Regulation of the carotenoid biosynthetic pathway in the green microalga Dunaliella salina and the diatom Phaeodactylum tricornutum

Tese dout. , Faculdade de Ciências do Mar e do Ambiente, 2007, Universidade do AlgarveCarotenoids are produced by all photosynthetic organisms where they play indispensable roles in light-harvesting and photoprotection. This thesis has focused on carotenoid biosynthesis in Dunaliella salina and Phaeodactylum tricornutum, two phylogenetically diverse algae. Both algae are able to maintain high photosynthetic rates under fluctuating light intensities. We examined the effect of several environmental stress conditions on carotenoid biosynthesis in D. salina and found that nutrient level, light intensity and salinity have a differential effect on carotenogenesis. We also found that the steady-state transcript levels of two key-enzymes involved in the early steps of carotenoid biosynthesis are coordinately up-regulated in carotenoid-accumulating D. salina cells, indicating that carotenoid biosynthesis in this alga may be partly regulated at the transcriptional level. Analysis of the P. tricornutum genome, as well as that from another diatom, Thalassiosira pseudonana, revealed that the genes involved in xanthophyll biosynthesis and xanthophyll cycling in diatoms have diversified greatly with respect to green algae and higher plants. We showed that the steady-state mRNA levels of the P. tricornutum carotenoid biosynthesis-related genes are increased upon nutrient stress and blue light, and we were able to establish that light of different spectral quality has a differential effect on the mRNA levels of these genes. By using transgenic P. tricornutum cell lines containing elevated levels of a putative diatom blue light cryptochrome photoreceptor, we demonstrated that this protein is involved in the transcriptional regulation of blue light-responsive genes, ultimately resulting in an enhanced accumulation of xanthophyll pigments and a significantly altered chromatic adaptation to blue light. In conclusion, the work reported in this thesis will facilitate future work on both the regulatory and biotechnological aspects of the carotenoid biosynthetic pathway in unicellular algae