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In the present study, 22 different bacteria were isolated from open ocean water from the Gulf of Mannar, India. Of the 22 isolates, 4 were identified as Vibrio spp. (VM1, VM2, VM3 and VM4) and found to produce siderophores (iron-binding chelators) under iron-limited conditions. Different media were found to have an influence on siderophore production. Maximum siderophore production was observed with VM1 isolate in MM9 salts medium at 48 h of incubation. The isolate was confirmed as Vibrio harveyi based on 16S rRNA gene sequencing and phylogenetic analysis. Fourier-transform infrared (FTIR) and (1)H nuclear magnetic resonance (NMR) spectra revealed the hydroxamate nature of the siderophore produced. Further characterization of the siderophore revealed it to be of dihydroxamate nature, forming hexadentate ligands with Fe(III) ions. A narrow shift in ultraviolet (UV)-Vis spectrum was observed on photolysis due to ligand oxidation. Growth-promotion bioassay with Aeromonas hydrophila, Staphylococcus aureus and E. coli confirmed the iron-scavenging property of the siderophore produced by Vibrio harveyi.

Pseudomonas putida (CMMB2) was isolated from open ocean water of Gulf of Mannar. The isolate was identified based on 16S rRNA gene sequencing and phylogenetic analysis. Chrome azurol sulphonate assay confirms siderophore production by the isolate. Nature of siderophore produced by the isolate was found to be of mixed type. Siderophore production was found to be inversely proportional to iron concentration of the medium. Maximum siderophore production was observed with MM9 medium. Siderophore production was found to be influenced by different carbon, nitrogen and amino acid sources. Optimization of MM9 medium nutrient composition by response surface methodology (RSM) enhances siderophore production. Application of RSM is one of the strategic attempts in cost effective siderophore production process. Presence of aromatic ring in the siderophore with (C-O) and (C=C) stretching was ascertained by FTIR spectral analysis. Mass spectral analysis revealed the presence of chromophore in the pyoverdine siderophore. Cell free supernatant and purified siderophore was found to inhibit the growth of bacterial and fungal pathogens.

In the present study, siderophore produced by the marine yeast Aureobasidium pullulans was characterized as hydroxamate by chemical and bioassays. The hydroxamate assignment was supported by the appearance of peaks at 1,647.21-1,625.99 cm(-1) and at 1,435.04 cm(-1) in the infrared spectrum. The purified siderophore exhibited specific growth-promoting activity under iron-limited conditions for siderophore auxotrophic probiotic bacteria. Cross-utilization of siderophore indicates a symbiotic relationship between the yeast A. pullulans and the selected probiotic bacterial strains. Statistical optimization of medium components for improved siderophore production in A. pullulans was depicted by response surface methodology. The shift in UV-Vis spectroscopy indicates the photoreactive property and subsequent oxidative cleavage of purified siderophore on exposure to sunlight.

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