The association between chloroplasts ofCodium fragileand the molluscElysia viridisII. Chloroplast ultrastructure and photosynthetic carbon fixation inE. viridis

Abstract

WhenCodium fragilechloroplasts in cells ofElysia viridiswere 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 inElysiaandcodiumwere of a similar order. The pathways of photosynthetic14C fixation inElysiawere not restricted compared to isolated chloroplasts. InCodium, incorporation of14C into sucrose occurs outside the chloroplast, as does incorporation into galactose inElysia. Substantial incorporation into galactose inElysiadoes not begin until about 20 to 30 min after the start of14C fixation, although a certain amount of14C galactose is detectable after 5 to 8 mins, indicating rapid release from chloroplasts. Chloroplasts isolated fromCodiumrelease only 2% of their fixed14C to the medium, mainly as glycolic acid. In animal homogenates, release of fixed14C 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 inElysia, unlike those inCodium, were not able to incorporate14C from CO2or other precursors into chlorophyll. InCodium, incorporation from CO2ceased 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 CO2into glycolipids and sulpholipids could not be detected inElysia. Chloroplasts inElysiaare unlikely to be completely autonomous.