Changes in the photosynthetic coefficients alpha and P-m of Planktothrix rubescens grown on light-dark cycles

The relationship between photosynthesis and irradiance was investigated in a strain of the cyanobacterium Planktothrix rubescens isolated from Lake Zu rich. using cultures grown on a 12:12-h light-dark cycle. From the photosyn thesis-irradiance (P/I) curves, values of the light-affinity coefficient, a lpha, and the maximum rate of photosynthesis, P-m, were determined: in diff erent cultures alpha ranged from 0.02-0.08 mu mol mg(-1) h(-1) (mu mol m(-2 ) s(-1))(-1) and P-m from 0.6-2.6 mu mol mg(-1) h(-1) (rates of O-2 product ion related to dry biomass). Comparisons made at different phases of the li ght-dark cycle in individual cultures showed that in the first hour of the light phase alpha rose by an average of 21% and P-m by 7%. In cultures that had been grown in many light-dark cycles, however, the rise occurred only if light was given at the beginning of the notional light phase; cultures k ept in darkness showed no rise. It is concluded that the observed increases in alpha and P-m after dawn were in response to the exogenous irradiance r ather than due to endogenous rhythms. These findings are discussed in relat ion to diel variations in the photosynthetic coefficients of natural popula tions of P. rubescens in Lake Zurich.