Carotenogenesis in the green alga Haematococcus pluvialis: Cellular physiology and stress response

The unicellular green alga Haematococcus pluvialis Flotow has recently aroused considerable interest due to its capacity to amass large amounts of the ketocarotenoid astaxanthin (3,3′-dihydroxy-β,β-carotene-4,4′-dione), widely used commercially to color flesh of salmon. Astaxanthin accumulation in Haematococcus is induced by a variety of environmental stresses which limit cell growth in the presence of light. This is accompanied by a remarkable morphological and biochemical 'transformation' from green motile cells into inert red cysts. In recent years we have studied this transformation process from several aspects: defining conditions governing pigment accumulation, working out the biosynthetic pathway of astaxanthin accumulation and questioning the possible function of this secondary ketocarotenoid in protecting Haematococcus cells against oxidative damage. Our results suggest that astaxanthin synthesis proceeds via cantaxanthin and that this exceptional stress response is mediated by reactive oxygen species (ROS) through a mechanism which is not yet understood. The results do not support in vivo chemical quenching of ROS by the pigment, although in vitro it was shown to quench radicals very efficiently. The finding that most of the pigment produced is esterified and deposited in lipid globules outside the chloroplast further supports this assumption. We have suggested that astaxanthin is the by-product of a defense mechanism rather than the defending substance itself, although at this stage one cannot rule out other protective mechanisms. Further work is required for complete understanding of this transformation process. It is suggested that Haematococcus may serve as a simple model system to study response to oxidative stress and mechanisms evolved to cope with this harmful situation.