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Recent studies have indicated that chemoautotrophic Epsilonproteobacteria might play an important role, especially as anaerobic or microaerophilic dark CO[sub2]-fixing organisms, in marine pelagic redoxclines. However, knowledge of their distribution and abundance as actively CO[sub2]-fixing microorganisms in pelagic redoxclines is still deficient. We determined the contribution of Epsilonproteobacteria to dark CO[sub2] fixation in the sulfidic areas of central Baltic Sea and Black Sea redoxclines by combining catalyzed reporter deposition-fluorescence in situ hybridization with microautoradiography using [[sup14]C]bicarbonate and compared it to the total prokaryotic chemoautotrophic activity. In absolute numbers, up to 3 x 10[sup5] [sup14]CO[sub2]-fixing prokaryotic cells ml[sup-1] were enumerated in the redoxcline of the central Baltic Sea and up to 9 x 10[sup4] [sup14]CO[sub2]-fixing cells ml[sup-1] were enumerated in the Black Sea redoxcline, corresponding to 29% and 12%, respectively, of total cell abundance. [sup14]CO[sub2]-incorporating cells belonged exclusively to the domain Bacteria. Among these, members of the Epsilonproteobacteria were approximately 70% of the cells in the central Baltic Sea and up to 100% in the Black Sea. For the Baltic Sea, the Sulfurimonas subgroup GD17, previously assumed to be involved in autotrophic denitrification, was the most dominant CO[sub2]-fixing group. In conclusion, Epsilonproteobacteria were found to be mainly responsible for chemoautotrophic activity in the dark CO[sub2] fixation maxima of the Black Sea and central Baltic Sea redoxclines. These Epsilonproteobacteria might be relevant in similar habitats of the world's oceans, where high dark CO[sub2] fixation rates have been measured. [ABSTRACT FROM AUTHOR]