Diversity and abundance of denitrifying and anammox bacteria from the Arabian Sea oxygen minimum zone

Abstract

Bacterial community involved in nitrogen transformations in the oxygen minimum zones (OMZ) with dissolved oxygen (DO) levels below 0.5 ml L−1 is ascribed to be responsible for the reported 40% losses of fixed nitrogen. The Arabian Sea (AS) OMZ is among the largest suboxic regions accounting for a substantial loss of fixed nitrogen. Sampling was carried out at the Arabian Sea Time Series (ASTS) location (17°0.126′ N, 67°59.772′E) during three different seasons to document the diversity and abundance of both denitrifying and anammox bacteria, based on their molecular markers. Quantitative PCR (qPCR) was done to assess the abundance and distribution of genes encoding nitrite reductase (nirS) in denitrifying bacteria and hydrazine oxidoreductase (hzo) in anammox bacteria. Copy numbers of these genes had their maxima in the core OMZ depths of 250 and 500 m. The preponderance of nirS to the tune of 0.35 × 106 copy numbers L−1 and hzo, of 1.5 × 106 L−1 is indicative of simultaneous denitrification and anammox processes. The abundance of hzo was higher than nirS during all three seasons implying probable dominance of anammox process at this sampling location. Phylogenetic analysis revealed that nirS gene was present in bacteria belonging to Gammaproteobacteria, Alphaproteobacteria and Deltaproteobacteria. All the hzo sequences obtained in this study apparently imply that Candidatus Scalindua is the dominant anammox bacteria at the ASTS location.