Steiner, G.; Saini, D.; Kapoor, A.; Gates, C.

In photosynthetic organisms, the process of photosystem II-cyclic electron flow (PSII-CEF) protects against degradation of the active D1 protein subunit of photosystem II in high light environments. By comparing the photophysiology of the extreme-light (2000 {micro}Ein/m2/s) desert alga Chlorella ohadii to the low-light (20 {micro}Ein/m2/s) aquatic alga Chlorella sp. NIES 642, we can monitor how natural adaptations resulting in differing levels of PSII-CEF affect the photosynthetic electron transport chain. Modeling of chlorophyll fast repetition rate fluorometry shows distinct backward transitions of the Kok cycle in C. ohadii, which are absent in NIES 642. Increase in PSII-CEF also has a positive effect on the occurrence of miss parameters in the water oxidizing complex. Fluorescent QA- reoxidation kinetics detail that the majority of reaction centers in C. ohadii are performing electron transfer to oxidized QB under saturating light conditions. A combination of kinetic observations excludes plastocyanin as a potential external electron carrier in the mechanistic path of PSII-CEF and suggests that photosystem I-cyclic electron flow works in tandem with PSII-CEF. The combination of these two alternative flow mechanisms expedite electron transfer downstream of PSII and optimize ATP production, respectively. Utilizing 77K spectrofluorometry, congruent photosystem stoichiometry is found between both Chlorella species, despite a 100-fold growth light intensity difference. Electrochromic shift measurements show that C. ohadii has diminished changes to both trans-membrane potential and {Delta}pH during operation of photosynthesis compared to NIES 642, and that excess addition of N,N-dicyclohexylcarbodiimide to Chlorella cells has an inhibitory effect on the photosynthetic electron transport chain.