Metal reduction by <i>Desulfitobacterium hafniense</i> DCB-2
I evaluated the ability of the Gram-positive dehalogenator Desulfitobacterium hafniense strain DCB-2 to reduce metals and to determine which would support growth as respiratory electron acceptors. Nine metals [As(V), Cd(II), Co(III), Cr(VI), Cu(II), Fe(III), Ni(II), Se(VI and IV), and U(VI)] were used to test growth. Slowed growth was found with 1 mM Cu(II), 1 mM Ni(II), 2.5 mM Se(VI), and 0.5 mM U(VI) and above. While growth was observed with all metals, use of metals as a sole terminal electron acceptor was found only with As(V), Cu(II), and Fe(III), although biomass yield indicated some benefit from Se(VI) and U(VI) as terminal electron acceptors. Valence state analyses detected reduction of As(V to III), Fe(III to II) (provided both as soluble ferric citrate and solid ferric oxide), Se(VI to 0 and IV to 0), and U(VI to IV). While Cu(II) and Co(III) showed the characteristic colors of reduction, changes in their valence state were below detection limits. Extracellular vesicles (containing RNA surrounded by both membrane and cell wall) were produced in response to Se(VI) and Se(IV) by budding from cells. The vesicles but not cells contained reduced Se and appeared to be a protection mechanism from Se toxicity. Both red and black crystalline Se(0) were formed. Microarrays containing probes for D. hafniense were used to investigate changes in gene expression when it was grown on nitrate, Fe(III), Se(VI), or U(VI) in comparison to growth by pyruvate fermentation. Gene expression observed during nitrate reduction can be characterized by two highly induced gene groups annotated for assimilatory sulfate reduction and N2 fixation-like function by the Nif1 operon. The most highly up-regulated genes during Se(VI) reduction were of unknown function. Comparing Fe(III) reduction and U(VI) reduction revealed many genes induced in common, especially those involved with coenzyme metabolism and energy metabolism, e.g. DMSO reductase and NADH dehydrogenase. Genes induced with U(VI) reduction suggested responses to uranium toxicity by rapid replication of DNA and an increase in motility through induction of flagellar genes and chemotaxis. Fe(III) reduction revealed the induction of genes that could produce of large amounts of extracellular material, which was supported by studies that showed Fe(III)-grown cells produced more bio film on silica and plastic coated beads than cells fermenting pyruvate. Genome analysis and expression studies suggested that this organism may grow as an acetogen, since the acetyl-CoA pathway (Wood-Ljungdahl pathway) was up-regulated during Fe(III) and U(VI) reduction. Growth studies demonstrated acetogenic growth with 30:30:40 gas combinations of H2:CO:N 2, H2:CO2:N2 and CO:CO2:N 2 as the only carbon and energy sources, consistent with using the acetyl-CoA pathway. This pathway is shared with Moorella thermoacetica, the closest sequenced acetogenic relative. To test similar physiologies, D. hafniense and M. thermoacetica were compared for growth and metal reduction. M. thermoacetica reduced Se(VI to 0) and As(V to III), but no growth or reduction occurred with soluble Fe(III).