A novel mechanism for regulating the excitation of photosystem II in a green alga

Photosynthetic organisms alter their photosynthetic pigment composition in response to changes in growth irradiance^1-3. Photo-adaptation maximizes light-harvesting when photon flux densities are low, and minimizes photo-oxidative damage to the photosynthetic machinery at high light levels. In chlorophytes, the major light-harvesting antenna is light-harvesting complex II (LHC II), a family of proteins binding chlorophyll a (Chl a), Chl b, and carotenoids, and accounting for 40-60% of total cell chlorophyll⁴. LHC II is associated principally with photosystem II, with reversible phosphorylation of LHC II regulating short-term adjustments in energy distribution to photosystem I⁵. Previous studies on green algae and higher plants have emphasized the longer-term adaptive importance of the inverse relationship between growth irradiance and the proportion of total cellular chlorophyll associated with LHC II^6-8. In higher plants the pigment composition of LHC II appears to be highly conserved, with Chl a/Chl b ratios between 1.0 and 1.2 (ref 7). In green algae, the pigment ratio of LHC II is more variable and values between 0.7 and 2.7 have been reported^9-11. We report here that in the unicellular marine chlorophyte, Dunaliella tertiolecta, the ratio is actually variable. Photoadapta-tion to high irradiance involves changes in the average composition and behaviour of LHC II; specifically, the Chl b content per polypeptide is halved and the efficiency of excitation transfer from carotenoid to Chl a declines. The result is a novel mechanism for regulating the effective absorption cross-section of photosystem II.