Photosynthesis, respiration, N 2 fixation and ammonium release were studied directly in Nodularia spumigena during a bloom in the Baltic Sea using a combination of microsensors, stable isotope tracer experiments combined with nanoscale secondary ion mass spectrometry (nanoSIMS) and fluorometry. Cell-specific net C- and N 2 -fixation rates by N. spumigena were 81.6±6.7 and 11.4±0.9 fmol N per cell per h, respectively. During light, the net C:N fixation ratio was 8.0±0.8. During darkness, carbon fixation was not detectable, but N 2 fixation was 5.4±0.4 fmol N per cell per h. Net photosynthesis varied between 0.34 and 250 nmol O 2 h -1 in colonies with diameters ranging between 0.13 and 5.0 mm, and it reached the theoretical upper limit set by diffusion of dissolved inorganic carbon to colonies (> 1 mm). Dark respiration of the same colonies varied between 0.038 and 87 nmol O 2h -1, and it reached the limit set by O 2 diffusion from the surrounding water to colonies (>1 mm). N 2 fixation associated with N. spumigena colonies (>1 mm) comprised on average 18% of the total N 2 fixation in the bulk water. Net NH 4+ release in colonies equaled 8-33% of the estimated gross N 2 fixation during photosynthesis. NH 4+ concentrations within light-exposed colonies, modeled from measured net NH 4+ release rates, were 60-fold higher than that of the bulk. Hence, N. spumigena colonies comprise highly productive microenvironments and an attractive NH 4 microenvironment to be utilized by other (micro)organisms in the Baltic Sea where dissolved inorganic nitrogen is limiting growth. © 2011 International Society for Microbial Ecology All rights reserved.