N-2-fixation, ammonium release and N-transfer to the microbial and classical food web within a plankton community

We investigated the role of N-2-fixation by the colony-forming cyanobacterium, Aphanizomenon spp., for the plankton community and N-budget of the N-limited Baltic Sea during summer by using stable isotope tracers combined with novel secondary ion mass spectrometry, conventional mass spectrometry and nutrient analysis. When incubated with N-15(2), Aphanizomenon spp. showed a strong N-15-enrichment implying substantial N-15(2)-fixation. Intriguingly, Aphanizomenon did not assimilate tracers of (NH4+)-N-15 from the surrounding water. These findings are in line with model calculations that confirmed a negligible N-source by diffusion-limited NH4+ fluxes to Aphanizomenon colonies at low bulk concentrations (<250 nM) as compared with N-2-fixation within colonies. No N-2-fixation was detected in autotrophic microorganisms