Caratterizzazione della fotosintesi e dell'allocazione del carbonio in specie di microalghe marine acclimatate a diversi regimi di illuminazione

Marine microalgae are a heterogeneous group of photosynthetic autotrophs. In addition to providing almost half of the oxygen on Earth, these organisms are extremely important carbon fixators. Their capability to live in a wide range of environments, and thus, tolerate a huge range of physical and chemical conditions, makes them great organisms to study. Here, we explore what occurs when marine microalgae from the green evolutionary lineage, such as Tetraselmis suecica and Dunaliella salina, and from the red lineage, such as Phaeodactylum tricornutum, are exposed to different light regimes. Light influences the photosynthetic reactions and the metabolic pathways within cells. To overcome any difficulties, marine microalgae have to counter-act the potential stressful impacts, and they do so by photoacclimating, or photoadapting, depending on whether the impact lasts from hours to days, or days to months, respectively. Microalgal species Tetraselmis suecica, Dunaliella salina and Phaeodactylum tricornutum were grown in batch cultures. “Control” conditions (CTR) corresponded to continuous light (24h) and light intensity of 100 μmol photons·m2·s-1. “Low light” conditions (LL) corresponded to continuous light (24h) and 10 μmol photons·m2·s-1, and in the third light condition, algae were exposed to 12 hours light and 12 hours dark with light intensity of 100 μmol photons·m2·s-1, thus, the “photoperiod” conditions (PP). Through the observed process of photoacclimation, the maximum growth rate is the greatest in all species under CTR conditions, and the most pronounced in the diatom P. tricornutum, that is also the species with the smallest dimensions. While the amount of carbohydrates under LL conditions diminishes in T. suecica and D. salina, it was not the case for P. tricornutum. Proteins substantially increase in all species acclimated to LL compared to the CTR. Giving the concomitant 2-fold increase of chlorophyll pigments and significantly higher photosynthetic efficiency for the photon capture in LL, we can conclude that cells acclimate by amplifying their photosynthetic apparatus (more abundant photosystems and antenna proteins). However, D. salina accumulates the most proteins under PP conditions. The lipids content increases in T. suecica and P. tricornutum, but in D. salina, it stays unvaried among all conditions. By examining the kinetics of photosynthesis, D. salina and P. tricornutum CTR have a higher non-photochemical quenching (NPQ) upon increasing illumination, while in T. suecica, NPQ is low. Additionally, the different parameters analysed for the characterization of the photosynthetic response show a peculiar kinetics in D. salina, hence leading to think that there is a complex control of the electronic transport. Physiological differences observed in the applied light conditions, both at a specific and interspecific level, have allowed to confirm the extreme diversification of the physiological responses of microalgae to environmental conditions, opening new possibilities of research, useful for a greater understanding of photosynthesis and metabolism in the different microalgal species.