Collection of dinoflagellates and other marine microalgae by centrifugation in density gradients of a modified silica sol 1

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.