agriculture, biomedical devices, tissue engineering, and controlled drug delivery systems. However, the commercial production of PHAs remains expensive due to the high cost of conventional carbon substrates, making agro-industrial waste an attractive low-cost feedstock for sustainable PHA production.

Despite the growing interest in microbial bioprocesses, conventional industrial fermentation systems predominantly rely on monoculture-based approaches, in which a single microbial strain performs a specific biochemical function [13]. Although monocultures have been successfully employed for the production of enzymes, organic acids, and other microbial metabolites, they often exhibit limitations such as incomplete substrate degradation, limited metabolic diversity, accumulation of inhibitory metabolites, and poor utilization of heterogeneous lignocellulosic biomass [14]. Since no single microorganism possesses all the enzymatic and metabolic capabilities required for complete biomass conversion, the efficiency of monoculture fermentation remains limited when processing complex agro-industrial residues [15].

To overcome these limitations, recent research has focused on the development of synergistic microbial consortia, in which two or more compatible microorganisms cooperate through complementary metabolic activities [16]. Within these consortia, enzyme-producing microorganisms hydrolyze lignocellulosic biomass into fermentable sugars, while other members convert these intermediates into valuable metabolites such as organic acids and polyhydroxyalkanoates. This metabolic division of labor enhances substrate utilization, improves fermentation stability, increases product yield, and more closely resembles naturally occurring microbial ecosystems [17]. Consequently, microbial consortia have emerged as promising tools for sustainable biomass valorization and integrated bioprocess development.

The successful establishment of a microbial consortium depends on the selection of compatible microorganisms possessing complementary physiological and metabolic characteristics [18]. Microorganisms isolated from agro-industrial waste and soil environments are particularly valuable because they are naturally adapted to degrade complex lignocellulosic substrates and tolerate diverse environmental conditions. Careful screening of these microorganisms for extracellular enzyme production, metabolite synthesis, and growth compatibility enables the construction of efficient and stable microbial consortia suitable for integrated fermentation processes [19].

Besides microbial selection, optimization of fermentation parameters including pH, temperature, incubation period, aeration, substrate concentration, and nutrient availability, is essential for maximizing enzyme production, organic acid synthesis, and PHA accumulation [20]. Appropriate optimization improves microbial growth, enhances metabolic efficiency, and increases product yield, thereby contributing to the development of economically viable and environmentally sustainable bioprocesses. These improvements are fundamental to the establishment of integrated microbial biorefineries capable of converting renewable biomass into multiple commercially important products [21].

Although significant advances have been achieved in industrial enzyme production, organic acid fermentation, and PHA biosynthesis, these processes have largely been investigated independently using monoculture systems [22]. Comparatively few studies have focused on integrating these three value-added products within a single synergistic microbial consortium utilizing agro-industrial waste as the primary feedstock. Therefore, the development of an efficient microbial consortium capable of simultaneously degrading lignocellulosic biomass while producing industrial enzymes, organic acids, and PHAs remains an important research challenge [23].

Accordingly, the present study aims to develop a synergistic microbial consortium for the integrated production of industrial enzymes, organic acids, and polyhydroxyalkanoates (PHAs) from agro-industrial waste. The study involves the isolation and characterization of efficient microorganisms, screening for enzyme-producing and PHA-producing isolates, development of compatible microbial consortia, optimization of fermentation conditions, and evaluation of their efficiency in converting agro-waste into multiple high-value bioproducts. The successful development of this integrated microbial system is expected to contribute to sustainable waste valorization, environmentally friendly bioprocessing, and the advancement of circular bioeconomy-based biorefinery technologies [24]

2. Materials and Methods

2.1 Sample Collection

Agro-industrial waste and rhizosphere soil samples were collected aseptically from agricultural fields and agro-processing sites. Samples were transferred to the laboratory in sterile polyethylene bags and processed within 24 h. These samples were used for the isolation of enzyme-producing bacterial strains.

2.2 Isolation and Screening of Enzyme-Producing Bacteria

Bacterial isolates were obtained by the serial dilution and spread plate method. Diluted samples were inoculated onto selective media for enzyme screening. Skim Milk Agar was used for protease-producing bacteria, Starch Agar for amylase-producing bacteria, and Carboxymethyl Cellulose (CMC) Agar for cellulase-producing bacteria [25].

