Microbial oxidation of arsenite at low temperatures

Arsenic is toxic to most living cells and has two soluble inorganic forms: arsenite (+3) and arsenate (+5), which are ubiquitous in the environment. Microbial metabolism of arsenic in the environment, contributes to its geochemical cycling. Prokaryotic oxidation of arsenite has been reported and characterised in moderate and thermal environments but not below 10°C. Giant Mine is a discontinued gold mine located 250 miles south of the arctic circle in the Northwest Territories, Canada. 230,000 tonnes of arsenic trioxide dust are stored underground at the site and infiltrating surface waters have become contaminated with >50 mM arsenic. Several microbial biofilms were found growing on the mine walls beneath seepage points of the arsenic-contaminated water. The diversity of arsenite oxidisers in two sub-samples (which differed in arsenite concentrations) of one biofilm were compared using a functional gene approach. The diversity of the two sub-samples did not differ but the relative abundance of the three identified clades did. An arsenite-oxidising bacterium, designated GM1, was isolated from the Giant Mine biofilm. GM1 was shown to be a member of the Polaromonas genus, had a growth range of 0-25°C and oxidised arsenite in the early-log phase of growth. GM1’s arsenite oxidase was constitutively expressed. The arsenite oxidase genes were partially sequenced and their role in arsenite oxidation confirmed by mutagenesis. The arsenite oxidase of GM1 was purified and partially characterised. It consists of two subunits (88 and 15 kDa) in a α1β1 conformation, and contained Mo and Fe as cofactors. The Vmax, Kcat and Km were the highest of any known arsenite oxidase. The GM1 arsenite oxidase functioned over a broad temperature range and was more active than that of the mesophile NT-26 at low temperatures. It was also found to be less stable than that of NT-26, as observed by circular dichroism spectroscopy