Antarctic photosynthesis: energy transfer and charge separation in the diatom Chaetoceros simplex

The polar oceanic environment poses extreme challenges to photosynthetic organisms, which have evolved atypical strategies to maintain efficient photosynthesis in cold temperatures. Here, the psychrophilic diatom Chaetoceros simplex (C. simplex) is studied in vivo in the dark-adapted state using steady-state and time-resolved fluorescence methods. Our results show that all fucoxanthin chlorophyll a/c protein (FCP) antenna transfer energy to photosystem I (PSI) or photosystem II (PSII), with no detached FCPs. PSI exhibits no fluorescence of 'red' forms of chlorophyll (chl) beyond 700 nm in both 279 K and 77 K conditions. Despite this, it apparently has a long decay time of ~85 ps indicating the presence of a large core-antenna supercomplex. PSII has an average lifetime of ~500 ps in open state (Q_A oxidized) and ~1220 ps in closed state (Q_A reduced). PSII of C. simplex has kinetics that are slightly slower than temperate diatoms, suggesting larger antenna. In addition, fucoxanthin (fx) molecules of FCP that absorb in the 500-550 nm range (fx-red) transfer more energy to PSII than fx that absorb in the blue range (fx-blue, 462 nm max absorption). A subpopulation of red-shifted, aggregated FCPs are detected at 77 K, that are active in energy transfer uphill at 279 K. Overall, our results indicate relatively larger antenna of PSI and PSII and an absence of red chls in PSI of cold-adapted species, compared to temperate species.