Phenotypic and Genetic Characterization of Putative Arsenic-Oxidizing Bacteria Isolated from the Lower Laguna Madre of Texas

Anthropogenic chemicals (e.g., arsenic) may be present in the Laguna Madre, a hypersaline estuary, due to agricultural runoff, illegal disposal, and other inputs along the USA-Mexico border. Arsenite-oxidizing bacteria transform arsenite (AsIII) to arsenate (AsV), which is a less bioavailable, less toxic form of arsenic. The objective was to isolate potential arsenite-oxidizing bacteria from Laguna Madre sediments, assess their tolerance toward arsenite, and characterize and identify the organisms. Sediment samples from the Laguna Madre were cultured on medium containing 100 µM sodium (meta) arsenite. Isolated bacteria were Gram-stained and biochemically profiled using API 20E^® test strips. Bacteria were tested for arsenite tolerance at varying concentrations. Microorganisms (n = 14) were identified through Polymerase Chain Reaction (PCR)-amplification of 16S rRNA genes. Resulting 16S rRNA sequences were queried against genome databases to identify the organisms. Results showed that most of the organisms were gram-negative and API 20E^® assays showed that the isolates were >95% similar; most had identical phenotypes. Cultures tolerated up to 1 mM arsenite. Although the bacteria were morphologically and phenotypically similar, identification of organisms revealed several gram-positive Bacillus species and a variety of gram-negative species, including opportunistic human pathogens (e.g., Vibrio, Enterobacter, Klebsiella). One isolated organism was identified as a member of the genus Psychrobacter suggesting it is a cold-loving organism. Its presence in subtropical Texas is intriguing. Isolates are currently being screened for known arsenite oxidation genes. We conclude that Laguna Madre sediments are inhabited by bacterial communities that are potentially important in arsenic detoxification in the ecosystem.