Isolation, screening, characterization, and optimization of bacteria isolated from calcareous soils for siderophore production

The most effective agricultural practice to prevent iron deficiency in calcareous soils is fertilizing with synthetic chelates. These compounds are non-biodegradable, and persistent in the environment; hence, there is a risk of leaching metals into the soil horizon. To tackle iron deficiency-induced chlorosis (IDC) in crops grown on calcareous soils, environmentally friendly solutions are needed rather than chemical application as it affects the soil health further. Hence, the present work focused on isolating and screening calcareous soil-specific bacteria capable of producing iron-chelating siderophores. Siderophore-producing bacteria (SPB) was isolated from the groundnut (Arachis hypogea L.) rhizosphere region, collected from Coimbatore district, Tamil Nadu, of which 17 bacterial isolates were positive for siderophore production assayed by chrome azurol sulphonate. The performance of SPB isolates was compared for siderophore kinetics, level of siderophore production, type of siderophore produced and iron-chelating capacity under 15 mM KHCO3. Four best performing isolates were screened, with average siderophores yield ranging ∼60–80% under pH 8, with sucrose as carbon source and NH2SO4 as nitrogen source at 37 °C. The four efficient SPB were molecularly identified as B. licheniformis, B. subtilis, B. licheniformis, and O. grignonense based on 16S rDNA sequencing. The simultaneous inhibition method showed T.viride has the highest antagonistic effect against S.rolfsii, and M.phaseolina with a reduction of mycelial growth by 69.3 and 65.1%, respectively, compared to control. Our results indicate that the optimized conditions enhanced siderophores chelation by suppressing the stem and root rot fungi, which could help in a cost-effective and environmentally friendly manner.