Rapid colour changes in Euglena sanguinea (Euglenophyceae) caused by internal lipid globule migration

The accumulation of red pigments under chronic stress is a response observed in most of oxygenic photoautotrophs. It is thought that the red pigments in the cell shield chlorophyll located underneath from the light. Among these red pigments, the of carotenoids is one of the most frequent cases. However, the synthesis degradation of carotenoids is a slow process and this response is usually only when the stress is maintained over a period of time. In the Euglenophyte sanguinea, this is due to the accumulation of a large amount of free and astaxanthin (representing 80% of the carotenoid pool). While reddening is a and sometimes irreversible process in other phototrophs, reducing the efficiency of harvesting by chlorophyll, in E. sanguinea it is highly dynamic, capable of shifting red to green (and vice-versa) in 10-20 min. This change is not due to de novo , but to the relocation of cytoplasmic lipid globules where astaxanthin . Thus, red globules migrate from the centre of the cell to peripheral when photoprotection is demanded. This protective system seems to be so that other classical mechanisms are not operative in this species. For example, the presence and operation of the diadino-diatoxanthin cycle, nonphotochemical (NPQ) is almost undetectable. Since E. sanguinea forms floating colonies, reddening can be observed at much greater scale than at a level, the mechanism described here being one of the fastest and most dramatic changes attributable to photosynthetic organisms at cell and landscape level. In , these data indicate an extremely dynamic and efficient photoprotective mechanism on organelle migration more than on carotenoid biosynthesis that prevents excess absorption by chlorophylls reducing the need for other protective processes related energy dissipation.