An autotrophic H 2 ‐oxidizing, nitrate‐respiring, T c( VII )‐reducing A cidovorax sp. isolated from a subsurface oxic‐anoxic transition zone

Summary Increasing concentrations of H 2 with depth were observed across a geologic unconformity and associated redox transition zone in the subsurface at the H anford S ite in south‐central W ashington, USA . An opposing gradient characterized by decreasing O 2 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 T c( VII ) reduction and for enrichment of H 2 ‐oxidizing denitrifiers capable of reducing T c( VII ). H 2 ‐driven TcO 4 − reduction was detected in sand incubated at all depths but was strongest in material from a depth of 17.1 m. A cidovorax spp. were isolated from H 2 ‐nitrate enrichments from colonized sand from 15.1 m, with one representative, strain JHL ‐9, subsequently characterized. JHL ‐9 grew on acetate with either O 2 or nitrate as electron acceptor (data not shown) and on medium with bicarbonate, H 2 and nitrate. JHL ‐9 also reduced pertechnetate ( TcO 4 − ) under denitrifying conditions with H 2 as the electron donor. H 2 ‐oxidizing A cidovorax spp. in the subsurface at H anford and other locations may contribute to the maintenance of subsurface redox gradients and offer the potential for T c( VII ) reduction.