An autotrophic H₂-oxidizing, nitrate-respiring, Tc(VII)-reducing Acidovorax sp. isolated from a subsurface oxic-anoxic transition zone

Increasing concentrations of H₂ with depth were observed across a geologic unconformity and associated redox transition zone in the subsurface at the Hanford Site in south-central Washington, USA. An opposing gradient characterized by decreasing O₂ and nitrate concentrations was consistent with microbial-catalysed biogeochemical processes. Sterile sand was incubated in situ within a multilevel sampler placed across the redox transition zone to evaluate the potential for Tc(VII) reduction and for enrichment of H₂-oxidizing denitrifiers capable of reducing Tc(VII). H₂-driven TcO₄ ^- reduction was detected in sand incubated at all depths but was strongest in material from a depth of 17.1m. Acidovorax spp. were isolated from H₂-nitrate enrichments from colonized sand from 15.1m, with one representative, strain JHL-9, subsequently characterized. JHL-9 grew on acetate with either O₂ or nitrate as electron acceptor (data not shown) and on medium with bicarbonate, H₂ and nitrate. JHL-9 also reduced pertechnetate (TcO₄ ^-) under denitrifying conditions with H₂ as the electron donor. H₂-oxidizing Acidovorax spp. in the subsurface at Hanford and other locations may contribute to the maintenance of subsurface redox gradients and offer the potential for Tc(VII) reduction.