Influence of environmental factors on benthic nitrogen fixation and role of sulfur reducing diazotrophs in a eutrophic tropical estuary

Abstract

Benthic nitrogen fixation in the tropical estuaries plays a major role in marine nitrogen cycle, its contribution to nitrogen budget and players behind process is not well understood. The present study was estimated the benthic nitrogen fixation rate in a tropical estuary (Cochin) and also evaluated the contribution of various diazotrophic bacterial communities. Nitrogen fixation was detected throughout year (0.1-1.11 nmol N g^-1 h^-1); higher activity was observed in post-monsoon. The nifH gene abundance was varied from 0.8 × 10⁴ to 0.6 × 10⁸ copies g^-1dry sediment; highest was detected in post-monsoon. The Cluster I and Cluster III were the dominant diazotrophs. Sulfur reducing bacterial phylotypes (Deltaproteobacteria) contributed up to 2-72% of total nitrogen fixation. These bacteria may provide new nitrogen to these systems, counteracting nitrogen loss via denitrification and anammox. Overall, the study explained the importance of benthic nitrogen fixation and role of diazotrophs in a monsoon influenced tropical estuarine environments.

Keywords: Cochin estuary; Diazotrophs; Sulfate reducing bacteria; nifH.