PII S0016-7037(00)00422-1 Bacterial dissimilatory reduction of arsenate and sulfate in meromictic Mono Lake, California RONALD S. OREMLAND, 1, *PHILIP R. DOWDLE, 1 SHELLY HOEFT, 1 JONATHAN O. SHARP, 1 JEFFRA K. SCHAEFER, 1 LAURENCE G. MILLER, 1 JODI SWITZER BLUM, 1 RICHARD L. SMITH, 2 NICHOLAS S. BLOOM, 3 and DIRK WALLSCHLAEGER 3 1 U.S. Geological Survey, Menlo Park, California 94025, USA 2 U.S. Geological Survey, Boulder, Colorado 80303, USA 3 Frontier Geosciences, Seattle, Washington 98109, USA (Received February 1, 2000; accepted in revised form April 4, 2000) Abstract—The stratified (meromictic) water column of alkaline and hypersaline Mono Lake, California, contains high concentrations of dissolved inorganic arsenic (200 mol/L). Arsenic speciation changes from arsenate [As (V)] to arsenite [As (III)] with the transition from oxic surface waters (mixolimnion) to anoxic bottom waters (monimolimnion). A radioassay was devised to measure the reduction of 73 As (V) to 73 As (III) and tested using cell suspensions of the As (V)-respiring Bacillus selenitireducens, which completely reduced the 73 As (V). In field experiments, no significant activity was noted in the aerobic mixolimnion waters, but reduction of 73 As (V) to 73 As (III) was observed in all the monimolimnion samples. Rate constants ranged from 0.02 to 0.3/day, with the highest values in the samples from the deepest depths (24 and 28 m). The highest activities occurred between 18 and 21 m, where As (V) was abundant (rate, 5.9 mol/L per day). In contrast, sulfate reduction occurred at depths below 21 m, with the highest rates attained at 28 m (rate, 2.3 mol/L per day). These results indicate that As (V) ranks second in importance, after sulfate, as an electron acceptor for anaerobic bacterial respiration in the water column. Annual arsenate respiration may mineralize as much as 14.2% of the pelagic photosynthetic carbon fixed during meromixis. When combined with sulfate-reduction data, anaerobic respiration in the water column can mineralize 32–55% of this primary production. As lakes of this type approach salt saturation, As (V) can become the most important electron acceptor for the biogeochemical cycling of carbon. Copyright © 2000 Elsevier Science Ltd 1. INTRODUCTION Arsenic has trace abundance in the Earth’s crust, yet it occurs widely in the environment, with localized high concentrations found in certain rocks, soils, and waters (Azcue and Nriagu, 1994). In nature it exists in four oxidation states: arsenate [As (V)], arsenite [As (III)], elemental arsenic, and arsine [As (-III)]. The later two occur only rarely, whereas As (V) and As (III) comprise the bulk of the inorganic speciation encountered in natural waters and sediments (Cullen and Reimer, 1989). Arsenate adsorbs strongly to mineral surfaces like ferrihydrite, whereas arsenite is much more mobile and toxic. Although the acute toxicity of arsenic to humans has been known since ancient times, both lethal and sublethal effects (“arsenicosis”) associated with chronic ingestion of arsenic-contaminated well water currently occur over large regions, such as in the Ganges Delta (Nickson et al., 1998). This has focused concerns on water quality standards. Sources of arsenic in the environment include anthropogenic ones such as drainage from abandoned mines and mine tailings, usage as pesticides and biocides, as well as natural ones derived from hydrothermal leaching or weathering of arsenic minerals in rocks. In stratified water bodies, arsenic typically exhibits a transi- tion from As (V) to As (III) in depth profiles transiting from oxic to anoxic waters (Peterson and Carpenter, 1983; Aggett and Kriegman, 1988; Seyler and Martin, 1989; Maest et al., 1992; Kuhn and Sigg, 1993). Similar trends with depth are also observed in sediment cores taken from arsenic-contaminated reservoirs (Ficklin, 1990). Although As (V) can be reduced to As (III) by the presence of strong chemical reductants like sulfides (Kuhn and Sigg, 1993; Newman et al., 1997a,b), direct biochemical reduction of As (V) is also possible. Two mech- anisms of biochemical reduction have been described: (1) a plasmid-encoded, detoxifying reductase (arsC enzyme) present in the cytoplasm of certain bacteria (e.g., Escherichia coli and Staphylococcus aureus), which reduces As (V) to As (III) for its rapid extrusion from the cell (Chen et al., 1986; Gladysheva et al., 1994; Ji et al., 1994), and (2) a respiratory (“dissimila- tory”) As (V) reductase present in the cell envelope of certain anaerobes that enables them to conserve the energy derived from the oxidation of organic substrates or H 2 (Krafft and Macy, 1998; Newman, D. K., Oremland, R. S., Dowdle, P. R., Morel, F. M. M., and Stolz, J. R., in prep.). Dissimilatory As (V) reduction is a newly discovered means of bacterial respi- ration, and several novel species of Gram-positive and Gram- negative Eubacteria have been isolated, which can achieve growth using As (V) as an electron acceptor (Ahmann et al., 1994; Laverman et al., 1995; Macy et al., 1996; Newman et al., 1997a,b; Switzer Blum et al., 1998; Stolz et al., 1999). Dissimilatory As (V) reduction (“DAsR”) to As (III) occurs in anoxic sediments that have been supplemented with milli- molar As (V) (Rittle et al., 1995; Dowdle et al., 1996; Ahmann et al., 1997; Harringon et al., 1998). However, no studies have been done that examine the capacity of microbes to carry out DAsR at ambient concentrations of As (V) (e.g., nanomolar to micromolar), which would require the use of a radioisotope that does not significantly alter these ambient concentrations. Al- *Author to whom correspondence should be addressed (roremlan @usgs.gov). Pergamon Geochimica et Cosmochimica Acta, Vol. 64, No. 18, pp. 3073–3084, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 + .00 3073