ALBERT

Description: 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-of-observation), subject to seasonal variations of temperature and inputs of organic substrates and sulfate. It includes (1) nine microbially-mediated reaction pathways involved in CH4 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 (H2) 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 H2 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 (SO42−) to be utilized for H2 oxidation, thereby promoting AOM.

Description: In this study the sulfur cycle in the organic-rich mud belt underlying the highly productive upwelling waters of the Namibian shelf is quantified using a 1D reaction-transport model. The model calculates vertical concentration and reaction rate profiles in the top 500 cm of sediment which are compared to a comprehensive dataset which includes carbon, sulfur, nitrogen and iron compounds as well as sulfate reduction (SR) rates and stable sulfur isotopes (32S, 34S). The sulfur dynamics in the well-mixed surface sediments are strongly influenced by the activity of the large sulfur bacteria Thiomargarita namibiensis which oxidize sulfide (H2S) to sulfate () using sea water nitrate () as the terminal electron acceptor. Microbial sulfide oxidation (SOx) is highly efficient, and the model predicts intense cycling between and H2S driven by coupled SR and SOx at rates exceeding 6.0 mol S m−2 y−1. More than 96% of the SR is supported by SOx, and only 2–3% of the pool diffuses directly into the sediment from the sea water. A fraction of the produced by Thiomargarita is drawn down deeper into the sediment where it is used to oxidize methane anaerobically, thus preventing high methane concentrations close to the sediment surface. Only a small fraction of total H2S production is trapped as sedimentary sulfide, mainly pyrite (FeS2) and organic sulfur (Sorg) (∼0.3 wt.%), with a sulfur burial efficiency which is amongst the lowest values reported for marine sediments (〈1%). Yet, despite intense SR, FeS2 and Sorg show an isotope composition of ∼5 ‰ at 500 cm depth. These heavy values were simulated by assuming that a fraction of the solid phase sulfur exchanges isotopes with the dissolved sulfide pool. An enrichment in H2S of 34S towards the sediment-water interface suggests that Thiomargarita preferentially remove H232S from the pore water. A fractionation of 20–30‰ was estimated for SOx (εSOx) with the model, along with a maximum fractionation for SR (εSR–max) of 100‰. These values are far higher than previous laboratory-based estimates for these processes. Mass balance calculations indicate negligible disproportionation of autochthonous elemental sulfur; an explanation routinely cited in the literature to account for the large fractionations in SR. Instead, the model indicates that repeated multi-stepped sulfide oxidation and intracellular disproportionation by Thiomargarita could, in principle, allow the measured isotope data to be simulated using much lower fractionations for εSOx (5‰) and εSR (78‰).