Characterization of mangrove-associated bacteria tolerance to stress, and their potential to increase NPK in OPMF biofertilizer, starch, and protein in duckweed

Biofertilizers offer an eco-friendly alternative to chemical fertilizers in agriculture, utilizing stress-tolerant plant growth-promoting microbes (PGPR) and sustainable carriers to enhance plant growth and yields. This study evaluates six mangrove-associated microbes (MAMs): Bacillus tropicus and Paenibacillus pasadenensis , Bacillus cereus , Bacillus thuringiensis , Acinetobacter radioresistens, and Enterobacter cloacae under environmental stressors, including extreme temperatures, salinity, and heavy metals. The findings of this study emphasize the resilience of B. cereus , B. thuringiensis , and B. tropicus under stress conditions, particularly at 37°C and in the presence of salt (NaCl) and heavy metals such as lead (Pb) and copper (Cu), as evidenced by their significant exopolysaccharide (EPS) production. Biofilm formation exhibited an inconsistent pattern among the bacterial strains; each strain responded differently to different stress levels. Flocculation activity showed insignificant formation under Pb exposure, NaCl, and 37°C, compared to non-stress conditions in B. cereus , B. thuringiensis , B. tropicus , and P. pasadenensis . These results suggest that these conditions neither enhance nor inhibit flocculation, indicating that the bacteria are resilient under moderate stress. Furthermore, analysis of the ability of these microbes to release Carbon (C), Nitrogen (N), Phosphate (P), and Potassium (K) from Oil Palm Mesocarp Fiber (OPMF) revealed that B. cereus significantly increased the P value ( P < 0.05 ) compared to other bacteria and the control. Then, each microbe was individually inoculated into OPMF, and its effect on duckweed growth, starch, and protein content was evaluated. Duckweed growth improved significantly ( P < 0.05 ) by day 15 when grown on OPMF inoculated with any of the microbes. A. radioresistens and E. cloacae inoculation resulted in the highest duckweed growth and protein content ( P < 0.05 ), respectively, compared to the control. The investigation into stress-tolerant MAMs under conditions of extreme and moderate temperatures, salinity, and heavy metals was chosen to explore potential threshold effects and better understand the biological mechanisms involved.