| Kinetics and nature of cell death were investigated in the cyanobacterium Anabaena flos-aquae (Lyngb.) Breb grown in a continuous culture under phosphorus limitation, light sub-/super-saturation, and exposure to a polychlorinated biphenyl. The specific death rate {dollar}(gamma),{dollar} was determined as the slope of a {dollar}rm vsb{lcub}x{rcub}sp{lcub}-1{rcub}{dollar} and D{dollar}sp{lcub}-1{rcub}{dollar} plot where v{dollar}rmsb{lcub}x{rcub}{dollar} and D are live cell fraction (or cell viability) and dilution rate, respectively.; Under phosphorus limitation, specific death rate was 0.0082 day{dollar}sp{lcub}-1{rcub}{dollar} with N{dollar}sb2{dollar} as the N source and 0.0042 day{dollar}sp{lcub}-1{rcub}{dollar} with nitrate as the N source. Specific death rate in the NH{dollar}sb4sp+{dollar} culture was not significantly different from zero. In both N{dollar}sb2{dollar} and nitrate cultures, the increase in the overall live cell fraction with growth rate was due entirely to an increase in live heterocysts, indicating that cell viability was related to heterocyst differentiation.; The Haldane function of non-competitive enzyme inhibition provided a good fit to the growth-irradiance relationship at light intensities above the saturation levels. When the maximum growth rate and the inhibition constant determined by this fit were forced in the Haldane equation, the resulting half-saturation constant for growth was in the range of the experimentally-observed value. The specific death rate was 0.047 day{dollar}sp{lcub}-1{rcub}{dollar} with light limitation and 0.103 day{dollar}sp{lcub}-1{rcub}{dollar} with light supersaturation. Live vegetative cells and heterocysts, either in absolute value or as a percent of total cells, showed a peak at the saturation intensity and decreased at lower and higher intensities.; When the effect of 2,5-{dollar}2spprime,5spprime{dollar}-tetrachlorinated biphenyl was investigated in a semicontinuous culture, cell death was observed only at concentrations above environmentally-relevant levels.; When a steady-state chemostat culture was entrained under a 12 h:12 h light/dark cycle, Anabaena flos-aquae exhibited a circadian rhythmicity in cell death as well as cell division. The phasing of the cell death was opposite to the cell division cycle. The rhythm persisted under continuous light and exhibited temperature compensation when grown at 15 and 20{dollar}spcirc{dollar}C. Therefore, both cell division and death are under the control of a circadian clock, and the death appears to be programmed death. |
