Microbially-driven nitrous oxide transformations and fluxes in organic-poor sediments from a tropical lagoon.

Microbially-driven transformations and fluxes of nitrous oxide (NO) in tropical organic-poor sediments (organic carbon, C < 0.79 %) differ from organic-rich sediments, but are largely overlooked and not well-constrained. We studied these transformations and fluxes, and identified their drivers, in organic-poor coastal sediments from the inlet regions of Pulicat lagoon, a tropical coastal ecosystem on the southeast coast of India. Intact core incubations were coupled with acetylene-inhibitor treatments to quantify fluxes of dissolved gases and nutrients at the sediment-water interface, and to understand the extent of NO production and consumption in these sediments. Bacteria that carry out these NO transformations were examined through 16S rRNA (V3-V4) sequencing. Lagoon inlets were profiled over two seasons that were representative of distinct temperature and salinity regimes, to analyze the influence of temperature and salinity on NO transformations. We found that NO consumption processes were significant, as a result of high rates of oxygen uptake in these sediments. Nitrate (NO) concentration at the sediment-water interface was found to be the key biogeochemical control on these consumption processes. The bacterial families found to be key to these processes in these sediments were Neisseriaceae, Burkholderiaceae, Desulfovibrionaceae, and Clostridiaceae. Both chemoheterotrophic and chemolithotrophic pathways were found to be crucial to bacterial metabolism in these sediments. We found that these organic-poor inlet sediments were a sink of NO (∼-4.36 × 10 mol year). Similar processes and mechanisms may be observed in similarly characterized ecosystems, and such ecosystems may therefore be valuable sinks of NO.