Photosynthesis and growth response of the oceanic picoplankter Pycnococcus provasolii Guillard (clone Ω48-23) (Chlorophyta) to variations in irradiance, photoperiod and temperature

Abstract

The growth, photosynthesis and respiration rates of the green picoplanktonic algae Pycnococcus provasolii Guillard were measured as a function of irradiance, temperature and photoperiod. The algae showed positive photoadaptation to low irradiance and from an analysis of the photosynthesis versus irradiance curves, it is suggested that this is achieved mainly by increasing the size of the photosynthetic units. In accordance with this conclusion, chlorophyll b to a ratios increased with decreasing photon flux density. The algae further compensated for low light energy supply by reducing the rates of respiration. Values of the initial slope of the growth versus irradiance curve were higher than average (0.0016–0.0022 h −1 (μ Em −2 · s −1) −1 at 20°C). It is thus concluded that P. provasolii Guillard is a well suited organism to grow at sites of low irradiance and this may explain its success in colonizing the pycnocline area of stratified oceanic waters. This capacity, however, was not accompanied by a reduced ability to photosynthesize at high irradiances. A 24 h light regime did not seem harmful to P. provasolii Guillard, however, light energy was utilized less efficiently under 24 h than under 12:12 h photoperiod. Values of Q 10 for P max and μ max were in the region of 2.