362
Using this method of analysis, we surveyed Synechococcus in the North Atlantic and Pacific
Oceans to better understand what regulates the distribution of the various pigment types (Olson
et al. 1990a). Although we found a large range in the relative PUB content of natural
populations, we could see two main groups. The large majority of cells in the open oceans
were of the blue-absorbing (high-PUB) type. Low-PUB populations were found only in coastal
waters, and these were limited to relatively shallow depths. In most cases the low PUB cells
were part of dual populations, so that high-PUB cells were present at all but a few stations.
This distribution of pigment types is consistent with the optical characteristics of the oceanic
euphotic zone, in that the cells best adapted to absorb blue light were found in the open ocean
(where blue light penetrates deepest), while the cells with PE adapted for green-light
absorption were limited to shallow, coastal waters where green light is available. It seems
likely that these findings will have an impact on the interpretation of remotely sensed ocean
color measurements, but this will depend on the light absorption by these numerous but very
small cells relative to the total absorption.
Campbell and Iturriaga (1988), using single-cell fluorescence excitation spectra obtained by
microspectrofluorometry, also concluded that Synechococcus in the Sargasso Sea had high
PUB:PEB ratios, and Wood et al. (1985) were able to discriminate between pigment types
with epifluorescence microscopy by examining each cell with two different filter sets. The
rapidity and objectivity of flow cytometry, however, allows us to put these observations on
a quantitative basis for large areas of the world ocean in a way that other methods cannot.
Prochlorophyte picoplankton: Flow cytometry has played an even more pivotal role in our
understanding of the picoplankton group currently designated "prochlorophytes," awaiting
detailed characterization. These cells were first observed in samples from the deep euphotic
zone, although we recognized that their apparent absence from most surface samples was due
to the detection limits of our instrument (Chisholm et al. 1988). More recently we have been
able to measure even the dim cells at the surface by using more sensitive instruments (Olson
et al. 1990b) (Fig. 7). The flow cytometric signatures of these cells were first recognized as
extremely numerous populations with very dim red fluorescence, no orange fluorescence, and
light scatter signals even smaller than Synechococcus. Even at depths where the flow
Using this method of analysis, we surveyed Synechococcus in the North Atlantic and Pacific
Oceans to better understand what regulates the distribution of the various pigment types (Olson
et al. 1990a). Although we found a large range in the relative PUB content of natural
populations, we could see two main groups. The large majority of cells in the open oceans
were of the blue-absorbing (high-PUB) type. Low-PUB populations were found only in coastal
waters, and these were limited to relatively shallow depths. In most cases the low PUB cells
were part of dual populations, so that high-PUB cells were present at all but a few stations.
This distribution of pigment types is consistent with the optical characteristics of the oceanic
euphotic zone, in that the cells best adapted to absorb blue light were found in the open ocean
(where blue light penetrates deepest), while the cells with PE adapted for green-light
absorption were limited to shallow, coastal waters where green light is available. It seems
likely that these findings will have an impact on the interpretation of remotely sensed ocean
color measurements, but this will depend on the light absorption by these numerous but very
small cells relative to the total absorption.
Campbell and Iturriaga (1988), using single-cell fluorescence excitation spectra obtained by
microspectrofluorometry, also concluded that Synechococcus in the Sargasso Sea had high
PUB:PEB ratios, and Wood et al. (1985) were able to discriminate between pigment types
with epifluorescence microscopy by examining each cell with two different filter sets. The
rapidity and objectivity of flow cytometry, however, allows us to put these observations on
a quantitative basis for large areas of the world ocean in a way that other methods cannot.
Prochlorophyte picoplankton: Flow cytometry has played an even more pivotal role in our
understanding of the picoplankton group currently designated "prochlorophytes," awaiting
detailed characterization. These cells were first observed in samples from the deep euphotic
zone, although we recognized that their apparent absence from most surface samples was due
to the detection limits of our instrument (Chisholm et al. 1988). More recently we have been
able to measure even the dim cells at the surface by using more sensitive instruments (Olson
et al. 1990b) (Fig. 7). The flow cytometric signatures of these cells were first recognized as
extremely numerous populations with very dim red fluorescence, no orange fluorescence, and
light scatter signals even smaller than Synechococcus. Even at depths where the flow