Spatio-temporal variations in culturable bacterial community associated with denitrification in the Arabian Sea oxygen minimum zone

Abstract

ABSTRACT The Arabian Sea (AS) oxygen minimum zone (OMZ) is a site of intense denitrification, contributing to 20% of the global oceanic denitrification, playing a significant role in the nitrogen cycle. In this study, the structure and diversity of culturable bacterial communities inhabiting the water column of the AS OMZ were investigated through phylogenetic analysis and nitrate-utilizing ability was studied through culture-based studies. A total of 248 isolates collected during pre-monsoon and post-monsoon season were analysed for 16S rRNA gene sequences. Phylogenetic analysis revealed that the bacterial isolates belonged to 24 genera comprising class Gammaproteobacteria (79%), Firmicutes (18.15%), Bacteroidetes (1.61%) and Alphaproteobacteria (1.2%). Further, the ability of the organisms to reduce nitrate was tested by measuring nitrate reduction rate and nitrate reductase enzyme assay. Around 67% of the bacterial isolates were found to be nitrate reducers. The nitrate reduction rate ranged between 0.86–3.4 µM NO3 − day−1 while nitrate reductase activity varied from 0.01–1.32 U ml−1. The presence of a high number of nitrate-reducing bacteria in this region signifies a link between functional diversity and ecosystem biogeochemistry of the OMZ region of the Arabian Sea.