This research seeks to determine the effect of coastal acidification on sediment denitrification. Sediment denitrification provides a critical ecosystem service by removing nitrogen from estuaries thereby combatting eutrophication. The effect of acidification on sediment denitrification is unknown. However, research on terrestrial ecosystems demonstrates pH as a regulator of denitrification: as pH decreases denitrification efficiency also decreases. Coastal acidification could inhibit denitrification which would decrease water quality and have significant impacts on economically important fauna (e.g., commercial/recreational fisheries, oyster aquaculture). Fuleiler and colleagues will use a multipronged biogeochemical-molecular approach to meet our objectives in the field and laboratory. Initial research will include a survey of the active microbial community in estuarine sediments to characterize the potentially active sediment microbial community under different pH regimes. Researchers will couple rates of sediment denitrification (i.e., N2 and N2O fluxes) to the activity of key functional genes (nirS, nirK, nosZ) and the denitrifying community at sites under different pH regimes. There will also be an experimental component, where researchers will alter water column pH to directly test coastal acidification impacts on sediment denitrification and the active microbial community. This research will ultimately help to improve water quality by providing novel insight on the impact of coastal acidification on sediment denitrification.