The measurement of phytoplankton biomass using flow-cytometric sorting and elemental analysis of carbon

Phytoplankton are one of a multitude of particle types in the marine environment that contribute to the particulate organic carbon pool. Current methods for estimating phytoplankton carbon (Cphyto) in the field rely on proxy measurements, such as chlorophyll or cell biovolume conversions. Other means of estimating Cphyto include remote‐sensing applications that convert optical properties, such as chlorophyll fluorescence or particulate backscattering, into biomass. These values, however, are not well constrained due to the variability of phytoplankton physiology (i.e., internal pigment concentrations) and the lack of field data for validating Cphyto retrievals. Here we introduce a method that uses flow cytometry to isolate phytoplankton from the total particle field. Phytoplankton samples are then analyzed for total carbon content. Analyses of laboratory cultures of Synechococcus, Micromonas pusilla, and Thalassiosira pseudonana indicate that this method allows elemental analysis of phytoplankton cells in the absence of unwanted particles. Tests of field samples show that the structure of the natural community is retained in a sorted sample. The acquisition of Cphyto data using this method will 1) benefit remote‐sensing applications by allowing more thorough validation of phytoplankton carbon estimates from space, 2) improve our knowledge of phytoplankton stoichiometry, and 3) provide a better understanding of phytoplankton physiological responses to environmental forces.