Slow and Fast Fluorescence Quenching of LHCII in Chlamydomonas Reinhardtii Cells

In this work we have studied the process of non-photochemical quenching (NPQ) in vivo in the model green alga C. reinhardtii. NPQ is the process that protects algae and plants from high light damage by dissipating large part of the absorbed energy as heat and which is activated by the low lumenal pH. Despite a large research effort the NPQ mechanism remains debated. The main obstacle to understand NPQ resides in the gap between in vitro and in vivo studies. On the one hand, the complexity of the thylakoid membrane makes it very difficult to obtain molecular information from in vivo experiments. On the other hand, a good in vitro system for the study of the quenching is not available. This has generated to a series of contrasting models that cannot be validated. We have developed a “minimal NPQ cell” which allows us to study the effect of the individual NPQ players in the membrane of living cells. We show that LHCII, the main antenna complexes of algae and plants, exists in different quenching states in the membrane depending on the growth conditions. This difference is suggested to be due to a difference in crowding. However, we also show that LHCII is not able to switch to the quenched conformation in response to pH. Instead, the presence of a very small amount of the protein LHCSR1 is sufficient to induce a large quenching (50%) in a membrane that contains only LHCII when the pH drops to 5.5. The quenching is very fast and completely reversible. Time-resolved fluorescence measurements at room and low temperature show that the quenching occurs in 360 ps and is not induced by additional clustering of LHCII in the membrane. Based on these results we propose a new model that links NPQ to long term acclimation responses.