Microbially mediated terminal electron accepting processes (TEAPs) to a large extent control the fate of redox reactive elements and associated reactions in anoxic soils, sediments, and aquifers. This review focuses on thermodynamic controls and regulation of H2-dependent TEAPs, case studies illustrating this concept, and the quantitative description of thermodynamic controls in modeling. Other electron transfer processes are considered where appropriate. The work reviewed shows that thermodynamics and microbial kinetics are connected near thermodynamic equilibrium. Free energy thresholds for terminal respiration are physiologically based and often near -20 kJ mol(-1), depending on the mechanism of ATP generation; more positive free energy values have been reported under "starvation conditions" for methanogenesis and lower values for TEAPs that provide more energy. H2-dependent methanogenesis and sulfate reduction are under direct thermodynamic control in soils and sediments and generally approach theoretical minimum energy thresholds. If H2 concentrations are lowered by thermodynamically more potent TEAPs, these processes are inhibited. This principle is also valid for TEAPS providing more free energy, such as denitrification and arsenate reduction, but electron donor concentration cannot be lowered so that the processes reach theoretical energy thresholds. Thermodynamics and kinetics have been integrated by combining traditional descriptions of microbial kinetics with the equilibrium constant K and reaction quotient Q of a process, taking into account process-specific threshold energies. This approach is dynamically evolving toward a general concept of microbially driven electron transfer in anoxic environments and has been used successfully in applications ranging from bioreactor regulation to groundwater and sediment biogeochemistry.

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Two highly enriched cultures containing Dehalococcoides spp. were used to study the effect of aceticlastic methanogens on reductive vinyl chloride (VC) dechlorination. In terms of aceticlastic methanogens, one culture was dominated by Methanosaeta, while the other culture was dominated by Methanosarcina, as determined by fluorescence in situ hybridization. Cultures amended with 2-bromoethanesulfonate (BES), an efficient inhibitor of methanogens, exhibited slow VC dechlorination when grown on acetate and VC. Methanogenic cultures dominated by Methanosaeta had no impact on dechlorination rates, compared to BES-amended controls. In contrast, methanogenic cultures dominated by Methanosarcina displayed up to sevenfold-higher rates of VC dechlorination than their BES-amended counterparts. Methanosarcina-dominated cultures converted a higher percentage of [2-14 C]acetate to 14 CO 2 when concomitant VC dechlorination took place, compared to nondechlorinating controls. Respiratory indices increased from 0.12 in nondechlorinating cultures to 0.51 in actively dechlorinating cultures. During VC dechlorination, aqueous hydrogen (H 2 ) concentrations dropped to 0.3 to 0.5 nM. However, upon complete VC consumption, H 2 levels increased by a factor of 10 to 100, indicating active hydrogen production from acetate oxidation. This process was thermodynamically favorable by means of the extremely low H 2 levels during dechlorination. VC degradation in nonmethanogenic cultures was not inhibited by BES but was limited by the availability of H 2 as electron donor, in cultures both with and without BES. These findings all indicate that Methanosarcina (but not Methanosaeta), while cleaving acetate to methane, simultaneously oxidizes acetate to CO 2 plus H 2 , driving hydrogenotrophic dehalorespiration of VC to ethene by Dehalococcoides.Contamination with chlorinated ethenes, an almost ubiquitous class of pollutants, presently poses a serious threat to groundwater quality in industrialized countries (1). Microbial reductive dechlorination is the major pathway of degradation and detoxification of chloroethenes under anaerobic conditions (26). Dehalorespiring bacteria (DRB; synonym, chlororespiring bacteria) are able to use the energy available from reductive dechlorination in a respiratory process (27, 28); however, sequential dechlorination may result in transient buildup of the highly toxic metabolite vinyl chloride (VC) (15,44). DRB growing with vinyl chloride as an electron acceptor are to date restricted to the Dehalococcoides group of bacteria (11,12,16,23). All known Dehalococcoides isolates require hydrogen (H 2 ) as a direct electron donor (3,23,34). However, in anaerobic substrate degradation, a considerable fraction of organic carbon is converted to methane via acetate. The latter may also be oxidized by sulfate-reducing, Fe(III)-reducing, or denitrifying bacteria (49), while known VC-dechlorinating DRB are unable to use acetate as a direct electron donor. H 2 production from acetate is an obligately syntrophic process that may...