INTRACELLULAR AND EXTRACELLULAR DISTRIBUTION OF PHOTOSYNTHETIC PROKARYOTES, PROCHLORON SP., IN A COLONIAL ASCIDIAN - ULTRASTRUCTURAL AND QUANTITATIVE STUDIES

Prochlorons are prokaryotic algae (or photosynthetic bacteria) found a s specific symbionts of some colonial ascidians in the family Didemnid ae, usually on tropical or subtropical marine coasts. In Lissoclinum p unctatum, they occur both inside as well as outside the tunic. In hist ological sections of the ascidian colony, the area occupied by Prochlo ron cells (PrC) is about 4.5%. Since more than 45% of these are presen t inside the tunic, mostly enclosed within certain free mesenchymal ce lls (tunic phycocytes), nearly half of the photosynthesis in the ascid ian colony is estimated to be carried out by such intracellular PrC. O ur observations indicate that tunic phycocytes originate from tunic ph agocytes thar have ingested the symbionts by endocytosis and retained them intact (Hirose et al., 1996). Apparently some tunic phagocytes ca n ingest two or three PrC at a time, while others can later ingest ano ther. Furthermore, some PrC can possibly survive and divide within sym biosomes. When phagocytic hemocytes of another symbiotic ascidian, L, patella, were incubated with their symbionts in vit ro, they become ph ycocyte-like by endocytosis of PrC. This observation further supports the origin of tunic phycocytes in L. punctatum.