385
Since the flow cytometer makes measurements on individual cells, we also have access to
information contained in the distribution of properties within each population. In the example
illustrated above, the distributions as well as the means in fluorescence per cell show some
dramatic changes with depth, including skewing of the distribution near the seasonal
thermocline (Fig. 21). This skewness is predicted to occur at a reflecting boundary layer using
a single cell mixing model (Dusenberry et al. 1990). We have also seen bimodal distributions
in the prochlorophytes near the nitracline, which we hypothesize to reflect a transition from
nutrient-limited to light-limited growth. The record of each individual cell's trajectory through
its environment represented by these measurements may provide insights into complex
processes, such as mixing, that are not possible in any other way.
Bacteria: High sensitivity flow cytometry has been used to characterize the heterotrophic as
well as the phototrophic prokaryotes in seawater (Robertson and Button 1989; Button and
Robertson 1989; Frankel et ai. 1990). Since most bacteria have no autofluorescent pigments,
this has been done using light scattering and the fluorescent DNA stain DAPI. In some
samples, flow cytometric analysis has revealed distinct subpopulations among the bacteria
(Button 1990), and there is potential for growth rate information in the DNA distributions
obtained. Cell cycle regulation has recently been studied in both heterotrophic marine bacteria
(Henderson 1990) and cyanobacteria (Armbrust et ai. 1989; Binder and Chisholm 1990) with
flow cytometry in the laboratory, thus we are beginning to develop the models necessary to
interpret field data in terms of growth rates. At present, even simple enumeration of bacteria
in seawater samples is typically carried out by epifluorescence microscopy, but the advent of
taxa-specific fluorescent probes (see below) should make flow cytometry an increasingly
useful tool in marine microbiology.
Probes: Advances in molecular biology are being applied with increasing frequency to
oceanographic questions (see chapter by DeLong, this volume). Our knowledge of the marine
bacterial community, for example, is very sketchy because most of the cells in a given water
sample cannot yet be cultured and the cells cannot easily be distinguished by morphology.
Molecular biological probes, however, can allow us to study specific cell types as well as total
bacterial biomass, and thus to ask more detailed questions about the distribution and function
Since the flow cytometer makes measurements on individual cells, we also have access to
information contained in the distribution of properties within each population. In the example
illustrated above, the distributions as well as the means in fluorescence per cell show some
dramatic changes with depth, including skewing of the distribution near the seasonal
thermocline (Fig. 21). This skewness is predicted to occur at a reflecting boundary layer using
a single cell mixing model (Dusenberry et al. 1990). We have also seen bimodal distributions
in the prochlorophytes near the nitracline, which we hypothesize to reflect a transition from
nutrient-limited to light-limited growth. The record of each individual cell's trajectory through
its environment represented by these measurements may provide insights into complex
processes, such as mixing, that are not possible in any other way.
Bacteria: High sensitivity flow cytometry has been used to characterize the heterotrophic as
well as the phototrophic prokaryotes in seawater (Robertson and Button 1989; Button and
Robertson 1989; Frankel et ai. 1990). Since most bacteria have no autofluorescent pigments,
this has been done using light scattering and the fluorescent DNA stain DAPI. In some
samples, flow cytometric analysis has revealed distinct subpopulations among the bacteria
(Button 1990), and there is potential for growth rate information in the DNA distributions
obtained. Cell cycle regulation has recently been studied in both heterotrophic marine bacteria
(Henderson 1990) and cyanobacteria (Armbrust et ai. 1989; Binder and Chisholm 1990) with
flow cytometry in the laboratory, thus we are beginning to develop the models necessary to
interpret field data in terms of growth rates. At present, even simple enumeration of bacteria
in seawater samples is typically carried out by epifluorescence microscopy, but the advent of
taxa-specific fluorescent probes (see below) should make flow cytometry an increasingly
useful tool in marine microbiology.
Probes: Advances in molecular biology are being applied with increasing frequency to
oceanographic questions (see chapter by DeLong, this volume). Our knowledge of the marine
bacterial community, for example, is very sketchy because most of the cells in a given water
sample cannot yet be cultured and the cells cannot easily be distinguished by morphology.
Molecular biological probes, however, can allow us to study specific cell types as well as total
bacterial biomass, and thus to ask more detailed questions about the distribution and function