Spatio-temporal variations in sulfur-oxidizing and sulfate-reducing bacterial activities during upwelling, off south-west coast of India

The Arabian Sea, off SW India, is becoming more anoxic in recent years. Poor ventilation affects microbial degradation of organic matter in the oxygen minimum zone (≤2.85mll−1 O2, ≤0.02μM NO2) and the anoxic marine zone (≤0.09mll−1 O2, ≥0.5μM NO2). We posit that one of the reasons at the microbial level could be due to a more prominent increase in sulfate-reducing activity (SRA), than sulfur-oxidizing activity (SOA). Hence, the objective was to measure the extent to which SOA can counter the effect of SRA. We, therefore examined these activities along with relevant environmental variables from 2009 to 2011 off Kochi (9.55°N–75.33°E) and Trivandrum (8.26°N–76.50°E), covering the three phases of upwelling. SRA was measured radiometrically using 35S, and SOA by iodometry. Off Kochi, the SOA of the water column increased 6× (194–1151μMd−1) and SRA 4× (13–54nMd−1) from phase I to III. Off Trivandrum, the increase in SOA was 1.7× (339–560μMd−1) and SRA 7× (24–165nMd−1) contributing to the build-up of reducing/oxidizing conditions. This increase in SOA moderates the effect of increase in SRA. Besides, the average concentrations of dissolved oxygen and nitrite off Trivandrum were 1.80±1.66, 1.48±1.55, 1.93±1.86mll−1 and 0.14±0.14, 1.69±0.67, 0.34±0.42μM during the three phases respectively. Hence, it is suggested that the coastal waters examined in this study could probably be between oxygen minimum zone (OMZ) and anoxic minimum zone (AMZ) in patches temporarily. The present paper highlights the interactions between sulfate-reducing and sulfur-oxidizing activities, during upwelling for the first time in these waters. These observations give an important and timely insight into the implications.