Depletion of oxygen in certain marine areas creates oxygen minimum zones (OMZs), which can alter the species composition and abundance. We have carried out high-throughput 16S rRNA gene amplicon profiling from the Bay of Bengal (BOB) OMZ and non-OMZ areas. Typically, a total of 35 families of micro-organisms were identified as biomarkers for OMZ and non-OMZ regions in the BOB. Our analysis has identified families Pseudoalteromonadaceae, OM60 and Synechococcaceae to be abundant in oxygenated water, whereas organisms belonging to families Pelagibacteraceae and Caulobacteraceae, which are involved in sulphur and nitrogen metabolism, were prominent in the OMZ areas. Predictive functional analysis for these identified bacteria clearly that suggested an abundance of microbes with assimilatory sulphurreducing genes (cysl and csH) in the non-OMZ, while bacteria involved in dissimilatory sulphate reduction (known to carry aprA and aprB genes) were enriched in the OMZ areas. Comparative analysis with OMZ areas from Peru and Chile revealed that OMZ areas in the BOB are characterized by specific and distinctive bacterial diversity. Overall, the current analysis provides valuable documentation about the bacterial populations and their characteristics, which can generate pointers for their functional significance in the BOB.

23Large oxygen depleted areas known as oxygen minimum zones (OMZ) have been 24 observed in the Arabian Sea and recent reports indicate that these areas are expanding at an 25 alarming rate. In marine waters, oxygen depletion may also be related to global warming 26 and the temperature rise, acidification and deoxygenation can lead to major consequences 27 wherein the plants, fish and other biota will struggle to survive in the ecosystem. 28The current study has identified the microbial community structure using NGS 29 based metagenomics analysis in the water samples collected at different depth from the 30 oxygen depleted and non-OMZ areas of Arabian Sea. Environmental variables such as 31 depth, site of collection and oxygen concentration appeared to influence the species 32 richness and evenness among microbial communities in these locations. Our observations 33 clearly indicate that population dynamics of microbes consisting of nitrate reducers 34 accompanied by sulphate reducers and sulphur oxidizers participate in the interconnected 35 geochemical cycles of the OMZ areas. In addition to providing baseline data related to the 36 diversity and microbial community dynamics in oxygen-depleted water in the OMZ; such 37 analysis can provide insight into processes regulating productivity and ecological 38 community structure of the ocean. 39Keywords: Arabian Sea, bacterial diversity, oxygen minimum zones, metagenomics, 40 sulphur and nitrogen metabolism, Goa, Mangalore, Calicut. 41 42

The oxygen minimum zone of the Arabian Sea (AS) and Bay of Bengal (BOB) is rich in organic matter and is an unusual niche. Bacteria present in the oceanic water play an important role in ecology since they are responsible for decomposing, mineralizing of organic matter and in elemental cycling like nitrogen, sulfur, phosphate. This study focuses on culturing bacteria from oxygen minimum zones (OMZ) and non-OMZ regions and their phylogenetic as well as the functional characterization. Genotypic characterization of the isolates using amplified rDNA based 16SrRNA sequencing grouped them into various phylogenetic groups such as alpha-proteobacteria, gamma-proteobacteria and unaffiliated bacteria. The cultivable bacterial assemblages encountered belonged to the genus Halomonas, Marinobacter, Idiomarina, Pshyctobacter and Pseudoalteromonas. Among the enzymatic activities, carbohydrate utilization activity was most predominant (100%) and microorganisms possessed amylase, cellulase, xylanase and chitinase. A large proportion of these bacteria (60%) were observed to be hydrocarbon consuming and many were resistant to ampicillin, chloramphenicol, kanamycin and streptomycin. The high diversity and high percentage of extracellular hydrolytic enzyme activities along with hydrocarbon degradation activity of the culturable bacteria reflects their important ecological role in oceanic biogeochemical cycling. Further assessment confirmed the presence of nitrogen reduction capability in these cultivable bacteria which highlights their importance in oceanic geochemical cycling.

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