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Pelagic redoxclines of the central Baltic Sea are dominated by the epsilonproteobacterial group Sulfurimonas GD17, considered to be the major driver of chemolithoautotrophic denitrification in this habitat. Autecological investigations of a recently isolated representative of this environmental group, Sulfurimonas gotlandica str. GD1T, demonstrated that the bacterium grows best under sulfur-oxidizing, denitrifying conditions. However, in the presence of bicarbonate, this strain is able to use pyruvate as both an additional carbon source and an alternative electron donor. These observations suggested that the environmental group GD17 actively metabolizes organic substrates in situ. To examine this possibility, we used RNA-based stable isotope probing ( RNA- SIP) on a natural redoxcline community provided with 13C-labeled pyruvate. While in this experiment, we were able to identify putative heterotrophic microorganisms, the uptake of 13C-pyruvate in GD17 nucleic acids could not be established. To resolve these contradictory findings, combined incorporation experiments with 14C- and 13C-labeled pyruvate were carried out in cells of strain GD1T cultivated under chemolithoautotrophic conditions, which favor pyruvate uptake rather than oxidation. An analysis of the labeled biomolecules revealed that pyruvate was mostly incorporated in cellular components such as amino acids, whose synthesis requires only minimal transformation. Carbon transfer into nucleic acids was not observed, explaining the inability of RNA- SIP to detect pyruvate incorporation by strain GD1T and the environmental group GD17. Together, these findings suggest that by integrating organic compounds such as pyruvate into cellular components S. gotlandica GD1T is able to replenish chemolithoautotrophic growth and thus ensure its survival in nutrient-limited habitats such as marine pelagic redoxclines. [ABSTRACT FROM AUTHOR]

Marine pelagic redoxclines are characterized by pronounced activities of chemolithoautotrophic microorganisms. As evidenced by the high dark CO2 fixation rates measured around the oxic–anoxic interface but also in the upper sulfidic zone, the accordant organisms participate in important biogeochemical transformations. Although Epsilonproteobacteria have been identified as an important chemoautotrophic group in these environments, detailed species-level information on the identity of actively involved prokaryotes is lacking. In the present study, active chemolithoautotrophic prokaryotic assemblages were identified in the sulfidic zone of a pelagic Black Sea redoxcline by applying rRNA-based stable isotope probing in combination with 16S rRNA gene single-strand conformation polymorphism analysis and 16S rRNA gene cloning. The results showed that a single epsilonproteobacterium, affiliated with the genus Sulfurimonas, and two different members of the gammaproteobacterial sulfur oxidizer (GSO) cluster were responsible for dark CO2 fixation activities in the upper sulfidic layer of the Black Sea redoxcline. Phylogenetically, these organisms were closely related to microorganisms, distributed worldwide, that are thought to be key players in denitrification and sulfide oxidation. Together, these findings emphasize the importance of chemolithoautotrophic members of the Sulfurimonas and GSO groups in the carbon, nitrogen, and sulfur cycles of oxic–anoxic pelagic transition zones. [ABSTRACT FROM AUTHOR]