Cryo-EM photosystem I structure reveals adaptation mechanisms to extreme high light in Chlorella ohadii.

Photosynthesis in deserts is challenging since it requires fast adaptation to rapid night-to-day changes, that is, from dawn's low light (LL) to extreme high light (HL) intensities during the daytime. To understand these adaptation mechanisms, we purified photosystem I (PSI) from Chlorella ohadii, a green alga that was isolated from a desert soil crust, and identified the essential functional and structural changes that enable the photosystem to perform photosynthesis under extreme high light conditions. The cryo-electron microscopy structures of PSI from cells grown under low light (PSI) and high light (PSI), obtained at 2.70 and 2.71 Å, respectively, show that part of light-harvesting antenna complex I (LHCI) and the core complex subunit (PsaO) are eliminated from PSI to minimize the photodamage. An additional change is in the pigment composition and their number in LHCI; about 50% of chlorophyll b is replaced by chlorophyll a. This leads to higher electron transfer rates in PSI and might enable C. ohadii PSI to act as a natural photosynthesiser in photobiocatalytic systems. PSI or PSI were attached to an electrode and their induced photocurrent was determined. To obtain photocurrents comparable with PSI, 25 times the amount of PSI was required, demonstrating the high efficiency of PSI. Hence, we suggest that C. ohadii PSI is an ideal candidate for the design of desert artificial photobiocatalytic systems.