The association between chloroplasts of Codium fragile and the mollusc Elysia viridis II. Chloroplast ultrastructure and photosynthetic carbon fixation in E. viridis

When Codium fragile chloroplasts in cells of Elysia viridis were examined by electron microscopy, no structural damage or alteration was observed beyond a general change in shape from ellipsoid in the plant to more oval in the animal; outer chloroplast envelopes were always intact. Some chloroplasts in animal cells were enclosed by a membrane external to the envelope but others were not; the origin of this membrane is unclear. Neither the mechanism of entry nor the ultimate fate of chloroplasts in animal cells is fully understood. Rates of photosynthetic carbon fixation by chloroplasts in Elysia and codium were of a similar order. The pathways of photosynthetic 14C fixation in Elysia were not restricted compared to isolated chloroplasts. In Codium, incorporation of 14C into sucrose occurs outside the chloroplast, as does incorporation into galactose in Elysia. Substantial incorporation into galactose in Elysia does not begin until about 20 to 30 min after the start of 14C fixation, although a certain amount of 14C galactose is detectable after 5 to 8 mins, indicating rapid release from chloroplasts. Chloroplasts isolated from Codium release only 2% of their fixed 14C to the medium, mainly as glycolic acid. In animal homogenates, release of fixed 14C from chloroplasts rises to 40%, mostly composed of glucose with some glycolic acid. In intact animals, at least 36% of the fixed carbon is released from the chloroplasts. Chloroplasts in Elysia, unlike those in Codium, were not able to incorporate 14C from CO2 or other precursors into chlorophyll. In Codium, incorporation from CO2 ceased in the presence of cycloheximide or choramphenicol, suggesting that complete biosynthesis of chlorophyll depends upon DNA in both the plant nucleus and the chloroplast. Incorporation from CO2 into glycolipids and sulpholipids could not be detected in Elysia. Chloroplasts in Elysia are unlikely to be completely autonomous.