Marine pelagic redoxclines are characterized by enhanced dark CO2 fixation rates. Hitherto, no detailed information on the composition and diversity of chemolithoautotrophic assemblages exist. Applying RNA-SIP Epsilonproteobacteria, closely related to Sulfurimonas denitrificans, and sulfur-oxidizing Gammaproteobacteria were identified as chemoautotrophs in sulfidic water layers of two different marine pelagic redoxclines. Furthermore, the mixotrophic uptake of pyruvate by a chemolithoautotrophic Epsilonproteobacterium isolated from a Baltic Sea redoxcline was successfully proven.