A freeze-substitution ultrastructural study of the cytoskeleton of the red alga Antithamnion kylinii (Ceramiales)

Ultrastructural examination of specimens of Antithamnion kylinii Gardner preserved by freeze-substitution revealed the presence of abundant microtubules and actin microfilaments. By contrast, and as is typical of red algae, neither cytoskeletal component was observed in specimens prepared by conventional chemical fixation. Microtubules were especially common close to the nucleus, but they were found throughout the peripheral cytoplasm and in strands of cytoplasm traversing the large central vacuole. Microtubules occurred individually rather than in bundles and generally were oriented parallel to the longitudinal axis of the cells. Microfilaments commonly formed sinuous bundles ranging from cylindrical and 0.2 to 0.5 mu m in diameter to sheet-like and up to 0.7 mu m wide. The bundles, which bent and branched, sometimes at large angles, occurred throughout the parietal layer of cytoplasm. Many organelles were observed in proximity to the microfilament bundles, and microfilaments often appeared to ensheath portions of calcium oxalate crystals present within the cytoplasm of cells of the indeterminate axes.