Changes in structure of isolated chloroplasts ofCodium fragile andCaulerpa filiformis in response to osmotic shock and detergent treatment

Summary The ultrastructure of chloroplasts from two genera of coenocytic green algae,Codium andCaulerpa, were examined after suspension in hypotonic solution and in detergent at various concentrations. The capacity of the suspensions to carry out CO₂-dependent and ferricyanide-dependent O₂ evolution was measured under the same conditions of osmotic strength and detergent concentration.The chloroplasts in the preparations were in the form of cytoplasts and gave rates of O₂ evolution comparable with those expected from undamaged chloroplasts. Suspension in hypotonic solution depressed the rate of CO₂-dependent O₂ evolution in both species, but this was partially restored in theCodium chloroplasts when these were re-suspended in iso-osmotic solutions. Major structural changes were observed only after suspension in buffer when theCodium chloroplasts lost their outer envelope, most of their stroma, and the thylakoids became swollen.Caulerpa chloroplasts were more variable in their response and, even when suspended in buffer only, the proportion of the plastids which had lost all of their stroma and thylakoid swelling was never as common as inCodium chloroplasts. However, once suspended in hyper-osmotic medium below 700 mosmolar,Caulerpa chloroplasts could not regain their capacity for CO₂-dependent O₂ evolution.Detergent treatment removed the cytoplast membrane but not the cytoplasmic material adhering to the chloroplast envelope. High concentrations of detergent were needed to cause loss of the chloroplast envelope, loss of stromal contents and unstacking of the thylakoids.Caulerpa chloroplasts were less sensitive to detergent than those ofCodium. There was no indication that specific structures such as the thylakoid organizing body were resistant to detergent action. The results show that exposure to hypotonic solutions and to detergent results in less damage to these chloroplasts than it would to those of higher plants. It is proposed that the basis of this unusual resistance is not due to the properties of the chloroplast membranes but to the presence of material which coats the organelles during isolation. This material is likely to be identical with the sulphated xylo-mannogalactan isolated from the vacuole contents of these algae and which has the visco-elastic properties essential to allow the organelles to resist disruption by osmotic forces and disintegration by detergents.