a b s t r a c tGradients of dissolved oxygen concentrations in stratified estuarine water columns directly influence microbial composition and metabolic pathways, resulting in vertical and axial chemical gradients of redox-active species. Understanding such microbial responses to changing geochemical conditions and elucidating the diversity of microbially-mediated processes are needed to comprehensively identify ecosystem functions. We tested the hypothesis that microbial metabolic processes in the vicinity of the oxic/anoxic interface in Chesapeake Bay would be elevated due to the coincidence of oxidized and reduced compounds. We measured rates of microbial redox processes associated with carbon cycles and quantified geochemical constituents. The transition from oxic to anoxic to oxic conditions was correlated with the pattern of dissolved nitrogen species, with the most reducing conditions comprising the highest concentrations of dissolved chemical compounds. The vertical distribution of community respiration was measured with changes in dissolved inorganic carbon concentrations and was related to water column stratification, with the highest median and variability (1.2 ± 1.4 mmol L À1 h À1 ) in the most stratified conditions. Rates of CO 2 fixation under dark conditions, a measure of chemoautotrophy, were elevated at the base of oxyclines and significantly correlated with a gradient of density and some reduced compounds. Although vertical interpolation must be made with caution due to high vertical variability, chemoautotrophic production averaged 0.5 mmol m À2 h À1 from May to August and added 5.8% to autochthonous organic carbon production in the mesohaline Chesapeake Bay. Overall, our results suggest that anaerobic community metabolism and chemoautotrophy in the oxic/anoxic interface exert a small impact on estuarine carbon cycles.
Intense annual spring phytoplankton blooms and thermohaline stratification lead to anoxia in Chesapeake Bay bottom waters. Once oxygen becomes depleted in the system, microbial communities use energetically favourable alternative electron acceptors for respiration. The extent to which changes in respiration are reflected in community gene expression have only recently been investigated. Metatranscriptomes prepared from near-bottom water plankton over a 4-month time series in central Chesapeake Bay demonstrated changes consistent with terminal electron acceptor availability. The frequency of respiration-related genes in metatranscriptomes was examined by BLASTx against curated databases of genes intimately and exclusively involved in specific electron acceptor utilization pathways. The relative expression of genes involved in denitrification and dissimilatory nitrate reduction to ammonium were coincident with changes in nitrate, nitrite and ammonium concentrations. Dissimilatory iron and manganese reduction transcript ratios increase during anoxic conditions and corresponded with the highest soluble reactive phosphate and manganese concentrations. The sulfide concentration peaked in late July and early August and also matched dissimilatory sulfate reduction transcript ratios. We show that rather than abrupt transitions between terminal electron acceptors, there is substantial overlap in time and space of these various anaerobic respiratory processes in Chesapeake Bay.
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