Diversity of nitrogen fixing bacterial communities in the coastal sediments of southeastern Arabian Sea (SEAS)

Abstract

The present study depicts diversity, abundance and activity of nitrogen (N2) fixing bacterial communities in the coastal sediments of southeastern Arabian Sea (SEAS). The diversity of N2 fixing bacterial community was determined by clone library analysis of nifH gene, the culturable N2 fixing bacteria was determined by 16s rRNA and nifH gene analysis. The nitrogen fixation rate in the sediments was also estimated by acetylene reduction assay method. The nifH clone library analysis revealed that fifteen putative diazotrophs, belonging to alpha-, beta-, gamma-, delta-, epsilon- Proteobacteria and Firmicutes. The predominant operational taxonomic units (OTUs) among these were related to α-Proteobacteria (with 16% of total sequences); β-Proteobacteria (10% sequences); γ-Proteobacteria (29% sequences); δ-Proteobacteria (11% sequences); ε-Proteobacteria (21% sequences); Firmicutes (3% of sequences) and percent of some sequences remained unknown. The 16s rRNA gene based identification of culturable N2 fixing bacteria revealed the presence of seven nitrogen fixing bacterial species namely Acinetobacter johnsonii, Rhizobium rosettiformans, Bacillus megaterium, B. circulans, B. flexus, B. oceanisediminis and B. subtilis. The observed N2 fixation rate in the sediments ranged from 0.12 to 0.36 nmol N g−1 h−1. Canonical correspondence analysis (CCA) revealed significant influence of the environmental variables such as sediment type, total nitrogen (TN), total organic matter (TOM), total organic carbon (TOC), iron (Fe), manganese (Mn), total sulfur (TS), nitrite (NO2) and nitrate (NO3) on the diversity of N2 fixing bacteria and N2 fixation rate. The observations of the current study provide a better understanding of N2 cycling in coastal sediments of the SEAS and sources of fixed nitrogen.