Elucidating the molecular response of microbial nitrogen fixation in estuarine sediments to hypoxia
Thermally–induced stratification and anthropogenic inputs of nutrients are major contributors to the onset of seasonal hypoxia in coastal estuaries. Hypoxia, defined by the U.S. Environmental Protection Agency as less than or equal to 2.3 mg L–1 dissolved oxygen (DO), has been recorded globally in over 400 systems. Hypoxia in estuarine systems is predicted to become more prevalent with climate change exacerbating eutrophication–driven hypoxic conditions. This study investigated impacts of hypoxic stress on microbes that drive nitrogen (N) fixation in estuarine sediments from Narragansett Bay, RI. Previous studies have indicated organic matter loading and depleted oxygen conditions may be driving factors of benthic N fixation in Narragansett Bay and demonstrated that diazotrophs may provide an additional and unanticipated source of N input to benthic ecosystems. Previous studies also indicated that anaerobes related to sulfate reducers (e.g. Desulfovibrio spp.) and the iron reducers, Geobacteraceae, have the potential to fix N in Narragansett Bay, as indicated by the detection of active expression of the gene (nifH) encoding a subunit of the protein complex that catalyzes N fixation, the nitrogenase iron protein. Interestingly, severely hypoxic conditions in Narragansett Bay appear to lead to an increase in nifH expression from these microbes. To elucidate the relationship between hypoxic stress, organic matter loading, and microbial N fixation, we performed a two–part study, coupling biogeochemical profiling of sediment cores (oxygen and total sulfide concentrations, pH, and carbon and N content analysis) with N fixation rate measurements and microbial analysis targeting nifH (quantitative PCR and RT–PCR). First, we collected sediment cores, capturing a natural DO gradient at a severely hypoxic site near a wastewater treatment plant (Greenwich Cove) and a site at the lower end of the nutrient gradient of Narragansett Bay that occasionally experiences hypoxia (Mid Bay). To understand the impact of spreading and intensifying oxygen depletion and elevated organic matter on the benthic community of diazotrophs, an incubation experiment was performed with Mid Bay sediment samples at the U.S. Environmental Protection Agency, Atlantic Ecology Division laboratory (Narragansett, RI). Mid Bay sediment cores were incubated under four treatments: normoxic, hypoxic, normoxic + organic matter, and hypoxic + organic matter. For the Narragansett Bay field study and incubation experiment, higher diazotrophic activity by sulfate reducers was observed, as measured by both total sulfide concentration and N fixation rates, under depleted DO. N fixation rates and total sulfide concentration were higher under the organic matter addition than the non-organic matter counterparts. Consistent with these data uncultivated sulfate reducers were the dominant group expressing nifH at the Greenwich Cove site and in the organic matter treated cores. Hypoxia, but not organic matter addition, stimulated nifH expression by uncultivated Geobacteraceae in the incubation experiment. N fixation rates and total sulfide concentration were higher under the organic matter addition than the non–organic matter counterparts during the incubation. This study indicates that two dominant groups of diazotrophs in Narragansett Bay sediments, uncultivated Geobacteraceae and sulfate reducers, may respond to different environmental drivers that may result from exacerbated hypoxia.^
Andraya Louise Ehrlich,
"Elucidating the molecular response of microbial nitrogen fixation in estuarine sediments to hypoxia"
Dissertations and Master's Theses (Campus Access).