ABSTRACT
Another end-member environment well worth mentioning is Searles Lake located in the Mohave Desert. This lake is an evaporated version of Mono Lake, having a salt-saturated brine (salinity = ∼340 g/L) with extraordinary levels of dissolved As (∼4 mM). Nonetheless, a full As redox cycle as mediated by microorganisms was detected in this environment (Oremland et al., 2005). A number of experiments were conducted that compared the biogeochemical behavior of sediments in Mono Lake with that of Searles (Kulp et al., 2006, 2007). Notably, sulfate reduction was detected in Mono Lake sediments, but could not be elicited from Searles Lake sediments. The weak bioenergetics associated with sulfate-reduction at salt saturation had been considered by Oren (1999) as the logical explanation for the general absence of sulfate-reduction in aquatic sediments poised at salt-saturation. However, experiments with both Mono and Searles materials demonstrated a sensitivity of sulfate-reduction (but not arsenate-reduction) to high borate levels rather than salinity. A novel extremophilic arsenate-respiring bacterium, Halarsenatibacter silvermanii was isolated from Searles Lake, which was able to achieve
1 INTRODUCTION
Although arsenic (As) has only trace abundance in the Earth’s crust, it can accumulate to high concentrations (micromolar to millimolar) in certain aquatic environments, notably hot springs and especially in hypersaline lakes located in desert regions that receive As-laden drainage from such springs. These environments can be considered as physical/chemical “extremes” of temperature, salinity, pH, and/or abundance of toxic substances (e.g., B) with regard their ability to host life. Hence, the microbes that inhabit these environments are not only adapted to living under these conditions, but in some cases can actually exploit the redox chemistry of the abundant As present therein for the purposes of deriving energy for growth. They achieve this by either by oxidizing arsenite [As(III)] (as an electron donor) or by respiring arsenate [As(V)] (as a terminal electron acceptor). This process has been previously reviewed in the literature (e.g., Oremland & Stolz, 2003), along with why the study of such extreme locales should be of relevance to the question of As mobilization in drinking water aquifers (Oremland & Stolz, 2005; Lloyd & Oremland, 2006). The biochemistry, biogeochemistry and microbiology were recently reviewed (Stolz et al., 2006; Oremland et al., 2009).
