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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