Abstract
Astaxanthin is a ketocarotenoid produced by photosynthetic microalgae. It is a pigment of high industrial interest in acquaculture, cosmetics, and nutraceutics due to its strong antioxidant power. Haematococcus pluvialis, a fresh-water microalga, accumulates high levels of astaxanthin upon oxidative stress, reaching values up to 5% per dry weight. H. pluvialis accumulates astaxanthin in oil droplets in the cytoplasm, while the chloroplast volume is reduced. In this work, we investigate the biochemical and spectroscopic properties of the H. pluvialis pigment binding complexes responsible for light harvesting and energy conversion. Our findings demonstrate that the main features of chlorophyll and carotenoid binding complexes previously reported for higher plants or Chlamydomonas reinhardtii are preserved under control conditions. Transition to astaxanthin rich cysts however leads to destabilization of the Photosystems. Surprisingly, astaxanthin was found to be bound to both Photosystem I and II, partially substituting β-carotene, and thus demonstrating possible astaxanthin biosynthesis in the plastids or transport from the cytoplasm to the chloroplast. Astaxanthin binding to Photosystems does not however improve their photoprotection, but rather reduces the efficiency of excitation energy transfer to the reaction centers. We thus propose that astaxanthin binding partially destabilizes Photosystem I and II.
Highlights
Haematococcus pluvialis is a photosynthetic fresh-water microalga which accumulates a high level of the ketocarotenoid astaxanthin[1,2,3,4]
In the case of cells grown in control conditions (G), monomeric and trimeric Light Harvesting Complexes (LHC) proteins isolated from H. pluvialis present features consistent with previous report for LHC proteins purified from A. thaliana or C. reinhardtii
While the different enzymes involved in β-carotene accumulation are localized in the plastids, the key enzyme β-carotene oxygenase (CRTO), which produces astaxanthin from β-carotene or zeaxanthin, was found both in the plastid and in lipid vesicles in the cytosol, despite its activity only having been previously reported in the cytosol compartment[20]
Summary
Haematococcus pluvialis is a photosynthetic fresh-water microalga which accumulates a high level of the ketocarotenoid astaxanthin (up to 5% DW)[1,2,3,4]. PSI and PSII are pigment binding proteins composed of a core complex and antenna proteins called Light Harvesting Complexes (LHC)[26,27,28,29]. Very little information is available regarding the photosynthetic complexes of the green alga H. pluvialis and how they are modulated during cyst formation and astaxanthin accumulation. It is still under debate if astaxanthin accumulation has some photoprotective function at the level of the chloroplast[12,22,34,35]. The aim of this work is to characterize the photosynthetic complexes in H. pluvialis and the possible role of astaxanthin in the photosynthetic apparatus during acclimation to high light and transition to the red stage
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