N2 fixation, measured as acetylene reduction, was studied in laboratory cultures and in natural assemblages (both
as a mixed population and as individually picked colonies) of the heterocystous cyanobacteria Aphanizomenon sp.
and Nodularia spp. from the Baltic Sea. During a diurnal cycle of alternating light and darkness, these organisms
reduced acetylene predominantly during the period of illumination, although considerable activity was also
observed during the dark period. In both laboratory cultures and natural populations N2 fixation was saturated
below a photon flux density of 600 μm−2 s−1. In cyanobacterial blooms in the Baltic Sea, nitrogenase activity was
mostly confined to the surface layers. Samples collected from greater depths did not possess the same capacity for
acetylene reduction as samples from the surface itself, even when incubated at the photon flux density prevailing
in surface waters. This suggests that, with respect to N2 fixation, Baltic cyanobacteria are adapted to the intensity
of illumination that they are currently experiencing.