Hydrogen production by Sulfurospirillum species enables syntrophic interactions of Epsilonproteobacteria.

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, Sulfurospirillum spp. The amount of hydrogen produced is different in two subgroups of Sulfurospirillum spp., represented by S. cavolei and S. multivorans. 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 E. coli) that is more abundant during fermentation. A syntrophic interaction is established between Sulfurospirillum multivorans and Methanococcus voltae 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.