High abundance and dark CO2 fixation of chemolithoautotrophic prokaryotes in anoxic waters of the Baltic Sea

We determined the abundance and distribution of chemolithoautotrophic prokaryotes in the redoxcline in two basins (Gotland Deep, Landsort Deep) of the central Baltic Sea by combining dark CO2 fixation measurements with flow cytometric cell sorting. Maximum CO2 fixation rates were recorded in sulfidic waters about 20 m below the chemocline. Flow cytometric analyses of deoxyribonucleic acid (DNA)–stained bacterioplankton revealed the existence of at least five different prokaryotic clusters in water samples collected below the chemocline. Dark CO2 fixation in these clusters was determined by flow cytometric sorting after anoxic incubations with NaHCO3 tracer. Two clusters, representing about 30% of total prokaryotes, were responsible for 65% to 100% of the total dark fixation. Calculated cell-specific CO2 fixation rates in the two basins ranged from 3.5 to 24.7 fg C cell21 d21 and suggested that these clusters are dominated by chemolithoautotrophic prokaryotes. Mean cell-specific fixation rates reached more than 10 fg C cell21 d21 in most cases, indicating relatively high growth rates (doubling times 1–2 d) of chemolithoautotrophic prokaryotes. Our results provide the first evidence of such high cellspecific CO2 uptake and abundance of chemolithoautotrophic prokaryotes in a pelagic marine environment. However, the identity of the organisms as well as the mechanisms fueling CO2 dark fixation in the anoxic zone remain unknown. Chemolithotrophic prokaryotes gain energy by oxidizing inorganic reduced compounds, such as ammonia or hydrogen sulfide, which originate from the degradation of organic material. This prokaryotic activity greatly impacts biogeochemical cycles, especially nitrogen and sulfur cycles in oligotrophic systems. In these environments, chemolithoautotrophs benefit from their ability to grow by assimilating carbon dioxide (CO2). The oxic–anoxic interface is an ideal biotope for organisms such as nitrifying or sulfur-oxidizing prokaryotes (e.g., Paerl and Pinckney 1996; Nealson 1997). However, due to the potentially low energy yield of several chemolithotrophic reactions, the compounds necessary for the reduction of CO2 are present, but the biomass and growth rates of these prokaryotes are often relatively low (Shively et al. 1998; Kelly 1999; Kelly