Refractive index of planktonic cells as a measure of cellular carbon and chlorophyll a content

Current methods for determining carbon content in individual planktonic cells from particle volume alone may involve large errors, and no routine technique exists for determining chlorophyll a content in individual phytoplankters. In this study the concept of using the refractive index of cells as a measure of intracellular concentrations of carbon and chlorophyll a is discussed. Specifically, the real part of the refractive index n (at light wavelengths where absorption is negligible or very small) is shown to correlate well with the intracellular concentration of carbon, Ci. The imaginary part of the refractive index n′ (in the red band of chlorophyll a) correlates well with the intracellular chlorophyll concentration, Chli. These relationships were found to be nearly identical for two species, a cyanobacterium Synechococcus and a diatom Thalassiosira pseudonana, over a two-fold range in Ci and Chli. This range was associated with interspecies differences and intraspecies variations in the cell properties over a day–night cycle. These observations and the underlying theoretical considerations suggest that the relationships Ci vs. n and Chli vs. n′ may be robust and hold for a variety of planktonic species regardless of interspecies and intraspecies variability in cellular carbon content, Chl a content, and cell size. In addition, these relationships may be applicable to single-particle analysis of natural water samples, which promises a unique capability to acquire information about the distribution of carbon and chlorophyll a among individual cells, different size classes, and taxonomic groups of planktonic microorganisms in the ocean. Further research with various planktonic species is needed to examine the generality of the relationships Ci vs. n and Chli vs. n′ before the approach can be implemented in field studies.