Functional organization of the chloroplast in the diatom Phaeodactylum tricornutum

The technique of protein A-gold immunoelectron microscopy was used to determine the distributions of two photosynthetic complexes in the thylakoid membranes of the diatom Phaeodactylum tricornutum. The fucoxanthin-chlorophyll a/c light-harvesting complex, believed to be associated with photosystem II, was found to be equally distributed among appressed and unappressed membranes, whereas photosystem I was slightly more concentrated in the latter. These results suggest that in diatoms, the two photosystems and their associated light-harvesting complexes are essentially equally distributed on the two types of membranes, in marked contrast to the lateral heterogeneity observed in higher plants and green algae. Furthermore, it was expected that in P. tricornutum, the nuclear-coded proteins of the fucoxanthin-chlorophyll a/c light-harvesting complex would be present in the vesicles of the periplastidal reticulum, which were postulated to be involved in protein transport into chloroplasts. Immunolabelling results, however, indicated that these proteins were absent from the periplastidal vesicles