PII S0016-7037(98)00106-9 Thermodynamic control on hydrogen concentrations in anoxic sediments TORI M. HOEHLER,MARC J. ALPERIN,DANIEL B. ALBERT, and CHRISTOPHER S. MARTENS Marine Sciences Department, University of North Carolina, 12-7 Venable Hall, CB 3300, Chapel Hill, North Carolina 27599, USA (Received December 2, 1997; accepted in revised form February 10, 1998) Abstract—Molecular hydrogen plays a central role in bacterially mediated anoxic sediment chemistry, as both an important electron transfer agent and a key thermodynamic control. We studied the response of hydrogen concentrations to changes in temperature, specific electron acceptor, sulfate concentration, and pH in a series of laboratory experiments using sediments from Cape Lookout Bight, North Carolina. Hydrogen concentrations were found to depend significantly on each of these factors in a fashion that suggests thermodynamic control. In general, the change in hydrogen concentrations was apparently driven by a necessity to maintain a constant G for the predominant terminal electron-accepting process. We hypothesize this situation derives from highly competetive conditions that force terminal metabolic bacteria to operate right at their thermodynamic limits. The response of hydrogen to individual controls in the laboratory experiments was reflected in relatively quantitiative fashion in down-core, seasonal, and inter-environmental variations observed in sediment cores from Cape Lookout Bight and the White Oak River, NC. Copyright © 1998 Elsevier Science Ltd 1. INTRODUCTION The biogeochemistry of aquatic sediments is dominated by bacterially mediated redox reactions that break down organic matter. At the heart of this bacterial chemistry is molecular hydrogen, which serves as both an important electron transfer agent and a primary thermodynamic control on most redox reactions. The important role of H 2 in anoxic sediments has long been appreciated (e.g., Zobell, 1947) but relatively little is understood about the factors that control its concentration. In this report, we examine the major environmental controls on H 2 concentrations in anoxic sediments. H 2 is produced under anaerobic conditions during bacteri- ally-mediated fermentation of organic matter (Dolfing, 1988): (CH 2 O) n + nH 2 O 3 nCO 2 + 2nH 2 (1) where (CH 2 O) n represents a generalized carbohydrate-like or- ganic matter. The H 2 can then be used by terminal metabolic bacteria to reduce a series of inorganic electron acceptors (see Zinder, 1993): 2nH 2 + mX ox 3 mX red + zH 2 O (2) where X ox and X red are the oxidized and reduced forms of an inorganic electron acceptor (e.g., SO 4 2- and S 2- ). Production and consumption of H 2 in natural systems are closely coupled, so that it is a free but short-lived intermediate that is generally present at very low concentrations (Zinder, 1993). One role of H 2 , then, is to link the oxidation of organic carbon to the reduction of inorganic compounds in a process termed inter- species hydrogen transfer (Iannotti et al., 1973). H 2 is also important by virtue of the thermodynamic control it exerts on a wealth of sedimentary reactions. In a stoichio- metric sense, it is generally the most important product of fermentation and reactant in terminal metabolism. Hence, vari- ations in the H 2 concentration affect the thermodynamics of both processes to a greater degree than is true of any other porewater constituent. In the case of fermentations, H 2 concen- trations that are too high can alter the products of (Iannotti et al., 1973; Chen and Wolin, 1977; Tewes and Thauer, 1980), inhibit (Dolfing, 1988), or even reverse (Lee and Zinder, 1988) selected reactions. Terminal metabolic reactions are affected when H 2 concentrations become too low. This may result in inhibition (Zinder, 1993), or (in the case of CO 2 reduction) possibly reversal (Zehnder and Brock, 1979; Hoehler et al., 1994) of H 2 -based terminal metabolism. Thus, H 2 serves not only to couple oxidative and reductive processes, but also to regulate the flow of carbon and electrons in virtually every step of organic matter breakdown (Fig. 1). A handful of pioneering studies have measured H 2 concen- trations in natural systems. These are predominantly based on freshwater, methanogenic ecosystems, with a few measure- ments in marine or estuarine environments that include other terminal metabolic reactions (e.g., sulfate reduction). These studies indicate that H 2 concentrations in aquatic sediments may depend on temperature (Conrad et al., 1987; Hoehler et al., 1994; Westermann, 1994) and the predominant terminal elec- tron acceptor (Lovley et al., 1982; Lovley and Goodwin, 1988; Hoehler et al., 1994). The latter observation demonstrates that H 2 concentrations in aquatic sediments are generally controlled by the terminal metabolic bacteria, as expected for H 2 -limited systems. The dependence of H 2 on both temperature (Conrad and Wetter, 1990) and electron acceptor (Lovley and Goodwin, 1988) is thought to relate to the effect these factors have on the G of terminal metabolic processes. In both cases, more neg- ative G values (more available free energy) are generally accompanied by lower H 2 concentrations. These observations suggest that the thermodynamics of terminal metabolic pro- cesses represent an important control on sedimentary H 2 con- centrations. In this regard, any environmental factor which affects the G of a terminal metabolic reaction represents a likely control on H 2 concentrations. We hypothesize that sedimentary H 2 con- centrations are controlled in quantitative and dynamic fashion by the thermodynamics of terminal metabolic reaction. We Pergamon Geochimica et Cosmochimica Acta, Vol. 62, No. 10, pp. 1745–1756, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 + .00 1745