Physiological studies on a coccoid marine blue-green alga (Cyanobacterium)

A coccoid marine cyanobacterium assignable to the genus Synechococcus has been isolated in axenic culture. This organism appears not to be a nitrogen fixer as it failed to reduce acetylene under either aerobic or anaerobic conditions. The specific growth rate was determined at six irradiance levels (5-, 15-, 30-, 50-, 100- and 150 μmol m^-2 s^-1) under continuous illumination and controlled temperature (20°C). The growth vs. incident irradiance curve and estimated light energy absorbed showed that growth saturates at a relatively low photon flux density and photosynthesis has a low compensation point. It attained its maximum division rate (1·4 divisions d^-1 = generation time, 18 h) at 30 μmol m^-2 s^-1. Such a low light-requiring planktonic cyanobacterium may be of importance in terms of both biomass and productivity towards the bottom of the photic zone.