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

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-2 h (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 (41mm) comprised on average 18% of the total N-2 fixation in the bulk water. Net NH4+ release in colonies equaled 8-33% of the estimated gross N-2 fixation during photosynthesis. NH4+ concentrations within light-exposed colonies, modeled from measured net NH4+ release rates, were 60-fold higher than that of the bulk. Hence, N. spumigena colonies comprise highly productive microenvironments and an attractive NH4+ microenvironment to be utilized by other (micro) organisms in the Baltic Sea where dissolved inorganic nitrogen is limiting growth.