Determination of the cross‐section absorption coeffcient of individual phytoplankton cells by analytical flow cytometry

An empirical relationship between the cellular cross‐section absorption coefficient (σ) and the Chl a fluorescence of individual plankton cells was established. Chl a fluorescence of single cells was measured in a flow cytometer with 488‐nm argon‐ion laser excitation. Absorption by dilute suspensions of cells was measured spectrophotometrically and normalized to cell number for computation of σ(488) (m2 cell−1). At the high excitation irradiances characteristic of lasers, a constant proportion of photons absorbed by photosynthetic pigments was re‐emitted as Chl a fluorescence; a strong correlation was observed between absorption at 488 nm and Chl a fluorescence (r2 = 0.93; P ≪ 0.001). The regression predicted the total phytoplankton absorption coefficient [aphyt(488) (m−1)] at 488 nm for mixtures of five monospecific cultures with <9% error. For field samples, discrepancies between the flow cytometric method and the filter pad spectrophotometric method for estimating aphyt(488) were greater. Relatively large, rare phytoplankton cells accounted for a disproportionate percentage of total aphyt(488). The coherence between absorption at 488 nm and at other wavelengths was high and provides a basis for extrapolating from σ(488) to single‐cell absorption coefficients at all visible wavelengths. The single‐cell spectral absorption coefficients can be used to compute total in situ photon absorption for modeling light‐limited photosynthetic rates of individual phytoplankton cells in the ocean.