Chromatic acclimation in phototrophic marine cryptophytes

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Chromatic acclimation (CA) is a fairly well understood process in certain cyanobacteria that optimizes photosynthetic light-harvesting under changing light conditions. The composition of light-harvesting phycobilisomes (PBS) is remodeled as the ambient light color changes. Of the previously defined CA types in cyanobacteria, those that change their ratio of green light-absorbing phycoerythrin (PE) to red light-absorbing phycocyanin (PC) under green and red illumination, respectively, are well known. CA4 in marine cyanobacteria, in which the light-absorbing phycobilin chromophores are exchanged within the PBP, is also quite well studied. More recently, a similar phenomenon has also been observed in certain cryptophyte algae. Cryptophytes are biflagellate eukaryotic microalgae found in a variety of aquatic habitats. Many species perform oxygenic photosynthesis using unique phycobiliproteins that, unlike those of cyanobacteria, are not organized in PBS but are soluble in the thylakoid lumen. The marine species Hemiselmis cryptochromatica has a major absorption peak at 569 nm due to the presence of the PC569. When growing under red light, this main absorption peak shifts towards 625 nm, while for cells growing under green light, the peak at 569 nm dominates. In this proposal, we aim to elucidate the underlying mechanism of this CA phenomenon in H. cryptochromatica. PC569 will be isolated from cells grown under different light qualities and analyzed for phycobilin and protein composition using a combination of UV-vis spectrometry, HPLC analysis, and mass spectrometry. The identified phycobilins will be compared to those synthesized by the five ferredoxin-dependent bilin reductases encoded in the H. cryptochromatica genome. These enzymes convert the linear tetrapyrrole biliverdin into various light-harvesting phycobilin chromophores. Since PC569 likely possesses one of the cryptophyte-specific acryoyl bilins, bilin 584, we will take initial steps to elucidate the biosynthetic pathway to these bilins. For this purpose, acryoyl-containing intermediates will be identified by specific inhibition of biosynthetic proteins. Subsequently, an enzyme assay will be established to create the prerequisites for enzyme isolation using traditional methods. Overall, this project will contribute to a better understanding of the adaptation of marine cryptophytes to changing light qualities.