This study reports the first direct measurements of the density and sinking rates of marine dinoflagellate cysts. Gyrodinium uncatenum, Gonyaulax tamarensis, and Scrippsiella trochoidea cysts had densities of 1.14, 1.24, and 1.32 g cm-3. No significant difference between cultured and natural cyst density was observed. Measured settling velocities ranged from 0.008 to 0.0 13 cm s-1 (6-l 1 m d-l). Settling rates calculated using the measured densities and reasonable assumptions about shape and orientation during descent were within lO-20% of measured sinking rates, confirming that cyst sinking can be described by a modification of Stokes' law for nonspherical particles in a viscous medium. The three types of cysts examined are more dense and fall faster than most vegetative phytoplankton cells. Removal of an outer layer of short calcite spines from S. trochoidea cysts reduced both cell density and radius by 7% and the sinking rate by 37%. The faster settling velocity of the spiny cyst is not considered a general effect common to all ornamented cysts but presumably applies only to those with numerous short spines or processes. Morphological features can thus markedly affect the rate of cyst deposition, but the adaptive significance of surface ornamentation remains unknown.

An improved medium is described, named “sorbitol seawater” (SSW), for the collection and concentration of marine phytoplankton by centrifugation into density gradients of silica. SSW is a partially synthetic seawater which is compatible with a modified colloidal silica (Percoll) and supports lowered but otherwise normal motility of several naked dinoflagellates, other fragile flagellates, diatoms, and a blue‐green alga. After isopycnic sedimentation into continuous or step gradients of Percoll in SSW, the organisms are concentrated into narrow zones specific to the banding densities of the alga and can be recovered more completely intact than from simple pelleting. Together with previously described methods, the medium should be useful in collecting morphologically and physiologically intact plankton from laboratory cultures and from marine waters.

Chloroplasts can be obtained by gentle lysis or mild shear of spheroplasts of vitamin B12-deficient Euglena gracilis and then purified by isopycnic sedimentation on gradients of Ludox AM or Percoll. The chloroplasts appear compact and highly refractile by phase contrast or Hoffmann contrast microscopy. Upon incubation with 13HIleucine or 135Slmethionine, the chloroplasts incorporate the amino acids into protein at rates that are 100-fold faster than we had previously observed with Euglena and up to 8-fold faster than with chloroplasts of spinach. Eugkna chloroplasts prepared by the current procedure are thus qualitatively superior to those previously available from Eugkna and at least as active in protein synthesis as chloroplasts from higher plants.Although Euglena gracilis offers many advantages in the study of chloroplast development, a serious limiting factor in pursuing the molecular biology of this process has been the difficulty in isolating pure, intact, and functional plastids from Euglena. The pioneer studies of Eisenstadt and Brawerman (5) were based on crude fractions obtained by differential centrifugation and differential flotation in sucrose. Although the plastids obtained were extensively contaminated with other subcellular particles and the integrity of the organelles was never assessed, their isolation procedures were nonetheless followed by a number of other laboratories. We now realize that the high osmotic pressures required for flotation in sucrose gradients are incompatible with the integrity of chloroplasts.Several investigators have used spheroplasts in an effort to isolate plastids under relatively mild conditions (3,8,11,13), but the chloroplasts obtained lacked stroma and the envelopes were broken or lost.We have described (18) a separation procedure in which the chloroplasts were separated by rate zonal centrifugation in isosmotic gradients of Ficoll. This method yielded chloroplasts that appeared intact by electron microscopy but were not functional. We then found (10) that pure, intact, and functional chloroplasts

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