Collaborative Research: Microbial Ecology of Nitrogen Cycling in the Oxygen Minimum Zone of the Arabian Sea

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ABSTRACTOCE- 0350720 / OCE- 0350683Although the Arabian Sea covers only 2 % of the area of the world ocean, it's well developed Oxygen Minimum Zone (OMZ) accounts for 20 % of the water column denitrification, a disproportionate and significant contribution to the marine nitrogen budget. The distribution of in situ denitrification activity in the OMZ is usually inferred from the correlation among indicator chemical variables, most importantly nitrate deficit, nitrite, oxygen, and suspended particulate material. The potential for oxygen, nitrate and organic carbon availability to regulate denitrifying bacteria is well known and easily demonstrated in culture and in sediment environments. Direct measurements of denitrification rates have been very difficult in the water column of the OMZ, and the factors that actually regulate the activity of denitrifying bacteria in this environment are therefore known only by inference. The recently recognized potential for anaerobic ammonia oxidation to account for part of the denitrification signal (in loss of fixed N) makes it abundantly clear that the scientific community lacks predictive understanding of factors controlling the N cycle of the OMZ. In this study, researchers at Princeton University and the University of Washington will investigate the community structure of the bacterial assemblage in the OMZ in order to evaluate the possible relationships between diversity and biogeochemistry. The team of scientists will assay gene expression for the gene that encodes the key enzyme in denitrification (nitrite reductase) as an indicator of denitrifying activity. Gene abundance and expression data will be compared with the distribution of the chemical variables that are used by convention to infer denitrification rates to investigate regulation of denitrifying activity. In addition to the usual chemical indicators, trace metal speciation such as copper bioavailability will be involved to regulate the denitrifying bacterial community composition and activity. Previous studies have demonstrated that denitrifying bacteria can be limited by copper in culture, and suggest that the tight complexation of copper with sulfides in the OMZ will lead to the accumulation of nitrous oxide that is characteristically observed in the OMZ. This copper hypothesis will be tested by comparing the denitrification rates and gene expression in treatments with and without copper additions. In terms of broader impacts, it is clear that this project that will build upon strong international collaboration among some of the premier marine nitrogen experts.