Plates were incubated at 37°C for 48 h. Colonies exhibiting clear hydrolysis zones were purified by repeated streaking on nutrient agar. Purified isolates were re-inoculated on the respective screening media to confirm enzyme production. The diameter of the hydrolysis zones was measured, and the three isolates showing the largest zones for each enzyme were selected for further evaluation.

Quantitative enzyme assays were performed for the shortlisted isolates. Based on enzyme activity, one bacterial isolate with the highest production of amylase, one cellulase-producing isolate, and one protease-producing isolate were selected for subsequent fermentation studies [26].

2.3 Preparation of Agro-Industrial Waste Substrates

Coconut leaves and rice paddy residues (rice husk or rice straw) were used as fermentation substrates. The substrates were washed thoroughly with distilled water, air-dried, and ground into small particles. Before fermentation, the substrates were subjected to suitable pre-treatment to improve substrate accessibility and facilitate microbial degradation.

2.4 Submerged Fermentation for Metabolite Production

The selected bacterial isolates were cultivated individually in submerged fermentation media containing pre-treated coconut leaves or rice paddy residues as the primary carbon source. Fermentation was carried out under controlled conditions of temperature, pH, and incubation period.

After completion of fermentation, the culture broth was centrifuged to separate the biomass. The cell-free supernatant was analyzed for the production of citric acid, lactic acid, and bioethanol using standard analytical procedures. Metabolite production obtained from the two substrates was compared to determine the most suitable substrate for each bacterial isolate.

2.5 Screening and Production of Polyhydroxyalkanoates (PHAs)

The selected bacterial isolates were screened for intracellular polyhydroxyalkanoate (PHA) accumulation using Sudan Black B, Nile Blue, and Nile Red staining methods. Stained cells were examined microscopically to confirm the presence of intracellular polymer granules.

PHA-positive isolates were cultivated under nitrogen-limited fermentation conditions to promote polymer accumulation. Following incubation, PHAs were extracted by solvent extraction and quantified to determine polymer yield and production efficiency [27].

2.6 Development of a Microbial Consortium

A microbial consortium was prepared by combining the selected amylase-, cellulase-, and protease-producing bacterial isolates in equal proportions. The consortium was inoculated into fermentation media containing pre-treated coconut leaves and rice paddy residues.

The performance of the consortium was compared with that of the individual bacterial isolates by evaluating substrate degradation, production of citric acid, lactic acid, bioethanol, and PHA accumulation. PHA production by the consortium was confirmed using Sudan Black B, Nile Blue, and Nile Red staining techniques.

2.7 Optimization of Fermentation Parameters

Fermentation parameters, including pH, temperature, substrate concentration, and incubation period, were optimized to improve enzyme production and metabolite synthesis. Optimization was first carried out for each selected bacterial isolate to determine the conditions required for maximum production of amylase, cellulase, and protease.

The optimized conditions were subsequently applied during submerged fermentation for enhanced production of citric acid, lactic acid, bioethanol, and PHAs using coconut leaves and rice paddy residues. The same parameters were further optimized for the microbial consortium to achieve efficient substrate degradation and simultaneous production of multiple bioproducts [28].

3. Observation and Result 

1. Isolation, Screening, and Selection of High Enzyme-Producing Bacterial Strains

Following serial dilution, soil samples were spread onto selective media, and well-isolated bacterial colonies were randomly selected and purified. The purified isolates were spot-inoculated onto starch agar, skim milk agar, and CMC agar plates for the primary screening of amylase, protease, and cellulase production, respectively. After incubation, several isolates produced distinct hydrolysis zones, indicating extracellular enzyme production. Based on the size of the hydrolysis zones, the most promising isolates were shortlisted and subjected to quantitative enzyme assays. Isolates exhibiting the highest enzyme activities were selected for further characterization and subsequent studie

A total of nine colonies from each selective medium were initially selected based on visible hydrolysis zones. Isolates were qualitatively assessed as low, medium, or high enzyme producers. The top three isolates showing prominent clearing zones were shortlisted for further quantitative analysis.”

