Changes in the photosynthetic coefficients α and Pm of Planktothrix rubescens grown on light-dark cycles

• DOI: 10.1007/pl00001359
• Publication Date: 2006-02-17T17:34:11Z
• Authors: I. P. Vaughan; P. A. Davis; A. E. Walsby
• Keywords: Darkness; Light intensity; Diel vertical migration

The relationship between photosynthesis and irradiance was investigated in a strain of the cyanobacterium Planktothrix rubescens isolated from Lake Zurich, using cultures grown on a 12:12-h light-dark cycle. From the photosynthesis-irradiance (P/I) curves, values of the light-affinity coefficient, α, and the maximum rate of photosynthesis, P m, were determined: in different cultures α ranged from 0.02-0.08 μmol mg-1 h-1 (μmol m-2 s-1)-1 and P m from 0.6-2.6 μmol mg-1 h-1 (rates of O2 production related to dry biomass). Comparisons made at different phases of the light-dark cycle in individual cultures showed that in the first hour of the light phase α 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 kept in darkness showed no rise. It is concluded that the observed increases in α and P m after dawn were in response to the exogenous irradiance rather than due to endogenous rhythms. These findings are discussed in relation to diel variations in the photosynthetic coefficients of natural populations of P. rubescens in Lake Zurich.