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

Abstract

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.