The traditional paradigm that rivers and terrestrial run-off are the major contributors of nutrients to coastal waters has been challenged by observations that nutrient fluxes originating from coastal aquifer subterranean estuaries can equal or even exceed that of other terrestrial sources. Within a coastal aquifer where organic carbon is scarce and ammonium is abundant, bacteria capable of anaerobic ammonium oxidation (anammox bacteria) may play a role in the removal of fixed nitrogen. We investigated the presence of anammox bacteria in a coastal groundwater system (Waquoit Bay, MA USA) using lipid biomarkers. From the distribution of sediment-bound ladderane phospholipids, biomarkers for viable anammox bacteria, we demonstrate the presence of these organisms in association with aqueous chemical transition zones within the aquifer. The distribution of ladderane fatty acids in contrast, provided insight into the historical distribution of anammox bacteria and temporal stability of that distribution. The results suggest that anammox communities have been present over a broad range of depths, most likely determined by changes in the depths of the redox transition zones over time, but that they are more prevalent in the upper portion of the subterranean estuary where ammonium and nitrate coexist.
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