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Two forms of arsenic are found in the environment: As(V) and As(III), the latter being more toxic, water-soluble, and mobile. Microorganisms may increase the mobility of arsenic by reducing As(V) to As(III); however, detoxification and immobilization can occur via the oxidation of As(III) to As(V). The US EPA has set a minimum contaminant level of 10 parts per billion (ppb) for arsenic in drinking water. The research objective was to confirm the presence of arsenic-tolerant bacteria in the Lower Laguna Madre of south Texas. Sediment samples were collected and inoculated into growth media which contained either 2 mM As(III) or 2 mM As(V) to enrich for As(III)-tolerant and As(V)-tolerant bacteria, respectively. Twenty six (26) As (III)-tolerant and 12 As(V)-tolerant cultures were obtained. Most isolates were small white colonies of Gram-positive rods. Biochemical tests using commercially-made test strips showed that As(V)-tolerant isolates displayed greater resource usage compared to As(III)-tolerant isolates but overall, few cultures demonstrated a wide-range of biochemical capabilities. Isolates with distinct morphological and biochemical phenotypes were subjected to Polymerase Chain Reaction (PCR) amplification and sequencing of the 16S rRNA genes to identify the bacteria. Closest sequence matches were to the eubacterial genera Mycoplasma, Salinispora, Frankia, and Pelodictyon. These results suggest that the Lower Laguna Madre is inhabited by a diverse group of microorganisms able to tolerate toxic concentrations of different arsenic species.

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Texas Journal of Science



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