Isolation and Genomic Characterization ofDesulfuromonas soudanensisWTL, a Metal- and Electrode-Reducing Bacterium from Anoxic Deep Subsurface Brine

ABSTRACT Reaching a depth of 713 m below the surface, the Soudan Underground Iron Mine (Soudan, Minnesota, USA) transects a massive Archaean (2.7 Ga) banded iron formation, providing a remarkably accessible window into the terrestrial deep biosphere. Despite carbon limitation, metal-reducing microbial communities are present in potentially ancient anoxic brines continuously emanating from exploratory boreholes on Level 27. Using graphite electrodes deposited in situ as bait, we enriched and isolated a novel halophilic iron-reducing Deltaproteobacterium, Desul-furomonas soudanensis strain WTL, from an acetate-fed three-electrode bioreactor poised at +0.24 V (vs. standard hydrogen electrode). Cyclic voltammetry revealed that D. soudanensis releases electrons at redox potentials approximately 100 mV more positive than the model freshwater surface isolate Geobacter sulfurreducens , suggesting that its extracellular respiration is tuned for higher potential electron acceptors. D. soudanensis contains a 3,958,620-bp circular genome, assembled to completion using single-molecule real-time (SMRT) sequencing reads, which encodes a complete TCA cycle, 38 putative multiheme c -type cytochromes, one of which contains 69 heme-binding motifs, and a LuxI/LuxR quorum sensing cassette that produces an unidentified N- acyl homoserine lactone. Another cytochrome is predicted to lie within a putative prophage, suggesting that horizontal transfer of respiratory proteins plays a role in respiratory flexibility among metal reducers. Isolation of D. soudanensis underscores the utility of electrode-based approaches for enriching rare metal reducers from a wide range of habitats.