Adaptive strategies of Zostera japonica photosynthetic electron transport in response to thermal stress

Seawater warming is emerging as a result of increasing global temperature. In this study, Zostera japonica (Ascherson and Graebner) collected from the intertidal zones of Changdao (37 degrees N, 120 degrees E) were used to investigate the responses of photosynthetic electron transport to thermal exposure in April 2016. As seawater temperature rose from 20 to 32 degrees C, increases of the relative variable fluorescence at the K-step to the amplitude of F-J-F-o (W-k) and the maximum photochemical efficiency (F-v/F-m) indicated inhibition of the oxygen-evolving complex (OEC) and photosystem II (PSII) reaction centers, respectively. When exposed to 32 degrees C for 4 h, both OEC and PSII reaction centers suffered irreversible damage, confirming that 32 degrees C is the upper critical seawater temperature for Z. japonica photosynthetic electron transport. As a thermal inhibition indicator, PIabs exhibited a time-dependent linear decrease, with irreversible damage of the PSII reaction center occurring once the value of PIabs dropped below 10.60. Based on results of the photosynthetic performance, thermal response strategies were summarized as: (1) an enhancement in the efficiency of the active PSII reaction centers; (2) an increase in the activity of the PSII electron acceptor side; (3) an enhancement in the activities of both PSI and the cyclic electron transport around PSI; (4) the alternation between PSII and PSI. Such adaptive strategies may balance the redox state of electron transport and regulate the distribution of excitation energy between the two photosystems, thereby protecting Z. japonica from ocean warming.