366
The study of prochlorophyte picoplankton exemplifies how individual cell analysis enabled us
to better utilize provocative but incomplete information from bulk water measurements.
Several sets of HPLC measurements had revealed "unusual" (red-shifted) chlorophyll pigments
at high concentrations (sometimes up to half the total chlorophyll) in tropical and sUbtropical
oceans (Gieskes et al. 1978, 1988; Gieskes and Kraay 1983, 1986; Vernet 1983; Goericke
1990), and size fractionation experiments (Gieskes and Kraay 1983) indicated the unusual
pigment was in the < 1 /km fraction. These pigments are almost certainly the same divinyl
chlorophylls that we find in the prochlorophytes, but without the means to see both the cells
and their pigments together it was not possible to complete the story. Similarly, what appears
to be the same cell as our prochlorophyte was previously reported in bulk water samples
examined by transmission electron microscopy (Johnson and Sieburth 1979) as a type of
Synechococcus. Without the clue given by fluorescence measurements of the living cells, no
other interpretation would be suggested. Finally, flow cytometric analysis made possible a
rapid and detailed study of the ecology of this organism, which appears to occupy a distinctly
different niche from that of its Synechococcus relatives (Olson et al. 1990b).
Flow cytometric studies of the picoplankton have been facilitated by the recent development
of a relatively effective preservation method for these cells, involving fixation with
glutaraldehyde and storage in liquid nitrogen (Vaulot et al. 1989). Although even this
technique is not quantitative for all species or for all parameters measured, it has already
allowed detailed studies involving many more samples (e.g., see Fig. 20) than were possible
to process when "real time" sample analysis was necessary.
Eukaryotic phytoplankton: The larger eukaryotic phytoplankton have not been studied in the
same detail as the picoplankters. This is partly because of sampling limitations and partly
because it is difficult to discriminate among the majority of the cell types with flow
cytometry, at least compared to traditional microscopic methods. For example, Li and Wood
(1988) demonstrated that by using flow cytometric analyses of chlorophyll fluorescence and
Coulter volume they could distinguish two kinds of ultraplanktonic (2-5 /km) eukaryotes,
whereas analysis of the same samples with epifluorescence microscopy suggested that a dozen
or more categories could be distinguished based on the size and shape of the chloroplasts.
(Note that flow cytometry of picoplankton also gives us only a relatively crude separation of
The study of prochlorophyte picoplankton exemplifies how individual cell analysis enabled us
to better utilize provocative but incomplete information from bulk water measurements.
Several sets of HPLC measurements had revealed "unusual" (red-shifted) chlorophyll pigments
at high concentrations (sometimes up to half the total chlorophyll) in tropical and sUbtropical
oceans (Gieskes et al. 1978, 1988; Gieskes and Kraay 1983, 1986; Vernet 1983; Goericke
1990), and size fractionation experiments (Gieskes and Kraay 1983) indicated the unusual
pigment was in the < 1 /km fraction. These pigments are almost certainly the same divinyl
chlorophylls that we find in the prochlorophytes, but without the means to see both the cells
and their pigments together it was not possible to complete the story. Similarly, what appears
to be the same cell as our prochlorophyte was previously reported in bulk water samples
examined by transmission electron microscopy (Johnson and Sieburth 1979) as a type of
Synechococcus. Without the clue given by fluorescence measurements of the living cells, no
other interpretation would be suggested. Finally, flow cytometric analysis made possible a
rapid and detailed study of the ecology of this organism, which appears to occupy a distinctly
different niche from that of its Synechococcus relatives (Olson et al. 1990b).
Flow cytometric studies of the picoplankton have been facilitated by the recent development
of a relatively effective preservation method for these cells, involving fixation with
glutaraldehyde and storage in liquid nitrogen (Vaulot et al. 1989). Although even this
technique is not quantitative for all species or for all parameters measured, it has already
allowed detailed studies involving many more samples (e.g., see Fig. 20) than were possible
to process when "real time" sample analysis was necessary.
Eukaryotic phytoplankton: The larger eukaryotic phytoplankton have not been studied in the
same detail as the picoplankters. This is partly because of sampling limitations and partly
because it is difficult to discriminate among the majority of the cell types with flow
cytometry, at least compared to traditional microscopic methods. For example, Li and Wood
(1988) demonstrated that by using flow cytometric analyses of chlorophyll fluorescence and
Coulter volume they could distinguish two kinds of ultraplanktonic (2-5 /km) eukaryotes,
whereas analysis of the same samples with epifluorescence microscopy suggested that a dozen
or more categories could be distinguished based on the size and shape of the chloroplasts.
(Note that flow cytometry of picoplankton also gives us only a relatively crude separation of