NONPHOTOCHEMICAL FLUORESCENCE QUENCHING AND THE DIADINOXANTHIN CYCLE IN A MARINE DIATOM

The diadinoxanthin cycle (DD-cycle) in chromophyte algae involves the interconversion of two carotenoids, diadinoxanthin (DD) and diatoxanth in (DT). We investigated the kinetics of light-induced DD-cycling in t he marine diatom Phaeodactylum tricornutum and its role in dissipating excess excitation energy in PS II. Within 15 min following an increas e in irradiance, DT increased and was accompanied by a stoichiometric decrease in DD. This reaction was completely blocked by dithiothreitol (DTT). A second, time-dependent, increase in DT was detected similar to 20 min after the light shift without a concomitant decrease in DD. DT accumulation from both processes was correlated with increases in n on-photochemical quenching of chlorophyll fluorescence. Stern-Volmer a nalyses suggests that changes in non-photochemical quenching resulted from changes in thermal dissipation in the PS II antenna and in the re action center. The increase in non-photochemical quenching was correla ted with a small decrease in the effective absorption cross section of PS II. Model calculations suggest however that the changes in cross s ection are not sufficiently large to significantly reduce multiple exc itation of the reaction center within the turnover time of steady-stat e photosynthetic electron transport at light saturation. In DTT poison ed cells, the change in nan-photochemical quenching appears to result from energy dissipation in the reaction center and was associated with decreased photochemical efficiency. D1 protein degradation was slight ly higher in samples poisoned with DTT than in control samples. These results suggest that while DD-cycling may dynamically alter the photos ynthesis-irradiance response curve, it offers limited protection again st photodamage of PS II reaction centers at irradiance levels sufficie nt to saturate steady-state photosynthesis.