The amylase activity of the selected bacterial isolates was evaluated using an iodine-based colorimetric assay, and enzyme activity was measured as optical density (OD) at 620 nm. Among the isolates, Bacillus sp. S3-9A showed the lowest OD value (0.77), indicating the highest starch hydrolysis and, therefore, the greatest amylase activity. In contrast, isolates S2-5A and S2-4A recorded higher OD values of 1.82 and 1.97, respectively, indicating comparatively lower amylase production. These results confirm the superior amylase-producing potential of Bacillus sp. S3-9A, which was selected for further fermentation and optimization studies.

The cellulase activity of the selected bacterial isolates was evaluated using a colorimetric assay, and enzyme activity was measured as optical density (OD) at 540 nm. Among the isolates, Bacillus subtilis S1-2C showed the lowest OD value (0.07), indicating the highest cellulase activity and maximum cellulose degradation. In comparison, isolates S2-5C and S3-8C recorded OD values of 0.13 and 0.15, respectively, indicating lower cellulase activity. These results demonstrate differences in cellulase-producing ability among the isolates and identify Bacillus subtilis S1-2C as the most efficient cellulase producer, making it suitable for further fermentation and optimization studies.

Protease Activity Analysis of Selected Bacterial Isolates

The protease activity of the selected bacterial isolates was evaluated using a casein-based colorimetric assay, and enzyme activity was measured as optical density (OD) at 660 nm. Among the isolates, Bacillus subtilis S2-7P showed the highest OD value (0.07), indicating the greatest protease activity and maximum casein hydrolysis. In comparison, isolates S2-5P and S2-6P recorded OD values of 0.04 and 0.03, respectively, indicating lower protease activity. Unlike the amylase and cellulase assays, where lower OD values indicate higher enzyme activity, higher OD values in the protease assay reflect greater production of soluble peptides during casein hydrolysis. Based on its superior proteolytic activity, Bacillus subtilis S2-7P was selected for further fermentation and optimization studies.

2. Substrate Pre-treatment and Fermentation for Metabolite Production

Pre-treated coconut leaf and rice paddy substrates were used as fermentation media and inoculated with the selected bacterial isolates. Growth was observed on both substrates following incubation. Fermentation broths were analyzed for the production of citric acid, lactic acid, and bioethanol. Quantitative analysis revealed variations in metabolite production among the isolates and between the two substrates, with one substrate demonstrating comparatively higher yields for specific metabolites.

Preparation and Pre-treatment of Agro-Waste Substrates

The agro-industrial wastes, coconut leaves and rice paddy husk, were collected, thoroughly washed with distilled water, dried, and ground into small particles to increase the surface area for microbial degradation. The powdered substrates were then pre-treated with 1% (w/v) sodium hydroxide by heating at 80°C for 2 hours. After treatment, they were washed with distilled water until a neutral pH was achieved and dried. This alkaline pre-treatment removed a portion of the lignin, improving the accessibility of cellulose and hemicellulose for microbial utilization.

Preparation of Fermentation Media

Submerged fermentation media were prepared using pre-treated coconut leaf or rice paddy as the primary carbon source. Each 100 mL of medium contained 1 g substrate, 0.5 g peptone, 0.5 g yeast extract, and 0.5 g sodium chloride. The pH was adjusted to approximately 7.0, and the media were sterilized by autoclaving at 121°C for 15 minutes.

Inoculation and Fermentation Conditions

Three previously selected high-performing bacterial isolates producing amylase, protease, and cellulase were used for submerged fermentation. Individual as well as consortium cultures were inoculated (1% v/v) into sterile fermentation media containing either coconut leaf or rice paddy substrate and incubated at 37°C for 24 hours under static conditions to facilitate metabolite production.

Qualitative Analysis of Metabolite Production

After incubation, fermented broth samples were subjected to qualitative analysis to detect the presence of key metabolites, including citric acid, lactic acid, and ethanol.

A. Detection of Citric Acid (Ferric Chloride Test)

Approximately 2 mL of fermented broth was taken in a test tube, and a few drops of ferric chloride solution were added. The formation of a brownish-yellow coloration indicated the presence of citric acid.

C. Detection of Ethanol (Iodoform Test)

A small volume of iodine solution followed by sodium hydroxide was added to the fermented sample and gently heated. The formation of a yellow precipitate (iodoform) will indicate the presence of ethanol.

