Quantitative analyses of denitrifying bacterial diversity from a seasonally hypoxic monsoon governed tropical coastal region

Abstract

Microbial community mediated denitrification leads to nitrate reduction and, its eventual removal from marine ecosystems. Therefore, it is necessary to understand the functional diversity in terms of the abundance and types of nitrate reductases in the microbial communities extant in the hypoxic/suboxic waters where organic matter oxidation is predominantly nitrate- reduction mediated. For this study, water samples from the surface, mid-depth and close to bottom were collected from three coastal locations along the west coast of India during Summer monsoon (SuM), Fall intermonsoon (FIM) and, Spring intermonsoon (SIM). The abundance of nitrate (narG), nitrite (nirS) and nitrous oxide (nosZ) reductase genes from these water samples were quantified by qPCR method. Besides the distinct spatio-temporal differences, the overall ranges of these three functional genes across the sampling sites ranged from 1.9 x 107 to 7.9 x 107 copies L-1 (narG), from 0.001 x 106 to 0.30 x 106 copies L-1 (nirS) and from 0.30 x 106 to 2.9 x 106 copies L-1 (nosZ). Functional gene sequencing of bacterial communities possessing these reductases were diverse and, dominated by members of Alpha-, Beta- and Gamma-proteobacteria, including taxonomic groups containing well known denitrifiers such as Pseudogulbenkiania, Kocuria, Pseudomonas, Herbaspirillum, Achromobacter, Rhodoferax, Mesorhizobium, Sinorhizobium, Shinella, Bradyrhizobiaceae, Rhodopseudomonas, Dinoroseobacter, Paracoccus, Rhodobacter, Ruegeria and Azospirillum sp. Assemblages of bacteria possessing narG, nirS, and nosZ indicated that complete denitrification could occur, particularly in the samples collected close to the bottom.