Freeze‐etch Ultrastructure of a Prochloron Species ‐ the Symbiont of Didemnum Molle

SUMMARY A species of Prochloron, a genus of photosynthetic prokaryotes containing chlorophylls a and b and lacking phycobilins, has been investigated by the freeze‐etch technique and thin sectioning. The thylakoids show a high level of stacking; particle distributions on fracture faces of the membranes resemble those of chloroplasts more than those of cyanophytes. Polyhedral bodies (carboxysomes?) are surrounded by a unit membrane which probably derives from the thylakoids. Vacuole‐like areas are seen in freeze‐etch replicas to contain a structured substance, probably a reserve material.