Relative real refractive index of marine microorganisms: a technique for flow cytometric estimation

Abstract

A flow cytometer was used to estimate the relative real refractive indices of six species of phytoplankton cells. Light scatter measurements at 90° and a near-forward region (1.5–19°) were made with hydrocarbon dispersions and a glass bead suspension having known refractive indices. The ratios of the intensity of light scatter in these two angular regions for these standards was found (and can be shown, theoretically) to be proportional to the refractive index of the material. Using these calibration materials, light scatter measurements of individual phytoplankton cells yielded estimates of the relative refractive indices of these algae. These measurements are well within the historically accepted range of algal refractive indices, however, a significant degree of interspecific variability exists. The resolution of this technique to measure relative real refractive index (±0.005) represents a new capability in the assessment of optical properties of marine particulates.