Diatoms, which are primary producers in the oceans, can rapidly switch on/off efficient photoprotection to respond to fast light-intensity changes in moving waters. The corresponding thermal dissipation of excess-absorbed-light energy can be observed as non-photochemical quenching (NPQ) of chlorophyll a fluorescence. Fluorescence-induction measurements on Cyclotella meneghiniana diatoms show two NPQ processes: qE1 relaxes rapidly in the dark while qE2 remains present upon switching to darkness and is related to the presence of the xanthophyll-cycle pigment diatoxanthin (Dtx). We performed picosecond fluorescence measurements on cells locked in different (quenching) states, revealing the following sequence of events during full development of NPQ. At first, trimers of light-harvesting complexes (fucoxanthin-chlorophyll a/c proteins), or FCPa, become quenched, while being part of photosystem II (PSII), due to the induced pH gradient across the thylakoid membrane. This is followed by (partial) detachment of FCPa from PSII after which quenching persists. The pH gradient also causes the formation of Dtx which leads to further quenching of isolated PSII cores and some aggregated FCPa. In subsequent darkness, the pH gradient disappears but Dtx remains present and quenching partly pertains. Only in the presence of some light the system completely recovers to the unquenched state.
Bibliographical noteFunding Information:
This work was supported by the HARVEST Marie Curie Research Training Network [ PITN-GA-2009-238017 to VUC, CB and HvA]. This project was also carried out within the research program of BioSolar Cells , co-financed by the Dutch Ministry of Economic Affairs (to VUC and HvA). We thank R.B.M. Koehorst (Wageningen UR), A. van Hoek (Wageningen UR), and Dr. A. Bader (Wageningen UR) for technical help with the measurements, C. Wolfs (Wageningen UR) for initial help with growing cells, and Dr. S. Laptenok (University of East Anglia) and J. Snellenburg (VU Amsterdam University) for help with the usage of the Glotaran software analysis toolkit. Dr. R. Croce and D. de Kort are acknowledged for critical reading of the manuscript and for many helpful suggestions.
- Light-harvesting complex
- Non-photochemical quenching
- Xanthophyll cycle