Adaptation of photosynthesis to irradiance and daylength in the red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Rhodophyceae, Cryptonemiales), a species from the deep sublittoral region

The crustose tetrasporophyte of the red alga Acrosymphyton purpuriferum (J. Ag.) Sjöst. (Rhodophyceae, Cryptonemiales), a species from the deeper sublittoral region, was cultured at a series of daylength and irradiance conditions and the pigment content and photosynthetic rates were compared. Chlorophyll-a and phycobilin contents varied little over the wide range of conditions tested (daylengths: 4, 8, 12 and 16 h; photon flux densities: 2.5, 5, 20, 40 and 80 μmol m−2 s−1). Nevertheless, the irradiance and daylength history of the plants distinctly influenced the shape of the photosynthesis–irradiance curve (PI-curve). The irradiance history mainly influenced the maximum photo-synthetic capacity (Pmax, which was highest in plants grown at the higher irradiance. This suggests an increased capacity of the enzymatic processes of photosynthesis at high irradiance. The daylength history of the plants influenced the initial slope (α) of the curve as well as Pmax. Pmax was highest and α steepest for plants that had been grown in the shorter days. This suggests an influence of daylength on photosynthetic efficiency through changed photosynthetic energy transduction. From the PI-curves the actual photosynthetic rate and the daily photosynthetic production at the various culture conditions were calculated. The influence of daylength and irradiance on these photosynthetic parameters was compared with their influence on tetrasporogenesis and thus the role of photosynthesis in the irradiance-dependency of the short day response in tetrasporogenesis was evaluated