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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

ABSTRACT

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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A reactive-transport model has been applied to investigate the dynamics of the sulfate-methane transition zone (SMTZ) in nearshore sediments of Aarhus Bay (Denmark). The sediments are influenced by seasonal variations of temperature and particulate organic carbon (POC) deposition flux at the sediment-water interface. Initially, the model was calibrated at steady state using field data collected at two sites (M1 and M5) in December 2004, and included a dynamic gas phase which determines the depth of the SMTZ. Simulations were then performed under transient conditions of heat propagation in the porous medium, which influenced the solubility of gaseous methane, the diffusion of solutes as well as the kinetic and bioenergetic constraints on redox conditions in the system. Results revealed important variations in local rates of anaerobic oxidation of methane (AOM) over a seasonal cycle due to temperature variation. Seasonal perturbations in POC depositional flux had no influence on AOM rates but did have a strong bearing on sulfate reduction rates in the surface layers of the simulations at both stations. At M5, where the SMTZ was located 63 cm below the sediment-water interface, depth integrated AOM rates varied between 76 and 178 nmol cm -2 d -1 . At M1, where the deeper SMTZ (221 cm) experienced less thermal variation, AOM rates varied relatively less (20 to 24 nmol cm -2 d -1 ). Furthermore, local and depth-integrated AOM rates over the year did not show a simple response to bottom water temperature but exhibited a hysteresis-type behavior related to time lags in solute transport and heat propagation. Overall, the solute concentration profiles were not very sensitive to the seasonal variability in rates or gas transport and the modeled vertical displacement of the SMTZ was limited to Ͻ1 cm at M1 and 2-3 cm at M5. The results suggest that the significantly larger apparent displacement observed in the field from repeated coring (80 cm and 16 cm at M1 and M5, respectively) must be attributed to other factors, of which spatial heterogeneity in gas transport rate appears to be the most likely.

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