Monophasic kinetics of electron donation from stromal reductants in unicellular halophytic alga Tetraselmis viridis: Relation of the function to characteristic structure of chloroplast membrane system

Redox conversions of P700, the primary donor of photosystem I (PSI), were investigated in cells of a halophytic alga Tetraselmis viridis Rouch. under irradiation with white light pulses that excite both photosystems of the chloroplast and with far-red light initiating photochemical reactions in PSI only. The P700⁺ dark reduction after irradiation with 50-ms pulse of white light comprised three kinetic components. The half-decay times and relative contributions of the fast, middle, and slow components were 38 ms (49%), 295 ms (26%), and 1690 ms (23%), respectively. The treatment with diuron, known to block electron transport between the photosystems, eliminated the middle exponential term having the half-decay time of 295 ms. After irradiation with far-red light, the kinetics of P700⁺ dark reduction comprised only two components with half-deacy times of 980 ms (72%) and 78 ms (31%). The component with a decay halftime of about 100 ms was fully inhibited after treating the cells with antimycin A, a specific inhibitor of ferredoxin-dependent cyclic electron flow around PSI. In addition, this kinetic component was strongly suppressed by methyl viologen known to inhibit this alternative pathway of electron transport. Both aforementioned reagents had no effect on the slow component of P700⁺ reduction; this component remained monophasic. Unlike higher plant chloroplasts, the chloroplasts of Tetraselmis viridis contained no stacked grana. Based on inhibitor analysis and electron microscopy data, it was concluded that the slow component of P700⁺ reduction in the cells of halophytic microalga reflects the electron donation to PSI from reductants localized in the chloroplast stroma. The monophasic kinetics of this process in the halophytic microalga, compared to the biphasic kinetic pattern in higher plants, is related to the lack of stacked grana in Tetraselmis viridis cells.