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The Baltic Sea receives large nitrogen inputs by diazotrophic (N2-fixing) heterocystous cyanobacteria but the significance of heterotrophic N2 fixation has not been studied. Here, the diversity, abundance and transcription of the nifH fragment of the nitrogenase enzyme in two basins of the Baltic Sea proper was examined. N2 fixation was measured at the surface (5 m) and in anoxic water (200 m). Vertical sampling profiles of >10 and <10 μm size fractions were collected in 2007, 2008 and 2011 at the Gotland Deep and in 2011 in the Bornholm Basin. Both of these stations are characterized by permanently anoxic bottom water. The 454-pyrosequencing nifH analysis revealed a diverse assemblage of nifH genes related to alpha-, beta- and gammaproteobacteria (nifH cluster I) and anaerobic bacteria (nifH cluster III) at and below the chemocline. Abundances of genes and transcripts of seven diazotrophic phylotypes were investigated using quantitative polymerase chain reaction revealing abundances of heterotrophic nifH phylotypes of up to 2.1 × 107 nifH copies l−1. Abundant nifH transcripts (up to 3.2 × 104 transcripts l−1) within nifH cluster III and co-occurring N2 fixation (0.44±0.26 nmol l−1 day−1) in deep water suggests that heterotrophic diazotrophs are fixing N2 in anoxic ammonium-rich waters. Our results reveal that N2 fixation in the Baltic Sea is not limited to illuminated N-deplete surface waters and suggest that N2 fixation could also be of importance in other suboxic regions of the world's oceans. [ABSTRACT FROM AUTHOR]

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]