Program

General Session 2: Marine & estuarine biogeochemistry

Poster

The mechanism and physiological role of excitation transfer among PSII units in four marine phytoplankters
GS2-51
Kui Xu* , Biology Department, Mount Allison University
Jessica Grant-Burt, Biology Department, Mount Allison University
Douglas A.Campbell, Biology Department, Mount Allison University
Presenter Email: x1k2345@126.com

Fast repetition and relaxation (FRR) chlorophyll fluorometry is widely used in photosynthesis and physiology research on ocean phytoplankton. The functional absorption cross section of Photosynthesis II (σPSII) is well studied, but the parameter for excitation transfer among PSII units (ρ) is less understood. We chose four marine phytoplankters (Prochlorococcus MIT 9313, Synechococcus sp. WH 8102, Ostreococcus tauri RCC 745 and Thalassiosira pseudonana CCMP 1335) grown under low light (30 μmol photons m-2 s-1) or high light (260 μmol photons m-2 s-1), to investigate the patterns and roles of ρ. ρ was consistently higher from cultures grown under low light than from cultures grown under high light, except for Ostreococcus. Upon illumination ρ decreased as the PSII reaction centers closed; ρ reached minimal levels once ~80% of PSII closed and PSIIETR saturated. We chose the saturating light which was sufficient to drive ρ to minimal level in each species and then investigated the recovery of ρ when this saturation light was turned off. We found that Prochlorococcus had rapid recovery of ρ, simultaneous with PSII re-opening after the end of illumination. Thalassiosira pseudonana and Ostreococcus showed slower recovery of ρ, demonstrating that exposure to saturating light induces a sustained down-regulation of connectivity that persists even when PSII centers reopen. In Synechococcus, the state II to state I transition upon growth light illumination increased ρ, in parallel with an increase in σPSII, which suggests a shorter distance among PSII units increasing the probability of excitation transfer among PSII units. These results show that dynamic changes in ρ are involved in phytoplankton adaptation to light and that different mechanisms mediate changes in ρ across species. We are now using temperature and DTT treatments to further explore these mechanisms.