Distinctions in photochemistry between Chlorella ohadii and Chlorella sorokiniana are dependent on growth light intensity.

Chlorella ohadii, currently the fastest-growing eukaryotic phototroph on record, is very closely related to the slower-growing Chlorella sorokiniana. C. ohadii also tolerates extreme light conditions in its normal growth and has been shown to perform better photoprotection comparatively. C. ohadii has previously been shown to tolerate this extreme light in part by performing cyclic electron flow around photosystem II. We have induced C. sorokiniana to grow rapidly under elevated CO2 and extreme light, reaching a doubling time of just under 1.7 hours at peak, and investigated how its photophysiological adaptations compared to the native behavior of C. ohadii. When grown in low light conditions, both strains have similar efficiency of their water oxidizing complex operation, but under extreme light conditions, we do not observe efficient oscillation of the water oxidizing complex in C. sorokiniana, indicating considerable recombination until exogenous electron acceptor is added. Similarly, there is a relatively higher rate of oxidation of the QB site, which can be explained by this recombination. However, the rate of photosystem II turnover is not distinguishable between the two species under these conditions, and total antenna expression is similar. Cytochrome b6f and plastocyanin stay significantly more reduced in extreme light grown C. sorokiniana, but PSI does not, implying elevated PSI-CEF as a more significant component of photoprotection in this species. A weaker initial electrochromic shift is observed in extreme light grown C. sorokiniana, but comparatively limited gradient breakdown precludes this being due to elevated ATP synthase activity. Overall, these discrepancies suggest the structural arrangement of the photosynthetic apparatus, specifically regarding photosystem II, is responsible for the improved efficiency and photoprotection of C. ohadii compared to its near relative.