| Summary: | Rice production primarily depends on chemical fertilizers, which has resulted in substantial world issues for instance water pollution, global warming, soil degradation, and pest problems. To address these issues, N2-fixing bacteria are recommended as a biofertilizer for sustainable rice production, reducing the reliance on chemical fertilizers. Since the indigenous microbes have better adaptability to both the biotic and abiotic factors, and cause less ecological disruption than exogenous microbes, hence, the goal of this study was to isolate N2-fixing bacteria from locally grown rice plants and optimize the bioprocessing for high biomass production of a newly isolated Bacillus sp. WICC B119. The experiment involved the isolation, and screening of N2-fixing bacteria, followed by medium optimization in the shake flask using one-factor-at-a-time (OFAT) and statistical method (Response Surface Methodology, RSM). The optimized medium composition that yielded higher biomass in the shake flask was further studied in batch mode using a 5-L bioreactor under uncontrolled pH conditions (initial pH 7) to assess the scalability of the selected isolate. In this study, ten isolated bacteria with high nitrogen-fixation capability were selected and identified from among 31 isolates. Out of the ten potential bacteria strains, Bacillus sp. WICC B119 was chosen for the optimization stage because of its high nitrogen-fixation capacity and non-pathogenic toward humans and animals. The optimization process, utilizing both OFAT and statistical strategies, discernible increased biomass by 50.20% and 65.01%, respectively, as compared to the un-optimized medium. Despite the statistically optimized medium yielded 9.86% more biomass than the OFAT-optimized medium, the OFAT-optimized medium (in g L-1: glucose, 30.0; soy flour, 20.0; MgSO4∙7H2O, 0.05; CaCl2∙2H2O, 0.05; MnSO4∙H2O, 0.03; FeSO4, 0.01) was selected for bioreactor study due to its cost-effectiveness. Further cultivation in the 5-L bioreactor yielded lower biomass (9.92 g L-1) than the shake flask level of OFAT-optimized medium (11.36 g L-1). Nonetheless, a 31.35% increase in biomass was noticed when compared to the un-optimized medium. The reduction in biomass from the shake flask to the 5-L stirred tank bioreactor using the OFAT-optimized medium was within the acceptable range. Based on the findings of this study, the biomass of the nitrogen-fixing Bacillus sp. WICC B119 has the potential to be scaled up and practicable for further research at the pilot scale and, eventually, at the industrial level.
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