Data Recording and Analysis

Observations from qualitative tests were recorded based on visible color changes or precipitate formation. Results were expressed as presence (+) or absence (–) of metabolites. Comparative analysis was performed between individual bacterial strains and the microbial consortium, as well as between the two agro-waste substrates, to evaluate differences in metabolite production efficiency.

The selected bacterial isolates successfully produced citric acid, lactic acid, and ethanol when grown on coconut leaf and rice paddy substrates. Qualitative tests using ferric chloride (FeCl₃), calcium carbonate (CaCO₃), and the iodoform test confirmed metabolite production. The individual amylase-, protease-, and cellulase-producing isolates produced citric acid and lactic acid on both substrates, while ethanol production was comparatively lower. Slight differences in metabolite production were observed among the isolates, reflecting variations in their fermentation efficiency.

The microbial consortium showed positive production of all three metabolites and performed better than the individual isolates, indicating enhanced substrate utilization through synergistic interactions. Among the two substrates, rice paddy supported higher metabolite production than coconut leaf, suggesting that it is a more suitable substrate for microbial fermentation. These findings highlight the potential of the microbial consortium for efficient bioconversion of agro-industrial waste into value-added products.

3. Identification and Production of Biodegradable Polymers (PHAs)

Selected bacterial isolates were screened for polyhydroxyalkanoate (PHA) production using Sudan Black B, Nile Blue, and Nile Red staining. Positive isolates showed intracellular PHA granules, while others were negative, indicating variation in PHA-producing ability. The positive isolates were further cultivated under nitrogen-limited conditions to promote PHA accumulation. Polymer extraction confirmed PHA production, and quantitative analysis revealed differences in PHA yield among the selected isolates.

Production and Extraction of Polyhydroxyalkanoates (PHAs)

Screening and Selection of PHA-Producing Bacterial Strain

Preliminary screening for polyhydroxyalkanoate (PHA) production was performed using Sudan Black B and Nile Blue staining. Bacterial isolates obtained from earlier screening were stained to detect intracellular PHA granules. The appearance of dark blue to black granules in Sudan Black B staining and fluorescence in Nile Blue staining indicated positive PHA accumulation. Based on these staining results, the PHA-positive bacterial isolate was selected for further production and characterization studies.Table 9. Qualitative screening of selected bacterial isolates for polyhydroxyalkanoate (PHA) production using Sudan Black B and Nile Blue staining. 

Inoculation and Fermentation Conditions

The selected PHA-positive bacterial isolate was inoculated into 100 mL of sterile PHA production medium at an inoculum size of 2% (v/v). The culture was incubated at 30°C with agitation at 150 rpm for 72 hours. Nitrogen-limited conditions in the presence of excess glucose promoted intracellular accumulation of polyhydroxyalkanoates (PHAs).

Result Interpretation

The selected isolate S3-9A (presumptive Bacillus sp.) exhibited good growth under nitrogen-limited fermentation conditions. The optimized culture conditions supported efficient bacterial growth and promoted intracellular accumulation of PHA.

Result Interpretation

A compact cream-colored bacterial pellet was successfully recovered after centrifugation. Washing effectively removed residual impurities, and the dried biomass was suitable for subsequent PHA extraction.

Result Interpretation

The solvent extraction procedure successfully recovered intracellular polymer from the bacterial biomass. Formation of a white precipitate following methanol addition confirmed the presence of polyhydroxyalkanoates.

Qualitative Assessment of PHA Production

PHA production was assessed qualitatively based on staining intensity and polymer recovery following solvent extraction. Since gravimetric estimation was not performed, polymer production was evaluated using qualitative observations.

Result Interpretation

The selected isolate produced a visible white polymer after solvent extraction, indicating successful intracellular accumulation of PHA. Qualitative observations confirmed the isolate’s ability to synthesize biodegradable polymer under nitrogen-limited conditions.

Confirmation of PHA Production

Sudan Black B staining revealed intracellular lipid granules, while Nile Blue staining confirmed the presence of fluorescent PHA granules. The successful recovery of a white polymeric precipitate further verified PHA production by the selecte

Analysis of Data

The observations obtained from staining methods and solvent extraction were compared qualitatively to evaluate the PHA-producing capability of the selected bacterial isolate.

