Arsenic transformation and mobilization from minerals by the arsenite oxidizing strain WAO

Analysis of arsenic concentrations in New Jersey well water from the Newark Basin showed up to 15% of the wells exceed 10 microg L(-1), with a maximum of 215 microg L(-1). In some geologic settings in the basin, this mobile arsenic could be from the weathering of pyrite (FeS2) found in black shale that contains up to 4% arsenic by weight. We hypothesized that under oxic conditions at circumneutral pH, the microbially mediated oxidation of sulfide in the pyrite lattice would lead to the release of pyrite-bound arsenic. Moreover, the oxidation of aqueous As(III) to As(V) by aerobic microorganisms could further enhance arsenic mobilization from the solid phase. Enrichment cultures under aerobic, As(III)-oxidizing conditions were established under circumneutral pH with weathered black shale from the Newark Basin as the inoculum source. Strain WAO, an autotrophic inorganic-sulfur and As(III)-oxidizer, was isolated and phylogenetically and physiologically characterized. Arsenic mobilization studies from arsenopyrite (FeAsS) mineral, conducted with strain WAO at circumneutral pH, showed microbially enhanced mobilization of arsenic and complete oxidation of released arsenic and sulfur to stoichiometric amounts of arsenate and sulfate. In addition, WAO preferentially colonized pyrite on the surface of arsenic-bearing, black shale thick sections. These findings support the hypothesis that microorganisms can directly mobilize and transform arsenic bound in mineral form at circumneutral pH and suggest that the microbial mobilization of arsenic into groundwater may be important in other arsenic-impacted aquifers.

The arsenite oxidase genes (aroAB) in novel chemoautotrophic arsenite oxidizers

Six novel bacterial strains were isolated from the environment which can oxidize arsenite [As(III)] to the less mobile form arsenate [As(V)] coupled to CO(2) fixation under either aerobic or denitrifying conditions. PCR primers were designed to the conserved molybdopterin domain of the large subunit of arsenite oxidase in order to identify the arsenite oxidase genes from these isolates. The amino acid sequences for the arsenite oxidases reported here were 72-74% identical to that of strain NT-26, the only previously reported autotrophic arsenite oxidizer. Indeed the autotrophic arsenite oxidase genes form a distinct phylogenetic group, separated from previously described heterotrophic arsenite oxidase genes, with the exception of the heterotroph Agrobacterium tumefaciens. The arsenite oxidase primers described here represent a powerful culture-independent tool to assess the diversity of arsenite oxidase genes in environmental bacteria.

Anaerobic arsenite oxidation by novel denitrifying isolates

Autotrophic microorganisms have been isolated that are able to derive energy from the oxidation of arsenite [As(III)] to arsenate [As(V)] under aerobic conditions. Based on chemical energetics, microbial oxidation of As(III) can occur in the absence of oxygen, and may be relevant in some environments. Enrichment cultures were established from an arsenic contaminated industrial soil amended with As(III) as the electron donor, inorganic C as the carbon source and nitrate as the electron acceptor. In the active enrichment cultures, oxidation of As(III) was stoichiometrically coupled to the reduction of NO(3) (-). Two autotrophic As(III)-oxidizing strains were isolated that completely oxidized 5 mM As(III) within 7 days under denitrifying conditions. Based on 16S rRNA gene sequencing results, strain DAO1 was 99% related to Azoarcus and strain DAO10 was most closely related to a Sinorhizobium. The nitrous oxide reductase (nosZ) and the RuBisCO Type II (cbbM) genes were successfully amplified from both isolates underscoring their ability to denitrify and fix CO(2) while coupled to As(III) oxidation. Although limited work has been done to examine the diversity of anaerobic autotrophic oxidizers of As(III), this process may be an important component in the biological cycling of arsenic within the environment.

Environmental microbes can speciate and cycle arsenic

Naturally occurring arsenic is found predominantly as arsenate [As(V)] or arsenite [As(III)], and can be readily oxidized or reduced by microorganisms. Given the health risks associated with arsenic in groundwater and the interest in arsenic-active microorganisms, we hypothesized that environmental microorganisms could mediate a redox cycling of arsenic that is linked to their metabolism. This hypothesis was tested using an As(V) respiring reducer (strain Y5) and an aerobic chemoautotrophic As(II) oxidizer (strain OL1 ) both isolated from a Superfund site, Onondaga Lake, in Syracuse, NY. Strains were grown separately and together in sealed serum bottles, and the oxic/anoxic condition was the only parameter changed. Initially, under anoxic conditions when both isolates were grown together, 2 mM As(V) was stoichiometrically reduced to As(III) within 14 days. Following complete reduction, sterile ambient air was added and within 24 h As(III) was completely oxidized to As(V). The anoxic-oxic cycle was repeated, and sterile controls showed no abiotic transformation within the 28-day incubation period. These results demonstrate that microorganisms can cycle arsenic in response to dynamic environmental conditions, thereby affecting the speciation, and hence mobility and toxicity of arsenic in the environment.

Microbial community responses to atrazine exposure and nutrient availability: linking degradation capacity to community structure

Repeated pesticide exposure may enhance biodegradation through selective enrichment of pesticide-metabolizing microorganisms, particularly when the compound is used as a C and energy source. The relationship between pesticide application history and degradation rate is unclear when the chemical is utilized as a nutrient source other than C. Atrazine, a poor source of C and energy, was chosen as a model compound because it can serve as an N source for some microorganisms. Soils with (H-soil) and without (NH-soil) prior s-triazine treatment history were repeatedly exposed to atrazine and a variety of C and N source amendments. Exposure to atrazine and inorganic-N availability were the dominant factors leading to the development of microbial communities with an enhanced capacity to degrade atrazine. The density of the atrazine-degrading microorganisms increased immediately, up to 1000-fold, with atrazine exposure in the H-soil, but comparable increases were not observed in the NH-soil until 12 weeks following laboratory acclimation, despite high rates of atrazine mineralization in these soils immediately following the acclimation period. Whole-soil fatty acid methyl ester (FAME) analysis showed that the application of alternative C and N sources in addition to atrazine resulted in a microbial community composition that was distinctly different from that in either the atrazinealone treatment or water controls for both the H- and NH-soils. These data suggest that the microbial communities in both soils were altered differently in response to the treatments but developed a similar enhanced capacity to mineralize atrazine.