Cell‐specific nitrogen‐ and carbon‐fixation of cyanobacteria in a temperate marine system (Baltic Sea)

Summary We analysed N 2 ‐ and carbon (C) fixation in individual cells of Baltic Sea cyanobacteria by combining stable isotope incubations with secondary ion mass spectrometry (SIMS). Specific growth rates based on N 2 ‐ and C‐fixation were higher for cells of Dolichospermum spp. than for Aphanizomenon sp. and Nodularia spumigena . The cyanobacterial biomass, however, was dominated by Aphanizomenon sp., which contributed most to total N 2 ‐fixation in surface waters of the Northern Baltic Proper. N 2 ‐fixation by Pseudanabaena sp. and colonial picocyanobacteria was not detectable. N 2 ‐fixation by Aphanizomenon sp., Dolichospermum spp. and N. spumigena populations summed up to total N 2 ‐fixation, thus these genera appeared as sole diazotrophs within the Baltic Sea's euphotic zone, while their mean contribution to total C‐fixation was 21%. Intriguingly, cell‐specific N 2 ‐fixation was eightfold higher at a coastal station compared to an offshore station, revealing coastal zones as habitats with substantial N 2 ‐fixation. At the coastal station, the cell‐specific C‐ to N 2 ‐fixation ratio was below the cellular C:N ratio, i.e. N 2 was assimilated in excess to C‐fixation, whereas the C‐ to N 2 ‐fixation ratio exceeded the C:N ratio in offshore sampled diazotrophs. Our findings highlight SIMS as a powerful tool not only for qualitative but also for quantitative N 2 ‐fixation assays in aquatic environments.