Result Interpretation

The selected isolate S3-9A (presumptive Bacillus sp.) showed positive results in all qualitative confirmation methods. Based on the staining characteristics and successful polymer recovery, the isolate was confirmed as a promising PHA-producing bacterium suitable for further optimization studies.

4. Development of a Synergistic Microbial Consortium

The three selected bacterial strains were successfully combined to develop a stable microbial consortium. Compared with the individual cultures, the consortium exhibited enhanced growth, improved substrate utilization, and greater degradation of agro-waste substrates. Simultaneous production of multiple metabolites, including extracellular enzymes, organic acids, and bioethanol, was observed, demonstrating higher overall productivity. PHA production by the consortium was also confirmed through positive Sudan Black B, Nile Blue, and Nile Red staining, indicating intracellular polymer accumulation within the mixed culture.

Integrated Production and Confirmation of Metabolites and Polyhydroxyalkanoates (PHAs) Using a Microbial Consortium

Development of the Microbial Consortium- The selected bacterial isolates, S3-9A (presumptive Bacillus sp.), S2-7P (presumptive Bacillus subtilis), and S1-2C (presumptive Bacillus subtilis), were combined in equal proportions to develop a microbial consortium. The consortium was inoculated into agro-waste-based fermentation medium and incubated aerobically at 30°C with agitation at 150 rpm for 48–72 hours. The culture was monitored for growth and compatibility among the constituent bacterial strains.

Result Interpretation

The selected bacterial isolates successfully formed a stable microbial consortium under the optimized fermentation conditions. Uniform turbidity and consistent growth indicated compatibility among the constituent strains. The consortium efficiently utilized both agro-waste substrates, demonstrating its suitability for integrated metabolite and biodegradable polymer production

Qualitative Confirmation of Metabolite Production

After fermentation, the culture supernatant obtained from consortium cultures grown on rice paddy and coconut leaf substrates was subjected to qualitative biochemical tests. Citric acid production was detected using the ferric chloride test, lactic acid by the calcium carbonate test, and ethanol by the iodoform test.

Result Interpretation

Qualitative biochemical analysis confirmed the production of citric acid and lactic acid by the microbial consortium on both agro-waste substrates. Ethanol production was not detected under the aerobic fermentation conditions employed in the study. Rice paddy supported comparatively higher metabolite production than coconut leaf, indicating better substrate utilization by the consortium.

Qualitative Confirmation of Polyhydroxyalkanoate (PHA) Production

Following metabolite production, the consortium culture was centrifuged to recover bacterial biomass. The pellet was washed and subjected to chloroform extraction, followed by methanol precipitation. The recovered polymer was further confirmed using Sudan Black B and Nile Blue staining to verify intracellular PHA accumulation.

The microbial consortium accumulated intracellular polyhydroxyalkanoates under nitrogen-limited conditions. Sudan Black B staining revealed characteristic intracellular granules, while Nile Blue staining confirmed PHA accumulation through fluorescence. Solvent extraction followed by methanol precipitation produced a white polymeric precipitate, confirming successful production of biodegradable polymer by the consortium.

Overall Performance of the Microbial Consortium

The overall performance of the microbial consortium was evaluated by comparing growth characteristics, agro-waste utilization, metabolite production, and qualitative confirmation of PHA production. The observations obtained from biochemical tests, staining techniques, and solvent extraction were collectively assessed to determine the efficiency of the consortium.

Result Interpretation

The developed microbial consortium consisting of S3-9A (presumptive Bacillus sp.), S2-7P (presumptive Bacillus subtilis), and S1-2C (presumptive Bacillus subtilis) exhibited stable growth and efficient utilization of agro-waste substrates. The consortium simultaneously produced citric acid and lactic acid while also accumulating intracellular polyhydroxyalkanoates, as confirmed by staining techniques and solvent extraction. The combined metabolic activities of the three bacterial isolates enhanced substrate degradation and overall bioconversion efficiency, demonstrating the potential of the consortium for sustainable production of value-added metabolites and biodegradable polymers from agro-waste.

5. Optimization of Process Parameters for Integrated Bioproduction

Optimization of fermentation conditions demonstrated that pH, temperature, and substrate concentration significantly influenced enzyme activity and metabolite production. Optimum conditions (pH 7, 30°C, and 2% substrate concentration) enhanced the production of enzymes, organic acids, and PHA, thereby improving the overall efficiency of the integrated bioprocess. 

