Phosphate solubilization by Bacillus isolates and its influence in a cyanobacterial co-culture

Aims: Phosphorus (P) and nitrogen (N) are essential nutrients required for the growth of crops, with scarce bioavailability in soil.The aim of this work was to study the inorganic P solubilization by three locally isolated bacteria of the genus Bacillus (San Luis, Argentina) and its influence in a co-culture with Tolypothrix tenuis, a diazotrophic and filamentous cyanobacterium used as a biofertilizer.Methodology and results: Morphological, biochemical and API50 CH tests clustered strains SL-3, SL-4 and SL-7 as members of the Bacillus subtilis group.The solubilization index (SI) on Pikovskaya's (PVK) agar showed that isolate SL-7 reached the highest solubilization value (SI=2.10),with better results than those obtained with Bacillus velezensis FZB42, a recognized plant growth-promoting bacterium (PGPB).In addition, T. tenuis was not able to solubilize tricalcium phosphate (TCP) on PVK agar.Individual cultures in Watanabe medium with TPC showed a reduction in the final biomass of 68.61% and their polyphosphate (poly-P) granules decreased considerably in size and quantity in relation to the cultures with soluble P, whose final biomass was 2.74 g/L.Co-cultures with strain SL-7 partially recovered the final biomass, reaching values of 1.37 to 1.42 g/L (p<0.01), while co-cultures with strain FZB42 showed no significant differences (p<0.05) in relation to the individual cyanobacterial culture with insoluble P. Conclusion, significance and impact of study:The results of this study indicate that the mixed inoculation of Bacillus sp.SL-7 and T. tenuis could be promising biofertilizers for overcoming the limitation of P and N compounds and boosting sustainable agriculture.