Refractive index of phytoplankton derived from its metabolite composition

The refractive index of phytoplankton is calculated from its metabolite composition. The mean index of the algal dry mass, relative to the index of water, seems to vary from 1.146 for green algae to 1.167 for coccolithophorids. This difference of 0.021 reduces to 0.007 and 0.003 when the partial water volumes of the algae are 0.6 and 0.8, respectively. The most important factor for the variation of the refractive index is the algal water content. If a reasonable average range of the partial water volume is 0.6 +/- 0.2, the corresponding range of the refractive index relative to sea water becomes 1.06 +/- 0.04. This result is in excellent agreement with estimates based on optical measurements of pure algal cultures. The normal dispersion of the refractive index is negligible, but anomalous dispersion in the vicinity of strong absorption bands may be of importance for the light scattering properties of small phytoplankton.