Despite the worldwide occurrence of marine hypoxic regions, benthic nitrogen (N) cycling within these areas is poorly understood and it is generally assumed that these areas represent zones of intense fixed N loss from the marine system. Sulfate reduction can be an important process for organic matter degradation in sediments beneath hypoxic waters and many sulfate-reducing bacteria (SRB) have the genetic potential to fix molecular N (N2). Therefore, SRB may supply fixed N to these systems, countering some of the N lost via microbial processes, such as denitrification and anaerobic ammonium oxidation. The objective of this study was to evaluate if N2 fixation, possibly by SRB, plays a role in N cycling within the seasonally hypoxic sediments from the Eckernförde Bay, Baltic Sea. Monthly samplings were performed over the course of one year to measure nitrogenase activity (NA) and sulfate reduction rates, to determine the seasonal variations in bioturbation (bioirrigation) activity and important benthic geochemical profiles, such as sulfur and N compounds, and to monitor changes in water column temperature and oxygen concentrations. Additionally, at several time points, the active N-fixing community was examined via molecular tools. Integrated rates of N2 fixation (approximated from NA) and sulfate reduction showed a similar seasonality pattern, with highest rates occurring in August (approx. 22 and 880 nmol cm−3 d−1 of N and SO42−, respectively) and October (approx. 22 and 1300 nmol cm−3 d−1 of N and SO42− respectively), and lowest rates occurring in February (approx. 8 and 32 nmol cm−3 d−1 of N and SO42−, respectively). These rate changes were positively correlated with bottom water temperatures and previous reported plankton bloom activities, and negatively correlated with bottom water oxygen concentrations. Other variables that also appeared to play a role in rate determination were bioturbation, bubble irrigation and winter storm events. Molecular analysis demonstrated the presence of nifH sequences related to two known N2 fixing SRB, namely Desulfovibrio vulgaris and Desulfonema limicola, supporting the hypothesis that some of the nitrogenase activity detected may be attributed to SRB. Overall, our data show that Eckernförde Bay represents a complex ecosystem where numerous environmental variables combine to influence benthic microbial activities involving N and sulfur cycling
A kinetic-bioenergetic reaction model for the anaerobic oxidation of methane (AOM) in coastal marine sediments is presented. The model considers a fixed depth interval of sediments below the zone of bioturbation (the window-ofobservation), subject to seasonal variations of temperature and inputs of organic substrates and sulfate. It includes (1) nine microbially-mediated reaction pathways involved in CH 4 production/consumption; (2) an explicit representation of five functional microbial groups; and (3) bioenergetic limitations of the microbial metabolic pathways. Fermentation of organic substrates is assumed to produce hydrogen (H 2 ) and acetate (Ac) as key reactive intermediates. Competition among the metabolic pathways is controlled by the relative kinetic efficiencies of the various microbial processes and by bioenergetic constraints. Model results imply that the functional microbial biomasses within the window-of-observation undergo little variation over the year, as a result of kinetic and thermodynamic buffering of the seasonal forcings.Furthermore, the microbial processes proceed at only small fractions of their maximum potential rates. These findings provide a theoretical justification for the approximation of steady-state microbial biomasses, which is frequently used in diagenetic modeling. In contrast, AOM rates show a strong seasonal evolution: AOM only becomes spontaneous in winter, when hydrogenotrophic sulfate reduction (hySR) sufficiently reduces the local H 2 concentration. The bioenergetic limitation of AOM is thus a critical factor modulating this process in seasonally-forced nearshore marine sediments. A global sensitivity analysis based on a 2-level factorial design reveals that AOM rates are most sensitive to the kinetic parameters describing hySR and acetotrophic methanogenesis (acME). The growth and substrate uptake kinetics of AOM are unimportant, whereas the threshold value of ATP energy conservation for AOM is the most sensitive thermodynamic parameter. These results confirm that anaerobic methane oxidizing microorganisms are metabolizing close to their thermodynamic limit, with the energetic balance being controlled by the relative rates of hySR and acME. The removal of Ac by acME primarily allows more sulfate (SO 4 2؊ ) to be utilized for H 2 oxidation, thereby promoting AOM.
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