IMMUNOCYTOCHEMICAL LOCALIZATION OF PHOTOSYSTEM-I AND THE FUCOXANTHIN-CHLOROPHYLL-A/C LIGHT-HARVESTING COMPLEX IN THE DIATOM PHAEODACTYLUM-TRICORNUTUM

Antiserum against two polypeptides of the major fucoxanthin-chlorophyll a/c light-harvesting complex of the diatom Phaeodactylum tricornutum and heterologous antiserum against purified photosystem I particles of maize were used to localize these two complexes on the thylakoid membranes of P. tricornutum. As in many chromophyte algae, the thylakoids are loosely appressed and organized into extended bands of three, giving a ratio of 2:1 for appressed versus non-appressed membranes. Immunoelectron microscopy demonstrated that the fucoxanthin-chlorophyll a/c light-harvesting complex, which is believed to be associated with photosystem II, was equally distributed on the appressed and non-appressed thylakoid membranes. Photosystem I was also found on both types of membranes, but was slightly more concentrated on the two outer non-appressed membranes of each band. Similarly, photosystem I activity, as measured by the photooxidation of 3,3'-diaminobenzidine, was higher in the outer thylakoids than in the central thylakoid of each band. We conclude that the thylakoids of diatoms differ from those of green algae and higher plants in their macromolecular organization as well as in their morphological arrangement.