364
cytometer could easily detect these cells, they could not be readily enumerated by
epifluorescence microscopy because upon illumination their fluorescence faded within a few
seconds (flow cytometric measurements are carried out in a few microseconds). The cells are
very abundant (up to 10 5 cells ml- I ) and appear at depths where the Synechococcus are no
longer able to grow. We have found them at nearly all areas of the oceans where we see
Synechococcus (Chisholm et al. 1988; Olson et al. 1990b), and their widespread distribution
has been more recently documented by others: they have now been observed in the
Mediterranean (Vaulot et al. 1990), the equatorial Pacific (sample courtesy of N. Price), off
Hawaii (unpublished data from samples provided by D. Vaulot), off Southern California
(Chisholm et al. 1988), in Monterey Bay (unpublished data from samples provided by K.
Buck and F. Chavez, MBARI) as well as in the North Atlantic (Chisholm et al. 1988; Neveux
et al. 1989; Li and Wood 1988; Veldhuis and Kraay 1990).
Because culturing these cells eluded us for several years, our initial characterization of them
had to be done using natural samples (Chisholm et al. 1988). The cells were much less
effectively excited by the 515 nm laser line than by the 488 nm line, suggesting the absence
of carotenoid accessory pigments which absorb efficiently at 515 nm and which would be
expected in non-chlorophytes. Since we knew the cells did not contain phycobiliproteins, we
suspected they might contain chlorophyll b. The first evidence that these cells were
prokaryotes came from transmission electron micrographs of a bulk water sample from 120
m in the Sargasso Sea, where flow cytometry indicated the prochlorophytes were 100-fold
more abundant than Synechococcus (Chisholm et al. 1988). Using flow cytometric analysis
to assess the community composition, we were sometimes able to obtain samples highly
enriched with prochlorophytes, either through filter fractionation (Fig. 8) or cell sorting, for
further analysis. This was possible in part simply because these cells can grow at greater
depths than Synechococcus. HPLC analysis of the pigments of the prochlorophyte-enriched
samples indicated that the cells contained both chI a and chI b, zeaxanthin, and a-carotene;
this, combined with their prokaryotic ultrastructure, is the reason for their designation as
prochlorophytes, after Lewin's (1981) original description.
The pigment analyses also revealed that the chlorophylls in these cells were "red-shifted"
(Soret absorption is shifted 8-10 nm toward longer wavelengths compared to normal
cytometer could easily detect these cells, they could not be readily enumerated by
epifluorescence microscopy because upon illumination their fluorescence faded within a few
seconds (flow cytometric measurements are carried out in a few microseconds). The cells are
very abundant (up to 10 5 cells ml- I ) and appear at depths where the Synechococcus are no
longer able to grow. We have found them at nearly all areas of the oceans where we see
Synechococcus (Chisholm et al. 1988; Olson et al. 1990b), and their widespread distribution
has been more recently documented by others: they have now been observed in the
Mediterranean (Vaulot et al. 1990), the equatorial Pacific (sample courtesy of N. Price), off
Hawaii (unpublished data from samples provided by D. Vaulot), off Southern California
(Chisholm et al. 1988), in Monterey Bay (unpublished data from samples provided by K.
Buck and F. Chavez, MBARI) as well as in the North Atlantic (Chisholm et al. 1988; Neveux
et al. 1989; Li and Wood 1988; Veldhuis and Kraay 1990).
Because culturing these cells eluded us for several years, our initial characterization of them
had to be done using natural samples (Chisholm et al. 1988). The cells were much less
effectively excited by the 515 nm laser line than by the 488 nm line, suggesting the absence
of carotenoid accessory pigments which absorb efficiently at 515 nm and which would be
expected in non-chlorophytes. Since we knew the cells did not contain phycobiliproteins, we
suspected they might contain chlorophyll b. The first evidence that these cells were
prokaryotes came from transmission electron micrographs of a bulk water sample from 120
m in the Sargasso Sea, where flow cytometry indicated the prochlorophytes were 100-fold
more abundant than Synechococcus (Chisholm et al. 1988). Using flow cytometric analysis
to assess the community composition, we were sometimes able to obtain samples highly
enriched with prochlorophytes, either through filter fractionation (Fig. 8) or cell sorting, for
further analysis. This was possible in part simply because these cells can grow at greater
depths than Synechococcus. HPLC analysis of the pigments of the prochlorophyte-enriched
samples indicated that the cells contained both chI a and chI b, zeaxanthin, and a-carotene;
this, combined with their prokaryotic ultrastructure, is the reason for their designation as
prochlorophytes, after Lewin's (1981) original description.
The pigment analyses also revealed that the chlorophylls in these cells were "red-shifted"
(Soret absorption is shifted 8-10 nm toward longer wavelengths compared to normal