Hydrogen production by Sulfurospirillum species enables syntrophic interactions of Epsilonproteobacteria

<jats:title>Abstract</jats:title><jats:p>Hydrogen-producing bacteria are of environmental importance, since hydrogen is a major electron donor for prokaryotes in anoxic ecosystems. Epsilonproteobacteria are currently considered to be hydrogen-oxidizing bacteria exclusively. Here, we report hydrogen production upon pyruvate fermentation for free-living Epsilonproteobacteria, <jats:italic>Sulfurospirillum</jats:italic> spp. The amount of hydrogen produced is different in two subgroups of <jats:italic>Sulfurospirillum</jats:italic> spp., represented by <jats:italic>S. cavolei</jats:italic> and <jats:italic>S. multivorans</jats:italic>. The former produces more hydrogen and excretes acetate as sole organic acid, while the latter additionally produces lactate and succinate. Hydrogen production can be assigned by differential proteomics to a hydrogenase (similar to hydrogenase 4 from <jats:italic>E. coli</jats:italic>) that is more abundant during fermentation. A syntrophic interaction is established between <jats:italic>Sulfurospirillum multivorans</jats:italic> and <jats:italic>Methanococcus voltae</jats:italic> when cocultured with lactate as sole substrate, as the former cannot grow fermentatively on lactate alone and the latter relies on hydrogen for growth. This might hint to a yet unrecognized role of Epsilonproteobacteria as hydrogen producers in anoxic microbial communities.</jats:p>