Light Perception in Marine Diatoms and Characterization of a Novel Phytochrome from <i>Phaeodactylum tricornutum</i>

Phytochromes are a family of red/far-red light photoreceptors widely distributed throughout the plant kingdom, among bacteria, fungi and even some non-photosynthetic bacteria. Because of the fast attenuation of red and far-red light in the water column, the existence of a red/far red light sensor as the phytochrome in marine organisms has long been a subject of discussion. In this thesis, I have started the characterization of the diatom phytochrome through biochemical and spectral approaches, molecular techniques, physiological studies and bio-informatics. This thesis reports the first evidence that the phytochrome, identified in the diatom genomes (named DPh), acts as photoreceptor for perceiving red/far-red light. The biochemical and spectral analyses of DPh from the diatom Phaeodactylum tricornutum have revealed that the absorption maxima of the Pr and Pfr forms of DPh (685 and 740 nm respectively) are more shifted toward the red end of the spectrum than the plant phytochromes. Studies through bio-informatic tools have shown that DPh has a domain organization similar to the bacterial phytochromes (Bphs) and that DPh, similarly to this phytochrome photoreceptor class, binds biliverdin as chromophore. The role of DPh as photoreceptor has also been studies by extensive analyses of red light-mediated gene expression. Several genes have been studied for their expression profiles in diverse light regimes (acute light response experiments with red and far red light treatments of different fluences), including genes involved in the chlorophyll and carotenoid biosynthesis, light harvesting and redox regulation. These studies have shown that a number of the studied genes are induced in various red/far-red light treatments, although it was not possible to reach a final and comprehensive conclusion about the role of DPh. In particular, it was not always clear if DPh acts as a photoreceptor and mediator in red/far-red light responses of these genes, although the results obtained offered important first hints into the complex gene regulation by red/far-red light in the marine environment. Preliminary biochemical analyses have also offered a glimpse in the complex regulation of DPh protein synthesis in cells grown in diverse light conditions. Curiously, DPh down-regulation by RNA interference in two diatom species induced the formation of abnormal cell chains and aggregates, possibly through altered cell division and neighbor perception processes. These data raise novel and interesting hypotheses about the role of a red light photoreceptor in controlling growth and life strategies in the oceans