CHEMOAUTOTROPHIC REDUCTION AND FUNCTIONAL GENE EXPRESSION IN SB(V)-REDUCING BACTERIAL COMMUNITIES FROM A CONTAMINATED MINE SITE

Antimony (Sb) and arsenic (As) are redox-sensitive toxic metalloids that commonly co-occur near geothermal systems and in association with hydrothermal ore deposits. As Group 15 neighbors on the periodic table, both metalloids share similar chemistries and are environmentally stable as their pentavalent forms [antimonate, Sb(V); or arsenate, As(V)] in oxygenated aqueous settings, or as their trivalent forms [antimonite, Sb(III); or arsenite, As(III)] in anoxic conditions. Bacterially mediated reduction of As(V) to As(III) is well established and serves as a respiratory process to support growth linked to the oxidation of both heterotrophic (organic) as well as autotrophic (inorganic) electron donors via enzymatic pathways that are encoded by the arrA functional gene in As(V)-reducing bacteria. Alternately, bacterial As(V) reduction can be mediated in other bacteria as an intercellular detoxification process via the arsC gene which confers As-resistance through a reductive efflux pathway. Reductive biotransformation of Sb(V) to Sb(III) has also been linked to heterotrophic growth in bacterial cultures, but chemoautotrophic growth linked to Sb(V) reduction is not well studied and the identification of structural genes responsible for Sb biotransformation and the potential involvement of genes which encode for As(V) reduction is not well established. This study reports H₂-dependent autotrophic bacterial reduction of up to 3 mM Sb(V) in microcosms and mixed cultures from Stibnite Mine, a historical As- and Sb-impacted mining site in Idaho. The observed Sb(V) reduction was stimulated by the presence of anthraquinone-2,6-disulfonate (AQDS) as an electron shuttle to facilitate Sb(V) reduction. RT-PCR methods were used to quantify the expression of known As-reduction functional gene products ArrA and ArsC during Sb reduction in these cultures and to demonstrate the role of these enzymatic pathways to geomicrobiological Sb(V) reduction in the contaminated wetland ecosystem.