Diversity of culturable nitrate-reducing bacteria from the Arabian Sea oxygen minimum zone

The subsurface waters of the northern Arabian Sea display a pronounced oxygen minimum layer associated with high nitrite maxima which provide an ideal niche for organisms that can respire nitrate. Culture-based studies elaborate the physiological characteristics of the organisms and their metabolic activities in biogeochemical cycles. In this study, the bacterial diversity and nitrate utilizing activity of the culturable heterotrophic bacteria inhabiting the water column oxygen minimum zone of Arabian Sea were investigated. Nitrate-reducing bacteria were isolated from the water column in the central Arabian Sea. Genotypic characterization of the isolates using 16 S rDNA gene sequencing grouped them into three phylogenetic groups i.e. Proteobacteria, Firmicutes, and Actinobacteria. Out of the 56 isolated bacteria, 45 strains belonged to Proteobacteria, 6 to Firmicutes and 5 to Actinobacteria. The nitrate reducing ability of the isolates was tested using Griess test. Thirty-six species belonging to genera Alcanivorax, Alteromonas, Halomonas, Pseudoalteromonas, Marinobacter, Bacillus, and Vibrio were positive for NO2-/NO3- reduction. Our results imply that cultivable bacteria capable of utilizing NO3- available in the system are present in the Arabian Sea oxygen minimum zone and the conditions existing therein must be favorable for their growth and functionality.