Denitrification rates of culturable bacteria from a coastal location turning temporally hypoxic

Abstract

Nitrate reduction plays a key role in the biogeochemical dynamics and microbial ecology of coastal upwelling systems. Seasonal upwelling in the eastern Arabian Sea entrains nitrate rich waters to the surface, the intense productivity that follows leads to oxygen depletion and then denitrification. Heterotrophic bacteria comprise diverse functional groups. Both aerobic and facultative anaerobic heterotrophic bacteria capable of nitrate reduction are important functional groups in the nitrogen cycle due to their ability to respire anaerobically using nitrogen oxides as electron acceptors, which are reduced to nitrous oxide and dinitrogen. However little is known from the coastal Arabian Sea on the types/species of culturable bacteria catalyzing this crucial step. To recognize the species composition of the culturable fractions involved in nitrate reduction and nitrous oxide production, as many as 360 cultures of native heterotrophic bacteria were isolated and screened for nitrate reduction. Isolates were identified by sequencing their 16S rRNA gene and the rate of nitrate reduction and production of nitrous oxide by representatives of 30 different species of bacteria were measured. Close to 77% (or 277 of 360) of the cultures were found to be nitrate reducers, which grouped into 30 different species. Further, the nitrate reduction rates and nitrous oxide production by these 30 representative nitrate reducing species were measured. Cultures were capable of nitrate reduction up to 2.40 (�10-14) mol cell-1 and 5.54 (�10-15) mol cell-1 of nitrous oxide. This high fraction of culturable bacteria reducing nitrate and producing N2O in low oxygen conditions ought to reflect their important role in biogeochemical processes in the tropical coastal water experiencing hypoxia during SuM (Summer monsoon) and FIM (Fall intermonsoon) in the monsoon affected regions.