• Anabaena flos-aquae;
  • cell death;
  • cyanobacterium;
  • dissolved organic carbon;
  • heterocysts;
  • light limitation;
  • light supersaturation;
  • photosynthesis;
  • specific death rate;
  • specific growth rate

The kinetics of population growth and death were investigated in Anabaena flos-aquae (Lyngb.) Bréb grown at light intensities ranging from limitation to photoinhibition (5 W·m−2 to 160 W·m−2) in a nutrient-replete turbidostat. Steady-state growth rate (μ, or dilution rate, D) increased with light intensity from 0.44·day−1 at a light intensity of 5 W·m−2 to 0.99·day−1 at 20 W·m−2 and started to decrease above about 22 W·m−2, reaching 0.56·day−1 at 160 W·m−2. The Haldane function of enzyme inhibition fit the growth data poorly, largely because of the unusually narrow range of saturation intensity. However, it produced a good fit (P < 0.001) for growth under photoinhibition. Anabaena flos-aquae died at different specific death rates (γ) below and above the saturation intensity. When calculated as the slope of a vx−1 and D−1 plot, where vx and D are cell viability (or live cell fraction) and dilution rate, respectively; γ was 0.047·day−1 in the range of light limitation and 0.103·day−1 under photoinhibition. Live vegetative cells and heterocysts, either in numbers or as a percentage of the total cells, showed a peak at the saturation intensity and decreased at lower and higher intensities. The ratio of live heterocysts to live vegetative cells increased with intensity when light was limiting but decreased when light was supersaturating. In cells growing at the same growth rate, the ratio was significantly lower under light inhibition than under subsaturation and the cell N:C ratio was also lower under inhibition. The steady-state rate of dissolved organic carbon (DOC) production increased with light intensity. However, its production as a percentage of the total C fixation was lowest at the optimum intensity and increased as the irradiance decreased or increased. The rate and percentage was significantly higher under photoinhibition than limitation in cells growing at the same growth rate. About 22% of the total fixed carbon was released as DOC at the highest light intensity. No correlation was found between the number of dead cells and DOC.