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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 (&gt;1 mm). Dark respiration of the same colonies varied between 0.038 and 87 nmol O(2) h(-1), and it reached the limit set by O(2) diffusion from the surrounding water to colonies (&gt;1 mm). N(2) fixation associated with N. spumigena colonies (&gt;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.

Carbon, nitrogen and O2 fluxes associated with the cyanobacterium Nodularia spumigena in the Baltic Sea