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

Current methods for determining carbon content in individual planktonic cel ls from particle volume alone may involve large errors, and no routine tech nique exists for determining chlorophyll a content in individual phytoplank ters. In this study the concept of using the refractive index of cells as a measure of intracellular concentrations of carbon and chlorophyll a is dis cussed. Specifically, the real part of the refractive index n (at light wav elengths where absorption is negligible or very small) is shown to correlat e well with the intracellular concentration of carbon, C-i. The imaginary p art of the refractive index n' (in the red band of chlorophyll a) correlate s well with the intracellular chlorophyll concentration, Chl(i). These rela tionships were found to be nearly identical for two species, a cyanobacteri um Synechococcus and a diatom Thalassiosira a pseudonana, over a two-fold r ange in C-i and Chl(i), This range was associated with interspecies differe nces and intraspecies variations in the cell properties over a day-night cy cle. These observations and the underlying theoretical considerations sugge st that the relationships C-i vs. n and Chl(i) vs, n' may be robust and hol d for a variety of planktonic species regardless of interspecies and intras pecies variability in cellular carbon content, Chi a content, and cell size . In addition, these relationships may be applicable to single-particle ana lysis of natural water samples, which promises a unique capability to acqui re information about the distribution of carbon and chlorophyll a among ind ividual cells, different size classes, and taxonomic groups of planktonic m icroorganisms in the ocean. Further research with various planktonic specie s is needed to examine the generality of the relationships C-i vs, n and Ch l(i) vs, n' before the approach can be implemented in field studies. (C) 19 99 Elsevier Science Ltd. All rights reserved.