Halophilic Phosphate-Solubilizing Microbes (Priestia megaterium and Bacillus velezensis) Isolated from Arabian Sea Seamount Sediments for Plant Growth Promotion.

Arabian Sea is a highly productive Ocean owing to deep upwelling with reports on phosphorus cycling in ocean sediments. In this study, microbes from sea mounts of the Arabian Sea at varying depths (400 m, 900 m) were screened to isolate and characterize phosphate-solubilizing bacteria (PSB) with plant growth-promoting properties. Out of the seven morphologically different PSBs, two bacterial strains with maximum phosphate solubilization index were identified as Priestia megaterium (H1) and Bacillus velezensis (H2) based on biochemical and molecular characteristics. Different factors influencing phosphatase production were optimized, which showed maximum solubilization at temperature of 30 °C (97.5 μg/mL), glucose as best carbon source (70 µg/mL), 1-M NaCl (114.1 µg/mL), and pH 8 (134.3 µg/mL) indicating their halophilic and alkaliphilic characteristics. Alkaline phosphatase enzyme was extracted and partially purified from both PSBs wherein H2 strains showed greater specific activity (24.83 U/mg). Metabolomics studies through HPLC revealed maximum production of gluconic acid (483.75 mg/L) in addition to lactic, oxalic, acetic, and succinic acid during solubilization. Biopriming effect of PSBs on tomato seed germination showed high germination index (80%) in consortia of both isolates which was also validated through root colonization by SEM analysis. Further studies using pot assay experiments also showed comparable results in marine PSB consortia with positive control (Phosphobacteria) for plant growth attributes including root height and weight. These findings suggest that the halophilic PSB strains from marine sediments could be used as potential bio-inoculants to enhance plant growth and combat saline stress for sustainable Agriculture.