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Keywords:

  • Freeze-fracture;
  • red algae;
  • ultrastructure;
  • phycobilisomes;
  • thylakoid membranes

summary

The ultrastructure and supramolecular organization of photosynthetic membranes in four species of marine red algae (Porphyra yezoensis, Porphyra leucosticta, Erythrocladia subintegra, Pterothamnion crispum) were studied both in replicas of rapidly frozen cells and in ultrathin sections. Because the thylakoids in red algal chloroplasts are not stacked, freeze-fractured thylakoid membranes exhibit only two types of fracture faces (EF and PF). In all four species the PF reveals numerous, tightly packed, but randomly distributed particles (density range from 3530 to 4970 particles μm−2 for Porphyra yezoensis). The particle size distribution is uniform in the two types of fracture faces with an average diameter of about 11.3–11.6 nm in all four algae.

The phycobilisomes of Porphyra yezoensis and Erythrocladia subintegra have the approximate shape of a compressed rotation ellipsoid with the following dimensions; Porphyra yezoensis, length 52.8±1.2 nm, thickness 24.4±0.6 run, height 34.6±1 nm; Erythrocladia subintegra, length 45.1±0.7 nm, thickness 23.1±0.5 nm, height 29.4±0.9 nm. In Porphyra yezoensis, Porphyra leucosticta and Erythrocladia subintegra both the phycobilisomes and EF particles are organized into rows with a spacing of 70–80 nm in Porphyra yezoensis, 65–75 nm in Porphyra leucosticta and 53.8–55 nm in Erythrocladia subintegra. EF particle complexes with four subunits were found in Porphyra yezoensis. In Pterothamnion crispum the shape of the phycobilisomes is hemiellipsoidal. The phycobilisomes seem to be organized into rows; their centre to centre periodicity is c. 60 nm. Porphyra yezoensis cells possess about 770±58 phycobilisomes μm−2 of thylakoid surface, whereas the number of EF particles is about 1270±203 μm−2 Thus, the ratio of phycobilisomes to EF particles is c. 0.6. In Erythrocladia subintegra the ratio of phycobilisomes to EF particles is about 0.7.

Of special interest is the ocurrence of a crystalline matrix in pyrenoids of Porphyra leucosticta and the absence of phycobilisomes in the intrapyrenoid thylakoids of Porphyra yezoensis and Erythrocladia subintegra.