The cyanobacterium was grown in continuous cultures with light periods of various duration at a constant irradiance. Growth at shorter light periods led to light-limited cultures. was the only photosynthetic parameter that reflected increasing pigment contents at shorter light periods; α and were maximal with light periods of 8 h and less. The dynamics of the carbohydrate pool with light/dark cycles are described in a concept of three maxima: a maximum accumulation rate, a maximum content and a maximum consumption rate. With shorter light periods the growth yield on carbon increased as did yield values for dark growth on carbohydrate. Rates of protein synthesis were equal in the light and dark for light periods > 8 h; with shorter light periods the rates of protein synthesis in the dark showed a severe drop. From the response of to changes in light/dark cycle we distinguished four ranges in which regulation of growth differed. The central role of the photosynthetic apparatus in the different responses to changes in either irradiance or light period is stressed.


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