Result Interpretation

Enzyme production varied with pH. All three isolates exhibited maximum enzyme activity at pH 7, indicating that neutral pH is most favorable for enzyme production. Reduced activity at pH 5 and pH 9 suggests that acidic and alkaline conditions adversely affected bacterial metabolism and enzyme synthesis

Result Interpretation

Temperature significantly influenced enzyme production. Maximum enzyme activity was observed at 30°C, while activity decreased slightly at 37°C and more noticeably at 25°C. These findings indicate that 30°C is the optimum incubation temperature for enzyme production by the selected bacterial isolates.

 Result Interpretation

The enzyme activity increased with substrate concentration up to 2%, after which a slight reduction was observed at 3%. This indicates that 2% substrate concentration provided the most favorable conditions for enzyme production, whereas higher substrate concentrations may have limited enzyme synthesis or substrate utilization efficiency.

Optimization of the process parameters demonstrated that neutral pH (7), an incubation temperature of 30°C, and a substrate concentration of 2% provided the most favorable conditions for enzyme production by S3-9A (presumptive Bacillus sp.), S2-7P (presumptive Bacillus subtilis), and S1-2C (presumptive Bacillus subtilis). Under these conditions, amylase and cellulase assays showed the lowest optical density values, whereas the protease assay exhibited the highest optical density, indicating maximum enzyme activity. These optimized conditions are consistent with the expected physiological characteristics of aerobic Bacillus species and provide a suitable basis for enhanced integrated bioproduction.

4. Conclusion

Rhizosphere soil proved to be a suitable source for the isolation of bacteria capable of producing extracellular enzymes, organic acids, and biodegradable polymers. Three bacterial isolates, S3-9A, S2-7P, and S1-2C, were selected based on their enzyme-producing ability. Based on morphological, cultural, and biochemical characteristics, S3-9A was identified as a presumptive Bacillus sp., while S2-7P and S1-2C were identified as presumptive Bacillus subtilis. These identifications remain tentative in the absence of 16S rRNA gene sequencing.

Among the selected isolates, S3-9A showed the highest amylase activity, S2-7P exhibited maximum protease activity, and S1-2C produced the highest cellulase activity. Enzyme production was highest at pH 7, 30°C, and 2% substrate concentration. The isolates utilized pre-treated coconut leaf husk and rice paddy husk as substrates and produced citric acid and lactic acid under submerged fermentation, whereas ethanol production was not observed under aerobic conditions.

The selected isolates also accumulated intracellular polyhydroxyalkanoates (PHAs), with isolate S3-9A showing the highest PHA-producing potential. A microbial consortium prepared from the three isolates showed improved substrate utilization and efficient degradation of agro-industrial waste. The consortium was also capable of producing organic acids and accumulating PHAs, indicating the suitability of mixed cultures for integrated bioprocessing.

The results indicate that the selected bacterial isolates and their consortium have potential for the conversion of agro-industrial waste into industrial enzymes, organic acids, and biodegradable polymers. Further work should include molecular identification of the isolates, quantitative estimation of metabolites and PHAs, optimization in bioreactor systems, and evaluation at pilot scale.

Recommendations and Future Prospects

Molecular identification of the selected bacterial isolates using 16S rRNA gene sequencing is recommended to confirm their taxonomic identity. Future studies should include quantitative estimation of citric acid, lactic acid, ethanol, and polyhydroxyalkanoates (PHAs) using appropriate analytical techniques, along with characterization of PHAs to evaluate polymer quality and composition.

Further optimization of fermentation parameters, including inoculum size, incubation time, aeration, agitation, and carbon-to-nitrogen ratio, may improve enzyme production and metabolite yield. Evaluation of the developed microbial consortium under pilot-scale and bioreactor conditions is necessary to assess its stability, productivity, and commercial feasibility. Additional agro-industrial residues should also be investigated as alternative low-cost substrates to improve process economics.

The industrial applicability of the produced enzymes and PHAs should be explored in sectors such as food processing, detergents, textiles, paper, biofuels, and biodegradable packaging. The integrated microbial process developed in this study provides a foundation for sustainable agro-waste valorization and may contribute to the development of economically viable and environmentally friendly biotechnological processes.

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