The microbial arsenic cycle in Mono Lake, California

Abstract

Significant concentrations of dissolved inorganic arsenic can be found in the waters of a number of lakes located in the western USA and in other water bodies around the world. These lakes are often situated in arid, volcanic terrain. The highest concentrations of arsenic occur in hypersaline, closed basin soda lakes and their remnant brines. Although arsenic is a well-known toxicant to eukaryotes and prokaryotes alike, some prokaryotes have evolved biochemical mechanisms to exploit arsenic oxyanions (i.e., arsenate and arsenite); they can use them either as an electron acceptor for anaerobic respiration (arsenate), or as an electron donor (arsenite) to support chemoautotrophic fixation of CO(2) into cell carbon. Unlike in freshwater or marine ecosystems, these processes may assume quantitative significance with respect to the carbon cycle in arsenic-rich soda lakes. For the past several years our research has focused on the occurrence and biogeochemical manifestations of these processes in Mono Lake, a particularly arsenic-rich environment. Herein we review some of our findings concerning the biogeochemical arsenic cycle in this lake, with the hope that it may broaden the understanding of the influence of microorganisms upon the speciation of arsenic in more common, less "extreme" environments, such as drinking water aquifers.

Sulfide Oxidation Coupled to Arsenate Reduction by a Diverse Microbial Community in a Soda Lake

“…Hydrothermal processes associated with volcanism in the watershed of Mono Lake, an endorheic lake in eastern California, have led to the accumulation of arsenic compounds (12,17). These compounds play an active role in the lake's biogeochemical cycles (26), serving both as electron acceptors for the oxidation of organic matter (23) and as electron donors (24). Arsenate reduction in Mono Lake appeared to be relatively insensitive to organic-carbon additions (11,26), suggesting that another electron donor, possibly sulfide, might be significant in this environment, a conclusion that was supported by geochemical evidence (12).…”

“…These compounds play an active role in the lake's biogeochemical cycles (26), serving both as electron acceptors for the oxidation of organic matter (23) and as electron donors (24). Arsenate reduction in Mono Lake appeared to be relatively insensitive to organic-carbon additions (11,26), suggesting that another electron donor, possibly sulfide, might be significant in this environment, a conclusion that was supported by geochemical evidence (12).Arsenate reduction coupled with sulfide oxidation was first demonstrated with an archived sample of Mono Lake bottom water (10) and was subsequently repeated with slurries of sediments from an arsenic-enriched playa basin south of Mono Lake (25). An organism (deltaproteobacterium strain MLMS-1) capable of chemoautotrophic growth by oxidizing sulfide with arsenate was isolated from the Mono Lake sample (10).…”

Natural organic matter and colloid-facilitated arsenic transport and transformation in porous soil media

“…Arsenic retention and transport in soil is primarily related to the content o f Fe and Al oxides (Fuller et al, 1993;Wilkie and Hering, 1996), redox potential (Deuel and Swoboda, 1972;Hess and Blanchar, 1976;Webb, 1966), pH, the type and content o f clay in the soil (Dickens and Hiltbold, 1967;Elkhatib et al, 1984), microbial communities (Jackson et al, 2003;Mukhopadhyay et al, 2002;Oremland et al, 2005;Oremland et al, 2004;Sohrin et al, 1997), and to a lesser extent upon exchangeable Ca and oxyanion competition (Manning and Goldberg, 1996b). Numerous studies have already been conducted on As adsorption onto oxy-hydroxides (Dzombak, 1990;Gupta and Chen, 1978;Lafferty and Loeppert, 2005;Oscarson et al, 1983;Tossell, 1997;Wilkie and Hering, 1996) and clay minerals (Goldberg, 2002;Goldberg and Glaubig, 1988;Griffin et al, 1977;Manning and Goldberg, 1997).…”

“…Microbes are widely recognized to be o f great importance to the transformation o f pollutant due to their omnipresence and their metabolic flexibility (Brown et al, 2004;McLean et al, 2002;Toes et al, 2004). Microbial activity may exert direct and indirect influence on As speciation and are attributable to many reactions that result in changes in speciation of As, oxidation o f As111 to Asv or reduction o f Asv to As111 (Jackson et al, 2003;Mukhopadhyay et al, 2002;Oremland et al, 2005;Oremland et al, 2004). These inorganic forms can also be biomethylated by certain microbes to gaseous arsines or to MMA and DMA, while other microbes can demethylate organic forms to inorganic species (Pongratz, 1998;Sohrin et al, 1997;Turpeinen et al, 2002).…”

“…values o f the various species o f arsenic are summarized in Table l.l (Lafferty and Loeppert, 2005). Microorganisms are attributable to many reactions that result in changes in speciation of arsenic; As111 can be oxidized to Asv, or Asv can be reduced to As111 (Jackson et al, 2003;Mukhopadhyay et al, 2002;Oremland et al, 2005;Oremland et al, 2004). These inorganic arsenic forms can also be biomethylated by certain microorganisms to gaseous arsines or to methylated species such as monomethyl arsonic acid (MMA) and dimethyl arsinic acid (DMA), while other microbes can demethylate organic forms to inorganic species (Mukhopadhyay et al, 2002;Sohrin et al, 1997) (Fig 1.3).…”