Limnol. Oceanogr., 44(3), 1999, 662–667

A combination of radiotracer and pore-water concentration measurements provide evidence for the occurrence of acetogenesis from CO2 in anoxic marine sediments that are ordinarily dominated by sulfate reduction and methanogenesis. In a month-long incubation experiment using sediments from Cape Lookout Bight, North Carolina, we measured H2 and acetate concentrations and monitored the incorporation of CO2 into CH4 and 14C-acetate. Depletion of pore-water sulfate resulted in a period of elevated H2 concentrations that made acetogenic CO2 reduction thermodynamically favorable. During this period, 14C-acetate was produced from CO2 at rates comparable to those of methanogenesis or sulfate reduction during their respective periods of dominance in the incubation. Maintenance of elevated but constant H2 concentrations immediately following sulfate depletion likely reflects control by acetogenic bacteria, suggesting they were the dominant consumers of H2 during this period. Acetogenic bacteria are virtually ubiquitous in anaerobic ecosystems as a result of their metabolic versatility. Schink (1994) notes that ‘‘there is hardly any transformation process in an anoxic environment in which homoacetogens do not participate or with which they do not compete.’’ These organisms produce acetate through both fermentation of organic compounds and reduction of CO2 (Drake 1994). Pure cultures of acetogens generally catalyze these two reactions simultaneously (Wood 1952), but it has been shown that growth can be based exclusively on either reaction (Daniel et al. 1990; Drake 1994; Schink 1994). Because molecular hydrogen can serve as the reductant for acetogenic CO2 reduction (2CO2 1 4H2 → CH3COOH 1 2H2O), the favorability and occurrence of the reaction are highly sensitive to ambient H2 concentrations. Low H2 concentrations, as might be maintained by proximal H2-consuming organisms, have been shown to inhibit (Cord-Ruwisch and Ollivier 1986; Heijthuijsen and Hansen 1986) and even reverse (Lee and Zinder 1988) acetogenic CO2 reduction. The basis for this inhibition (or reversal) is almost certainly thermodynamic: decreased H2 concentrations translate to a smaller thermodynamic driving force (DG less negative) for the reaction. Progressively lower H2 concentrations (more positive DG values) would at first inhibit acetogenic CO2 reduction and then cause the reverse process to become energetically favorable. Such thermodynamic inhibition of acetogenic CO2 reduction is expected to prevail in most sediment ecosystems, where the activity of terminal metabolic bacteria (e.g., methanogens and sulfate reducers) maintains H2 concentrations at very low levels (Lovley and Goodwin 1988). For example, at 108C, Hoehler et al. (1994) measured H2 concentrations of 0.16 and 1.82 nM for sulfate-reducing and methan1 Present address: NASA-Ames Research Center, Mail Stop 2394, Moffett Field, California 94035.