PHOTOSYNTHETIC SOLAR CONVERSION EFFICIENCY IN WILD TYPE AND TRUNCATED Chl ANTENNA TRANFORMANT (Chl b-less MUTANT) OF THE GREEN ALGA Chlamydomonas reinhardtii

The assembly, organization and function of the photosynthetic apparatus was investigated in wild type and a chlorophyll (Chl) b-less mutant of the unicellular green alga Chlamydomonas reinhardtii, generated via DNA insertional mutagenesis. Comparative analyses were undertaken with cells grown photoheterotrophically (acetate), photomixotrophically (acetate and HCO 3 - ) or photoautotrophically (HCO 3 - ). It is shown that lack of Chl b diminished the photosystem-II (PSII) functional Chl antenna size from 320 Chl ( a and b) to about 95 Chl a molecules. However, the functional Chl antenna size of PSI remained fairly constant at about 290 Chl molecules, independent of the presence of Chl b. Western blot and kinetic analyses suggested the presence of inner subunits of the Chl a-b light-harvesting complex of PSII (LHCII) and the entire complement of the Chl a-b light-harvesting complex of PSI (LHCI) in the mutant. It is concluded that Chl a can replace Chl b in the inner subunits of the LHCII and in the entire complement of the LHCI. Growth of cells on acetate as the sole carbon source imposes limitations in the photon use efficiency and capacity of photosynthesis. These are manifested as a lower quantum yield and lower light-saturated rate of photosynthesis, and as a lower variable to maximal (Fv/Fmax) chlorophyll fluorescence yield ratios. This adverse effect probably originates