Vol. 68, No. 10
Anaerobic Oxidation of Arsenite in Mono Lake Water and by a Facultative, Arsenite-Oxidizing Chemoautotroph, Strain MLHE-1
Ronald S. Oremland,1* Shelley E. Hoeft,1 Joanne M.Santini,2 Nasreen Bano,3 Ryan A. Hollibaugh,3 and James T. Hollibaugh3
U.S. Geological Survey, Menlo Park, California 940251; La Trobe University, Victoria 3086, Australia2; and University of Georgia, Athens, Georgia 306023
Received 21 May 2002/Accepted 16 July 2002
Arsenite [As(III)]-enriched anoxic bottom water from Mono Lake, California, produced arsenate [As(V)] during incubation with eithernitrate or nitrite. No such oxidation occurred in killed controls or in live samples incubated without added nitrate or nitrite. A small amount of biological As(III) oxidation was observed in samples amended with Fe(III) chelated with nitrolotriacetic acid, although some chemical oxidation was also evident in killed controls. A pure culture, strain MLHE-1, that was capable of growth with As(III) asits electron donor and nitrate as its electron acceptor was isolated in a deﬁned mineral salts medium. Cells were also able to grow in nitrate-mineral salts medium by using H2 or sulﬁde as their electron donor in lieu of As(III). Arsenite-grown cells demonstrated dark 14CO2 ﬁxation, and PCR was used to indicate the presence of a gene encoding ribulose-1,5-biphosphate carboxylase/oxygenase. StrainMLHE-1 is a facultative chemoautotroph, able to grow with these inorganic electron donors and nitrate as its electron acceptor, but heterotrophic growth on acetate was also observed under both aerobic and anaerobic (nitrate) conditions. Phylogenetic analysis of its 16S ribosomal DNA sequence placed strain MLHE-1 within the haloalkaliphilic Ectothiorhodospira of the -Proteobacteria. Arseniteoxidation has never been reported for any members of this subgroup of the Proteobacteria. Arsenic (As) is a known carcinogen in drinking water, occurring therein primarily as arsenate [As(V)] or as arsenite [As(III)], with the latter oxyanion having greater toxicity and hydrologic mobility than the former (10). Although various chemical agents can drive the redox reactions occurring between the As(V)and As(III) species (7, 31), it has been shown that speciﬁc microbiological detoxiﬁcation mechanisms can achieve this as well (5, 30). Recent discoveries have revealed that the biochemical reduction or oxidation of these two inorganic As species can also be coupled with energy conservation in some prokaryotes. Thus, respiratory (dissimilatory) reduction of As(V) is found in several phylogeneticallydiverse anaerobic Bacteria and Crenarchaeota (28), while the aerobic oxidation of As(III) provides energy for the rapid growth of chemoautotrophic bacteria isolated from gold mines (34, 35). Signiﬁcant lithotrophic oxidative processes need not necessarily be coupled biochemically with oxygen. Such reactions can occur under anoxic conditions, provided that the oxidant has a higher electrochemicalpotential than the reductant. Examples of this phenomenon are the bacterial oxidation of Fe(II) with nitrate (44) and the oxidation of phosphite by sulfate reducers (36). Here we explore the potential of oxidants such as nitrate and Fe(III) to be coupled with the microbial oxidation of As(III) to As(V). Arsenic-rich Mono Lake, California, was selected as the source of materials for investigation.The high content of
* Corresponding author. Mailing address: U.S. Geological Survey, MS 480, 345 Middleﬁeld Rd., Menlo Park, CA 94025. Phone: (650) 329-4482. Fax: (650) 329-4463. E-mail: firstname.lastname@example.org. 4795
dissolved inorganic arsenic (200 M) in this stratiﬁed soda lake (pH 9.8; salinity, 70 to 90 g liter 1) is derived from hydrothermal sources feeding into the lake coupled with...