Photosynthetic acclimation and photoprotective mechanism of Haematococcus pluvialis (Chlorophyceae) during the accumulation of secondary carotenoids at elevated irradiation

The photosynthetic acclimation of Haematococcus pluvialis Flotow to elevated irradiance and its photoprotective mechanisms were investigated. High light caused a remarkable increment in carotenoid content per cell. Cellular and volumetric chlorophyll contents were significantly increased after four days of high light treatment. Net photosynthesis of high light treated cells was decreased but their dark respiration was increased during the accumulation of secondary carotenoids (SC). The inactivation of reaction centers was observed in high light treated cells, and their normalized complementary area and turnover number were higher than those under low light. The PS II activity of red cells from high light was decreased by 17% compared with green cells front low light but their PS I activity was significantly increased. The K-step Could not be observed in the fluorescence transients of red cells, indicating that the oxygen-evolving, complex was not affected during SC accumulation. Haemotococcus phuvialis could protect itself against strong irradiance through the D1 protein repair cycle and the xanthophyll cycle. The D1 protein repair cycle was the most important protective mechanism in H. phuvialis and its operation Could alleviate photoinhibition by 49% in green cells and by 53 similar to 55% in red cells. The xanthophyll cycle could contribute to the protection of green cells subjected to strong irradiance but its role was negligible in red cells. F root F-o values were decreased in green cells and red cells by 45% and 32 similar to 34% respectively after 2.5 hours of photoinhibitory treatment. However. this may not necessarily indicate that the accumulated SC in red cells might play a photoprotective role.