LAUSR

Abstract Oxygen minimum zones (OMZs) are unique, widely spread and well-studied features of the global ocean, varying in seasonality and intensity. The Northern Indian Ocean contains OMZs in the Arabian Sea (AS-OMZ) and the Bay of Bengal (BB-OMZ) having unique biogeochemical features. OMZ water column harbours distinct microbial communities that play vital roles in ocean biogeochemical cycles. Sulphur cycling processes facilitated by OMZ microbial communities are poorly understood with regards to different microbial groups involved, spatially and temporally. Here we sampled different stations in the AS-OMZ and BB-OMZ across dissolved oxygen gradients, to obtain culturable sulphur-oxidising bacterial (SOB) diversity using modified agar-based media. Phylogenetic analysis of the isolates placed the majority of them in Phylum Proteobacteria, followed by Actinobacteria, Firmicutes, and Bacteroidetes. Analyses of thiosulphate oxidation potential of isolates revealed that an α-proteobacteria, Citreicella thiooxidans and a β-proteobacterial species, Achromobacter xylosoxydans oxidised thiosulphate to sulphate within 48 h, suggesting that this potential is present within a narrow group of bacteria. This study sheds light on the functioning of the microbial community in the sulphur cycle of the Indian Ocean OMZs.