EVIDENCE FOR A LACK OF PHOTOSYSTEM SEGREGATION IN CHLAMYDOMONAS REINHARDTII (CHLOROPHYCEAE)

The distribution of photosystems I and II (PSI and PSII) in cells of Chlamydomonas reinhardtii Dangeard was studied by immunogold electron microscopy using cultures grown autotrophically at moderate irradiance and harvested in the middle of the light period. Sections of Lowicryl-embedded cells were labeled with monospecific heterologous antisera raised against the reaction center proteins of PSI (CP1-e) or the core antenna proteins of PSII (CP40 and CP47). All three antisera labeled both the appressed and the nonappressed thylakoid membranes at essentially similar densities. Labeling with both PSI and PSII antisera was slightly more concentrated over the outer nonappressed membranes of the thylakoid bands (1.7- to 2.4-fold with anti-CP1- e and 1.5- to 1.8-fold with anti-CP47 and anti-CP40). However, since appressed membranes comprised 73% of the total thylakoid membranes, 50%–62% of the PSI and 58%–65% of the PSII labeling were localized on appressed membranes. We conclude that photosystem distribution in C. reinhardtii is similar to that reported for other algae and different from the lateral heterogeneity observed in higher plants.