Bay of Bengal: An Oxygen Minimum Zone at the Tipping Point

Oxygen minimum zones (OMZs) host N₂ producing microbial populations responsible for a major percentage of the fixed nitrogen (N) loss from the oceans. While, the Bay of Bengal (BoB) contains regions with oxygen (O₂) concentrations below detection of standard methods, there have been no prior reports of N₂ production. We sampled the BoB in January 2014 during the winter monsoon, measuring in situ O₂ with a highly sensitive STOX (switchable trace oxygen) sensor. Sampling was carried out for natural abundance NO₃^- isotopes, molecular characterization of microbial populations and experiments exploring the aerobic and anaerobic microbial turnover of N species. Unlike recognized OMZs, the BoB has minimum O₂ concentrations in the 10 to 100 nM range. Despite this measurable O₂, the BoB OMZ houses microbial populations similar to other OMZs, capable of N₂ production through both anammox and denitrification. Isotope labeling experiments with ¹⁵NO₂^- detected anammox up to 6.2 nM N d^-1, but were below detection with ¹⁵NH₄⁺ at all stations, indicating the great potential for anammox in the BoB OMZ waters, but the process is limited by NO₂^- availability. Denitrification was also detected in ¹⁵NO₂^- experiments, up to 0.9 nM N d^-1. Active N cycling with very minor N loss was also indicated by NO₃^- isotopes. The general lack of NO₂^- accumulation (< 180 nM) and low in situ N₂ production rates are likely a result of limited organic matter availability, which in turn limits the drawdown of O₂ and allows a close coupling of NO₃^- reduction and NO₂^- oxidation. The potential for N₂ production, is however high, and the BoB would become an important player in the marine N cycle if these last traces of O₂ were removed, for example, by anthropogenic forcing or climate change.