FUNCTIONAL-CHARACTERIZATION OF ISOLATED PLASTIDS FROM 2 MARINE DIATOMS

A protocol for the isolation of intact plastids from two marine centri c diatoms, Odontella sinensis (Greville) Grunow and Coscinodiscus gran ii Gough, has been worked out. The cells were broken in a Yeda Press, and the intact plastids were purified by centrifugation in Percoll gra dients. Electron microscopy indicates that at least one of the four en velope membranes is present in the isolated plastids. The plastids are photosynthetically active as proven by CO2 fixation which was measure d by light-dependent oxygen evolution. Rates up to 50 mu mol O-2.(mg C hl)(-1).h(-1), i.e. about 40% of the in vivo rate of photosynthesis we re obtained. The inhibition of CO2 fixation by external phosphate and the ability of the plastids to reduce added 3-phosphoglycerate photosy nthetically indicate the presence of a phosphate translocator in the e nvelope of the diatom plastids. Light-dependent O-2 evolution upon add ition of nitrite indicates the presence of nitrite reductase in these plastids. Purified envelope membranes of Odontella plastids analyzed b y sodium dodecyl sulfate-polyacrylamide gel electrophoresis contain po lypeptides similar to those of the envelope of higher-plant chloroplas ts. However, there are additional bands present, which in part may be constituents of the two additional envelope membranes (''chloroplast e ndoplasmic reticulum'') and in part may represent additional component s of the inner membranes.