(ii) Light-harvesting versus light-dissipation

Light is a necessary substrate for photosynthesis, but its absorption by pigment molecules such as chlorophyll can cause severe oxidative damage and result in cell death. The excess absorption of light energy by photosynthetic pigments has led to the evolution of protective mechanisms that operate on the time scale of seconds to minutes and involve feedback regulated de-excitation of chlorophyll molecules in photosystem II (qE). Despite the significant contribution of eukaryotic algae to global primary production, little is known about their qE mechanism, in stark contrast to flowering plants. Data that have been obtained in cooperation the group of Prof. K. Niyogi show that Chlamydomonas reinhardtii mutants diminished or -deficient in qE have an impact in the expression of lhcsr genes, either due to RNAi-technology or due to knock-down of two of the three genes encoding LHCSR (formerly called LI818) (Peers et al., 2009). This protein is an ancient member of the light-harvesting complex superfamily, and orthologs are found throughout photosynthetic eukaryote taxa, except red algae and vascular plants. The qE capacity of Chlamydomonas is dependent on environmental conditions and is inducible by growth in high light. LHCSR-RNAi or knock-out mutants have a reduced fitness in a shifting light environment compared to wild-type cells, demonstrating that LHCSR is required for survival in a dynamic light environment. Therefore the data indicate that plants and algae employ different proteins to dissipate harmful excess energy and protect the photosynthetic apparatus from damage. In the future more insight into the NPQ mechanism and its regulation in algae has to be gathered to understand the evolution of light management in algae and vascular plants.

Peers, G., Truong, T.B., Ostendorf, E., Busch., A., Elrad, D., Grossman, A.R., Hippler, M*. and Niyogi, K.K.*
* co-corresponding authors (2009)
An ancient light-harvesting protein is critical for the regulation of algal photosynthesis.
Nature, 462(7272), 518-21