VIRUS‐LIKE PARTICLES AND NUCLEAR INCLUSIONS IN THE RED ALGA PORPHYRIDIUM PURPUREUM (BORY) DREW ET ROSS 1

SUMMARY Ultrastructural examination of the unicellular red alga Porphyridium purpureum has revealed cytoplasmic and nuclear inclusions termed concentrosomes. These bodies are morphologically distinct from the irregular membranous inclusions previously reported by others as concentric bodies. In thin section, the morphology of concentrosomes varies from a simple set of 3 or 4 (or rarely 5) concentrically arranged, electron‐opaque, circular profiles to elongate, sinuous forms and particulate aggregations, the majority clearly within the nucleus but separated front the nucleoplasm by what appears to be the nuclear envelope. Although simple concentrosomes may be observed in either the cytoplasm or the nuclei, the more elaborate forms occur only in nuclei. In addition to the concentrosomes, subspherical to polygonal virus‐like particles of approximately 40 nm diameter have been observed in P. purpureum. These particles are characterized by an electron‐opaque perimeter that, in approximately equal numbers, surrounds an “empty” or an opaque core. Dense arrays of the virus‐like particles appear in the cytoplasm but not in the nuclei. Similarities between certain forms of the concentrosomes and the virus‐like particles are suggestive of an ontogenetic relationship. The infrequency with which either the concentrosomes or the virus‐like particles are observed has hampered attempts to verify a developmental sequence or to establish unequivocally the infectious nature of the virus‐like particles.