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 CO 2 fixation measurements with flow cytometric cell sorting. Maximum CO 2 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 CO 2 fixation in these clusters was determined by flow cytometric sorting after anoxic incubations with NaH 14 CO 3 tracer. Two clusters, representing about 30% of total prokaryotes, were responsible for 65% to 100% of the total dark fixation. Calculated cell‐specific CO 2 fixation rates in the two basins ranged from 3.5 to 24.7 fg C cell −1 d −1 and suggested that these clusters are dominated by chemolithoautotrophic prokaryotes. Mean cell‐specific fixation rates reached more than 10 fg C cell −1 d −1 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 cell‐specific CO 2 uptake and abundance of chemolithoautotrophic prokaryotes in a pelagic marine environment. However, the identity of the organisms as well as the mechanisms fueling CO 2 dark fixation in the anoxic zone remain unknown.