HR: 0800h
AN: H21C-1028    [Abstracts]
TI: Role of Sulfide, Selenate and Nitrate in Arsenite Oxidation in Mono Lake, CA
AU: * Fisher, J C
EM: jenf@uga.edu
AF: University of Georgia, Department of Marine Sciences University of Georgia, Athens, GA 30606-3636 United States
AU: Hollibaugh, J T
EM: aquadoc@uga.edu
AF: University of Georgia, Department of Marine Sciences University of Georgia, Athens, GA 30606-3636 United States
AB: Arsenic is an important chemical constituent in Mono Lake where it occurs naturally at sufficient concentrations ($\sim$200 $\mu$M) to provide a source of energy for microbial metabolism (through oxidation or dissimilatory reduction). Arsenite and arsenic-thiol compounds are the dominant forms of arsenic present in seasonally anoxic regions of the water column when sulfide is present, but are rapidly converted to arsenate throughout the water column after holomixis. Experiments were conducted on surface water samples in April and August of 2004 to measure potential rates of aerobic As(III) oxidation to As(V) and to study the effect of co-oxidation of reduced S compounds. The presence of significant levels of arsenate ($\sim$20 �M) in anoxic bottom waters suggests that anaerobic oxidation of arsenite may also occur in Mono Lake. Anaerobic arsenite oxidation experiments were conducted on samples collected from low oxygen (April) or anoxic (August) bottom waters. Selenate was tested as an electron acceptor for arsenite oxidation in April experiments, and separate experiments with nitrate and selenate were conducted in the August experiment. Surface water collected from Mono Lake in April and August of 2004 was enriched with arsenite or arsenite and sulfide and exposed to air for the duration of the experiment. Average rates of As(III) oxidation (arsenate production) were $\sim$25 $\mu$M/day in the arsenite treatments; 2 mM As(III) was converted to As(V) in 45 days. Rates increased 3-4-fold during the experiment, suggesting adaptation of the microbial community to the added arsenite. Oxidation was significantly faster in the arsenite + sulfide treatments ($\sim$200 $\mu$M/day). Similar results were observed in the August experiments, with much faster rates of arsenite oxidation in treatments with both arsenite and sulfide. No significant arsenate production was measured in killed controls, indicating that abiotic arsenite oxidation is slow. If similar rates occur in situ, all As(III) present in anoxic waters could easily be oxidized within a few days during turnover. Thus biological As(III) oxidation may be an important process over short time scales in Mono Lake. Anaerobic oxidation of arsenite (1 mM) using selenate (1 mM) as an electron acceptor occurred at a rate of $\sim$10 $\mu$M/day in April experiments. Rates were significantly faster in experiments conducted in August ($\sim$200 $\mu$M/day), possibly due to seasonal shifts in the microbial community or lake water chemistry. In both experiments, after 0.5-1 mM arsenate accumulated, arsenate reduction became the dominant arsenic metabolism, yielding a net decrease in arsenate concentrations. The oxidation of arsenite using selenate as the terminal electron acceptor represents a novel pathway for arsenite oxidation (and selenate reduction). Potential rates of anaerobic arsenite oxidation using nitrate (5 mM) as an electron acceptor were also measured in August samples. After a 5 day lag period, arsenite was oxidized at a rate of nearly 1 mM/day. Although the potential for anaerobic As(III) oxidation exists, this process is less significant than aerobic oxidation since nitrate and selenate are found at considerably lower concentrations in Mono Lake ($<$10 $\mu$M each). The results of these experiments show that 1) rapid biological oxidation of arsenite in Mono Lake can occur in both oxic and anoxic waters, 2) the observed potential rates of aerobic arsenite/thioarsenite oxidation are sufficient to oxidize all reduced arsenic species present within 1 week of turnover, and 3) at least two potential biological pathways of anaerobic arsenite oxidation exist in Mono Lake.
DE: 1845 Limnology
DE: 1030 Geochemical cycles (0330)
DE: 1065 Trace elements (3670)
DE: 1615 Biogeochemical processes (4805)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting