358
such as this, however, are more difficult to interpret because several processes are likely to
co-occur (i.e., removal and production of particles), and because the heterogeneity of natural
particle assemblages may make discrimination among particle classes difficult.
Flow cytometry has also allowed us to demonstrate food selection by grazers on the basis of
food "quality" within a single species of phytoplankter (Cowles et al. 1988). For this
experiment, diatoms were grown in N-limited chemostats at high and low growth rates. Even
though both kinds of cells were the same size, they could be distinguished from each other
by their chlorophyll fluorescence: the fast-growing cells had more chlorophyll than the
slow-growing, severely N-limited ones (Fig. SA). When a mixture of the two types was
grazed by copepods, more of the fast-growing cells were removed (Fig. SB), indicating that
the grazers could sense some difference at the single-cell level.
Direct detection of particles ingested by individual grazers is an area where flow cytometry
promises to be a particularly useful tool in terms of both quantitation and speed. In a flow
cytometric study of grazing by the ciliate Tetrahymena pyriformis, it has been shown that cell
fluorescence from ingested polystyrene beads increases linearly as a function of bead ingestion
over a range from 1 to 150 beads (Lavin et al. 1990). Similarly, phycoerythrin-containing
Synechococcus cells have been used to measure feeding by the chlorophyll-containing
chrysophyte Ochromonas (Cucci et al. 1989). Grazers that ingest Synechococcus cells become
distinctively labelled by their increased orange fluorescence. Non-fluorescent particles can also
be stained for use in direct grazing studies; for example, grazing on fluorescently labelled
bacteria is currently measured using epifluorescence microscopy (Sherr et al. 1987) and
should be easily detected and quantified using flow cytometry.
Gerritsen et al. (1987) compared flow cytometric and microscopic analyses of grazing. They
found that the flow cytometer was faster and required less sample preparation, but pointed out
that background fluorescence could in some cases bias the results obtained by flow cytometry.
Background fluorescence is a problem that must be dealt with in many flow cytometric
studies. With the aid of a microscope, the human eye can easily distinguish the subtle
differences in color and shape between the discrete ingested food particles within the grazer
and the dim overall background fluorescence of detritus or the grazer itself, while the flow
such as this, however, are more difficult to interpret because several processes are likely to
co-occur (i.e., removal and production of particles), and because the heterogeneity of natural
particle assemblages may make discrimination among particle classes difficult.
Flow cytometry has also allowed us to demonstrate food selection by grazers on the basis of
food "quality" within a single species of phytoplankter (Cowles et al. 1988). For this
experiment, diatoms were grown in N-limited chemostats at high and low growth rates. Even
though both kinds of cells were the same size, they could be distinguished from each other
by their chlorophyll fluorescence: the fast-growing cells had more chlorophyll than the
slow-growing, severely N-limited ones (Fig. SA). When a mixture of the two types was
grazed by copepods, more of the fast-growing cells were removed (Fig. SB), indicating that
the grazers could sense some difference at the single-cell level.
Direct detection of particles ingested by individual grazers is an area where flow cytometry
promises to be a particularly useful tool in terms of both quantitation and speed. In a flow
cytometric study of grazing by the ciliate Tetrahymena pyriformis, it has been shown that cell
fluorescence from ingested polystyrene beads increases linearly as a function of bead ingestion
over a range from 1 to 150 beads (Lavin et al. 1990). Similarly, phycoerythrin-containing
Synechococcus cells have been used to measure feeding by the chlorophyll-containing
chrysophyte Ochromonas (Cucci et al. 1989). Grazers that ingest Synechococcus cells become
distinctively labelled by their increased orange fluorescence. Non-fluorescent particles can also
be stained for use in direct grazing studies; for example, grazing on fluorescently labelled
bacteria is currently measured using epifluorescence microscopy (Sherr et al. 1987) and
should be easily detected and quantified using flow cytometry.
Gerritsen et al. (1987) compared flow cytometric and microscopic analyses of grazing. They
found that the flow cytometer was faster and required less sample preparation, but pointed out
that background fluorescence could in some cases bias the results obtained by flow cytometry.
Background fluorescence is a problem that must be dealt with in many flow cytometric
studies. With the aid of a microscope, the human eye can easily distinguish the subtle
differences in color and shape between the discrete ingested food particles within the grazer
and the dim overall background fluorescence of detritus or the grazer itself, while the flow