No abstract

There are regions in the world oceans where oxygen saturation is at its lowest, evident at depths between shelf to upper bathyal zone. These regions are known as Oxygen Minimum Zones (OMZs), which reportedly support phylogenetically diverse microbes. In this study, we aimed to characterize prokaryotic diversity in the water samples collected from 43, 200 and 1000 m depth of the Bay of Bengal Time Series location (BoBTS-18.0027°N, 89.0174°E) in the OMZ region. Illumina sequencing generated 3,921,854 reads of 16S rRNA gene amplicons, which corresponded to 5778 operational taxonomic units. The distribution of bacteria at class level varied with depth and oxygen concentration. a-Proteobacteria was found in abundance in 43 m and 1000 m depth water samples. c-Proteobacteria was prominently detected in oxygen-depleted depths of 200 m and 1000 m. AB16 (Marine Group A, originally SAR406) was restricted at dissolved oxygen concentration of 1.5 lM at 200 m. Archaeal members were observed in low abundance (2%), with a high occurrence of phylum Euryarchaeota at 43 m, while Crenarchaeota was detected only at 200 m depth. Select bacterial cultures were screened for their ability to reduce nitrate in vitro, to obtain insights into their possible role in the nitrogen cycle. A total of 156 bacterial isolates clustered majorly with Alcanivorax, Bacillus, Erythrobacter, Halomonas, Idiomarina and Marinobacter. Among them, 11 bacterial genera showed positive nitrate reduction in the Griess test. A large percentage (63.55%) of 16S rRNA gene amplicons corresponded to unidentified OTUs at genus or higher taxonomic levels, suggesting a greater undiscovered prokaryotic diversity in this oxygen depleted region.

No abstract

The dissolved oxygen concentration in ocean water is the main driver of nutrient and energy flow patterns within marine ecosystems (Diaz & Rosenberg, 2008;Diaz et al., 2009). Oxygen deficiency in ocean water selects for microbial groups capable of utilizing alternative respiratory substrates including nitrate, nitrite, sulfate, or carbon dioxide (Zehnder & Stumm, 1988). Within oxygen-deficient waters, the use of nitrite or nitrate as alternative electron acceptors results in the production of nitrous oxide and nitrogen gas (Lam & Kuypers, 2011). This leads to denitrification or loss of fixed nitrogen.The Arabian Sea (AS) oxygen minimum zone (OMZ) is one of the largest pelagic low-oxygen environments in the open ocean, which is a major part of global ocean-led denitrification, which extends from ~100 to 1,200 m of depth (Jayakumar et al., 2009). Here the oxygen concentration drops to 0.1 ml/L (4.5 μmol/kg) or lower, compared to the average ocean dissolved oxygen concentration that ranges between 6 and 8 ml/L (270-360 μmol/kg). These OMZ regions are of microbiological importance as major biogeochemical processes are catalyzed by the microbial communities which inhabit them