High cell-specific rates of nitrogen and carbon fixation by the cyanobacterium Aphanizomenon sp at low temperatures in the Baltic Sea

Aphanizomenon is a widespread genus of nitrogen (N-2)-fixing cyanobacteria in lakes and estuaries, accounting for a large fraction of the summer N-2-fixation in the Baltic Sea. However, information about its cell-specific carbon (C)- and N-2-fixation rates in the early growth season has not previously been reported. We combined various methods to study N-2-fixation, photosynthesis and respiration in field-sampled Baltic Sea Aphanizomenon sp. during early summer at 10 degrees C. Stable isotope incubations at in situ light intensities during 24 h combined with cell-specific secondary ion mass spectrometry showed an average net N-2-fixation rate of 55 fmol N cell(-1) day(-1). Dark net N-2-fixation rates over a course of 12 h were 20% of those measured in light. C-fixation, but not N-2-fixation, was inhibited by high ambient light intensities during daytime. Consequently, the C: N fixation ratio varied substantially over the diel cycle. C-and N-2-fixation rates were comparable to those reported for Aphanizomenon sp. in August at 19 degrees C, using the same methods. High respiration rates (23% of gross photosynthesis) were measured with C-14-incubations and O-2-microsensors, and presumably reflect the energy needed for high N-2-fixation rates. Hence, Aphanizomenon sp. is an important contributor to N-2-fixation at low in situ temperatures in the early growth season.