Abstract

<b>Linear discriminant analysis (LDA) of single-cell fluorescence excitation spectra (λ<sub>em</sub> = 680 nm) for five species of marine phytoplankton was used to determine whether intra-species variation among single cells precluded discrimination among species. Single-cell spectra were recorded in an optical trap with a custom-built spectral fluorometer. For nitrogen (N)-replete cells, separation of all five species (<i> Emiliania huxleyi</i>, a coccolithophore,</b> <b><i>Thalassiosira pseudonana</i>, a diatom,</b> <b><i>Dunaliella tertiolecta</i>, a chlorophyte,</b> <b><i>Amphidinium carterae</i>, a dinoflagellate, and</b> <b><i>Rhodomonas salina</i>, a cryptophyte) was possible using only a portion of the excitation spectra (570-610 nm). This wavelength region gave perfect classification of species with a minimum Fisher ratio of 62. For four species (<i> E. huxleyi</i>,</b> <b><i>T. pseudonana</i>,</b> <b><i>D. tertiolecta,</i></b> <b>and</b> <b><i>A. carterae</i>), variations in fluorescence excitation spectra as cells were starved of N did not impact the classification process adversely within the chosen spectral window.</b> <b><i>R. salina</i></b> <b>cells grown with and without N showed significant differences in their fluorescence excitation spectra but could still be classified if a different spectral window (490-570 nm) was used. Overall, we conclude that intra-species variation among single-cell fluorescence excitation spectra does not preclude discrimination among species.</b>

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