Measurements of cell density of three freshwater phytoplankters by density gradient centrifugation1

Density gradients of a modified silica sol (Percoll) were used to measure the densities of three freshwater phytoplankters of simple geometry. To check the validity of these measurements cell densities were calculated from a modification of Stokes’ law, which describes the dependence of sinking velocity (V) on cell shape ( ϕ ), nominal diameter ( D ), and density ( p′ ): The ability to independently measure cell density and sinking velocity allows the determination of the coefficient of form resistance ( ϕ ) and the elucidation of the relationships between sinking velocity, cell shape, and cell density. Values of ϕ calculated for approximately spherical Chlorococcum sp. were 1.02, 1.06, and 1.04, while for Chlorella vulgaris, a prolate spheroid, values were 0.98 and 1.07. Disk‐shaped Cyclotella meneghiniana gave a ϕ value of 1.03.