The Loss of the Circadian Rhythm in Photosynthesis in an Old Strain of Gonyaulax polyedra

Cultures of Gonyaulax polyedra Stein maintained in the laboratory for 15 to 20 years, including an axenic strain isolated in 1960, have gradually lost the ability to survive in darkness. G. polyedra (70A), isolated in 1970 and maintained in a 12:12 light:dark cycle, now tolerates continuous darkness for a much shorter time than a strain isolated in 1981. I have compared the properties of strain 70A with those of this newer strain (81N), to investigate changes in Gonyaulax with length of time in culture, which may account for poor survival in darkness. When grown in continuous light (13, 12, or 4.5 watts per square meter), strains 70A and 81N have similar growth rates, yields, cell diameters, protein contents, C/N ratios, respiration rates, pigment complements, and photosynthetic rates. When entrained by a light:dark cycle (12L:12D), 70A showed no photosynthesis rhythm, although such a rhythm was formerly present. However, the circadian rhythms in bioluminescence and cell division were normal in both strains. Thus, the circadian clock is apparently still intact in 70A as in 81N. The rate of photosynthesis in strain 70A was constant at a low level, the consequent smaller accumulation of photosynthetic products probably accounting for the limited survival in darkness. The defect in strain 70A may be the loss of a component either directly affecting P(max) or necessary for transduction from the circadian clock to photosynthesis.