The importance of hydrogen and formate transfer for syntrophic fatty, aromatic and alicyclic metabolism

Summary We used a combination of genomic, transcriptional and enzymatic analyses to determine the mechanism of interspecies electron transfer by two model syntrophic microorganisms, S yntrophomonas wolfei and S yntrophus aciditrophicus . Both organisms contain multiple hydrogenase and formate dehydrogenase genes, but lack genes for outer membrane cytochromes and nanowire formation. Syntrophically grown cells and cell‐free extracts of S . aciditrophicus and S . wolfei had both hydrogenase and formate dehydrogenase activities. Butyrate metabolism and CH 4 production by washed cell suspensions of S . wolfei and M ethanospirillum hungatei were inhibited by hydrogenase inhibitors (cyanide and carbon monoxide), but not by a formate dehydrogenase inhibitor (hypophosphite). Syntrophic benzoate oxidation and CH 4 production by washed cell suspensions of S . aciditrophicus and M . hungatei were inhibited by hypophosphite, but not cyanide and carbon monoxide. All three inhibitors halted syntrophic cyclohexane‐1‐carboxylate metabolism. Two hydrogenase genes, hydA1 and hydA2 , were more highly expressed when S . wolfei was grown syntrophically. S . aciditrophicus expressed multiple hydrogenase and formate dehydrogenase genes during syntrophic benzoate and cyclohexane‐1‐carboxylate growth, one of which ( fdhA2 ) was highly differentially expressed during syntrophic benzoate growth. Thus, these syntrophic microorganisms have flexible metabolisms that allow them to use either H 2 or formate transfer depending on the substrate involved.