A red-shifted LHCII in Chlamydomonas priscui allows for efficient light harvesting under an Antarctic lake

The psychrophilic green alga Chlamydomonas priscui was isolated from an Antarctic lake, where it has adapted to low light, low temperature, and high salinity environment. How photosynthetic light harvesting adjusts to such conditions remains an important question. Here, we present biochemical, biophysical, and phylogenetic analyses of the major light-harvesting complex II (LHCII) of C. priscui (CpLHCII). Compared to the LHCII of the mesophilic model alga C. reinhardtii (CrLHCII), CpLHCII has a chlorophyll (Chl) a:b ratio of ~1 (CrLHCII Chl a:b = ~1.24), with an intermediate red Chl a species replaced by Chl b and altered spectral tuning of Chl a. These changes cause an overall red shift in absorption, apparently driven by specific mutations in the primary sequence of CpLHCII. Consequently, CpLHCII shows enhanced energy transfer efficiency, in particular for energy harvested in the blue-green region directed to Chl a. These characteristics indicate a light-harvesting system with reduced energy loss, with respect to CrLHCII. We propose that the unique properties of CpLHCII aids survival in the extreme, spectrally-limited light conditions posed by C. priscui's ecological niche and suggest that these features could inform strategies to optimize light harvesting in agriculture and biotechnology.