disguised as a sulfate reducer growth of the deltaproteobacterium desulfurivibrio alkaliphilus this study demonstrates that the deltaproteobacterium desulfurivibrio alkaliphilus can grow chemolithotrophically by coupling sulfide oxidation to the dissimilatory reduction of nitrate and nitrite to ammonium key genes of known sulfide oxidation pathways are absent from the genome of d alkaliphilus instead the genome contains all of the genes necessary for sulfate reduction including a gene for a reductivetype dissimilatory bisulfite reductase dsr despite this growth by sulfate reduction was not observed transcriptomic analysis revealed a very high expression level of sulfatereduction genes during growth by sulfide oxidation while inhibition experiments with molybdate pointed to elemental sulfurpolysulfides as intermediates consequently we propose that d alkaliphilus initially oxidizes sulfide to elemental sulfur which is then either disproportionated or oxidized by a reversal of the sulfate reduction pathway this is the first study providing evidence that a reductivetype dsr is involved in a sulfide oxidation pathway transcriptome sequencing further suggests that nitrate reduction to ammonium is performed by a novel type of periplasmic nitrate reductase and an unusual membraneanchored nitrite reductaseimportance sulfide oxidation and sulfate reduction the two major branches of the sulfur cycle are usually ascribed to distinct sets of microbes with distinct diagnostic genes here we show a more complex picture as d alkaliphilus with the genomic setup of a sulfate reducer grows by sulfide oxidation the high expression of genes typically involved in the sulfate reduction pathway suggests that these genes including the reductivetype dissimilatory bisulfite reductases are also involved in asyetunresolved sulfide oxidation pathways finally d alkaliphilus is closely related to cable bacteria which grow by electrogenic sulfide oxidation since there are no pure cultures of cable bacteria d alkaliphilus may represent an exciting model organism in which to study the physiology of this process