A desert Chlorella sp. that thrives at extreme high-light intensities using a unique photoinhibition protection mechanism.

While light is the driving force of photosynthesis, excessive light can be harmful. Photoinhibition is one of the key processes that limit photosynthetic productivity. A well-defined mechanism that protects from photoinhibition has been described. Chlorella ohadii is a green micro-alga, isolated from biological desert soil crusts, that thrives under extreme high light (HL). Here, we show that this alga evolved unique protection mechanisms distinct from those of the green alga C. reinhardtii or plants. When grown under extreme HL, a drastic reduction in the size of light harvesting antennae occurs, resulting in the presence of core PSII, devoid of outer and inner antennas. This is accompanied by a massive accumulation of protective carotenoids and proteins that scavenge harmful radicals. At the same time, several elements central to photoinhibition protection in C. reinhardtii, such as psbS, LHCSR, PSII protein phosphorylation and state-transitions are entirely absent or were barely detected. In addition, a carotenoid biosynthesis related protein (CBR) accumulates in the thylakoid membranes of HL cells and may function in sensing HL and protecting the cell from PI. Taken together, a unique photoinhibition protection mechanism evolved in C. ohadii, enabling the species to thrive under extreme-light intensities where other photosynthetic organisms fail to survive.