Photosynthetic Acclimation And Photoprotective Mechanism Of Haematococcus Pluvialis (Chlorophyceae) During The Accumulation Of Secondary Carotenoids At Elevated Irradiation

The unicellular green alga Haematococcus pluvialis Flotow has been widely known by its ability to accumulate notable amounts of secondary carotenoids (SC), mainly astaxanthin esters. Astaxanthin is used commercially as a pigmentation source in fish aquaculture and in the poultry industry, but it is also recognized as having potential clinical applications due to its higher antioxidant activity than P-carotene and a-tocopherol. Astaxanthin accumulation in H. pluvialis occurs in response to various environmental stressors such as high light, nitrogen and phosphate starvation, salt stress, high temperature, and reactive oxygen species.Most of the work published so far on H. pluvialis has been focused on the growth and carotenogenesis of this alga. However, far less attention has been paid to the physiological changes of H. pluvialis during SC accumulation. The xanthophyll cycle and the damage-repair cycle of PS II reaction centers are two major non-photochemical quenching mechanisms in higher plants and green algae. Under strong irradiance, both of them could contribute to protect the photosynthetic apparatus. However, far less attention has been paid to these mechanisms in H. pluvialis. Though some photoprotective mechanisms have been suggested, the function of astaxanthin in red cells of H.pluvialis is still questioned. Furthermore, there is little information about the relative importance of non-photochemical quenching mechanisms mentioned above. The aim of the present study was therefore to investigate the photoprotective mechanism of H. pluvialis and the photosynthetic acclimation tc elevated irradiance.High light caused a remarkable increment in carotenoids 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. The inactivation of reaction centers was observed in high light treated cells, and their normalized complementary area and turn-over numberwere higher than those under low light. The PS II activity of red cells from high light was decreased by 17% compared with green cells from 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. H. pluvialis 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. pluvialis and its operation could alleviate the photoinhibition by 49% in green cells and 53-55% in red cells. The xanthophyll cycle could contribute to protect green cells subjected to strong irradiance but its roles could be neglected in red cells. The Fv/Fo values were decreased respectively by 45% in green cells and 32-34% in red cells after 2.5 hours of photoinhibitory treatment. However, this could not necessarily indicate that the accumulated SC in red cells might play a photoprotective role.