An ultrastructural study of hypnozygotes of Alexandrium species (Dinophyceae)

Light, scanning and transmission electron microscopy were carried out on Alexandrium tamarense and A. fundyense hypnozygotes (cysts) from cultures and marine sediments. Transmission electron microscopy protocols were adapted to improve the quality of ultrathin sections. Cell contents of hypnozygotes were reduced compared to vegetative stages and were largely made up of storage vesicles in a dense, granular matrix. Chloroplasts and other organelles (Golgi bodies, endoplasmic reticulum and mitochondria) were observed as whorls of undifferentiated membranes and the nucleus was compressed with strongly condensed, granular chromosomes. Two types of accumulation bodies were found, some composed of dense amorphous material and others containing polygonal crystalline inclusions. Both types contained numerous membrane profiles. In cross section, the hypnozygote wall was made up of three layers divided by membranes: an outer layer with a thin electron-dense distal surface and membrane that formed the interface with the environment; a wide middle layer of striated material and membrane (possibly involved in deposition of cyst wall material); and a narrow unstructured inner layer and membrane lying close to the cytoplasmic membrane of the cell. Comparative analysis of cyst wall structure with other dinoflagellate species showed this three layered structure is common among refractive cysts. Energy dispersive X-ray analysis of the cyst wall surface demonstrated that the principal components of the cyst wall were sulphur and silica.