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 (N₂)-fixing cyanobacteria in lakes and estuaries, accounting for a large fraction of the summer N₂-fixation in the Baltic Sea. However, information about its cell-specific carbon (C)- and N₂-fixation rates in the early growth season has not previously been reported. We combined various methods to study N₂-fixation, photosynthesis and respiration in field-sampled Baltic Sea Aphanizomenon sp. during early summer at 10°C. Stable isotope incubations at in situ light intensities during 24 h combined with cell-specific secondary ion mass spectrometry showed an average net N₂-fixation rate of 55 fmol N cell^-1 day^-1. Dark net N₂-fixation rates over a course of 12 h were 20% of those measured in light. C-fixation, but not N₂-fixation, was inhibited by high ambient light intensities during daytime. Consequently, the C:N fixation ratio varied substantially over the diel cycle. C- and N₂-fixation rates were comparable to those reported for Aphanizomenon sp. in August at 19°C, using the same methods. High respiration rates (23% of gross photosynthesis) were measured with 14C-incubations and O₂-microsensors, and presumably reflect the energy needed for high N₂-fixation rates. Hence, Aphanizomenon sp. is an important contributor to N₂-fixation at low in situ temperatures in the early growth season. [ABSTRACT FROM AUTHOR]