Nitrogen fixation by Baltic cyanobacteria is adapted to the prevailing photon flux density

N-2 fixation, measured as acetylene reduction, was studied in laboratory cu ltures and in natural assemblages (both as a mixed population and as indivi dually picked colonies) of the heterocystous cyanobacteria Aphanizomenon sp . and Nodularia spp. from the Baltic Sea. During a diurnal cycle of alterna ting light and darkness, these organisms reduced acetylene predominantly du ring the period of illumination, although considerable activity was also ob served during the dark period. In both laboratory cultures and natural popu lations N-2 fixation was saturated below photon flux density of 600 mu m(-2 ) s(-1). In cyanobacterial blooms in the Baltic Sea, nitrogenase activity w as mostly confined to the surface layers. Samples collected from greater de pths did not possess the same capacity for acetylene reduction as samples f rom the surface itself, even when incubated at the photon flux density prev ailing in surface waters. This suggests that, with respect to N-2 fixation, Baltic cyanobacteria are adapted to the intensity of illumination that the y are currently experiencing.