Photosynthetic apparatus organization and function in the wild type and a chlorophyll b-less mutant of Chlamydomonas reinhardtii. Dependence on carbon source

The assembly, organization and function of the photosynthetic apparatus was investigated in the wild type and a chlorophyll (Chl) b-less mutant of the unicellular green alga Chlamydomonas reinhardtii, generated via DNA insert ional mutagenesis. Comparative analyses were undertaken with cells grown ph otoheterotrophically (acetate). photomixotrophically (acetate and HCO3-) or photoautotrophically (HCO3-). It is and HCO shown that lack of Chl b dimin ished 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 si ze of PSI remained fairly constant at about 290 Chl molecules, independent of the presence of Chl b. Western blot and kinetic analyses suggested the p resence 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. G rowth of cells on acetate as the sole carbon source imposes limitations in the photon-use efficiency and capacity of photosynthesis. These are manifes ted as a lower quantum yield and lower light-saturated rate of photosynthes is, and as lower variable to maximal (F-v/F-max) chlorophyll fluorescence y ield ratios. This adverse effect probably originates because acetate shifts the oxidation-reduction state of the plastoquinone pool, and also because it causes a decrease in the amount and/or activity of Rubisco in the chloro plast. Such limitations are fully alleviated upon inclusion of an inorganic carbon source (e.g. bicarbonate) in the cell growth medium. Further, the w ork provides evidence to show that transformation of green algae can be use d as a tool by which to generate mutants exhibiting a permanently truncated Chi antenna size and a higher (per Chi) photosynthetic productivity of the cells.