Microbial regulation of arsenic bioavailability and toxicity in soils

Abstract

The biogeochemical cycle of arsenic (As) is very complex. The current practice of measuring total arsenic concentrations, especially in soil environments, ignores any differences in arsenic form or bioavailability, and therefore misrepresents the risk of arsenic to human health. Biotic and abiotic transformations of arsenic change its mobility, availability, and toxicity. A key reaction is arsenite (As(III)) oxidation to arsenate (As(V)), a form that is less bioavailable and fifty times less toxic. The role of microorganisms may be key to understanding As(III) oxidation rates in soils. This research studies how As(III) oxidation rates may be altered when soil microorganisms have had previous arsenic exposure. My primary hypothesis is: Previous exposure to arsenic affects the soil microbial population, allowing it to more quickly oxidize, and therefore detoxify, additional arsenite challenges. Microcosms of soils with different histories of arsenic contamination were amended with As(III). As(III) oxidation rates were compared by monitoring the bioavailable (water-extractable) arsenic concentrations using high performance liquid chromatography--hydride generation--atomic fluorescence spectrometry. Although As(III) oxidation occurred in all biologically active soils, reaction rates were positively correlated with previous As exposure, even with fewer and less diverse microorganisms in the chronically contaminated soils. 16SrDNA denaturing gradient gel electrophoresis analysis detected an emerging band with sequence homology to Thiomonas sp. NO115 after the highest contaminated soil was given the highest amendments. Bacterial isolates from that soil also showed different As transformation abilities, even when under the same selection pressures. To determine if faster As(III) oxidation rates could be induced in an arsenic-naive soil, control soils were given multiple doses of As(III). Changes in As(III) oxidation rate were dependent on both the number of previous doses and the challenge dose. When a soil inoculated with microorganisms extracted from a pre-exposed soil was challenged with 500ppm As(III), As(III) disappearance was more similar to an adapted soil, showing that it was the change in the microbial population that was responsible for the change in the As(III) oxidation rate. By studying how microbial populations adapt to environmental stressors like As(III), we may better understand not only the microorganisms, but also the biogeochemistry of arsenic.