374
wavelengths might reduce this source of error, as well as allow some information to be gained
about the accessory pigment composition of the individual cells (e.g., Olson et al. 1989).
Measurements of single-cell fluorescence should be useful in modeling photosynthetic rates
in the sea and in studies of bulk water absorption. Effects of nutrient limitation and absorption
by photoprotective pigments in cells near the surface remain to be investigated.
Light Scatter: The combination of forward and side scatter signals normally measured in flow
cytometry can yield useful information about the size and refractive index of individual
particles (see chapter by G. Salzman, this volume). Using polystyrene particles (spherical,
homogenous particles with constant refractive index), Ackleson and Spinrad (1988) have
shown that measured forward and side scatter values agreed well with differential scattering
cross-sections computed from Mie theory. The ratio of forward to side scatter can be used as
an indicator of refractive index (Spinrad and Brown 1986), as demonstrated by analyzing
droplets of oils with different refractive indices. Ackleson and Spinrad (1988) combined these
approaches by plotting forward against side scatter, and comparing the data for measurements
of several oil suspensions against Mie computation results assuming either constant particle
size and varying refractive index or vice versa (Fig. 14). The measured data for each
suspension of oil droplets (which covered a range of sizes) fell on a line parallel to the
computed curves for constant refractive index but not to those for constant size. This approach
has been used to evaluate the size and refractive index of both cultures (Ackleson and Spinrad
1988) and natural populations of phytoplankton (Ackleson et al. 1988), with the calculated
results close to expected values. This is encouraging, considering that the calculations assumed
spherical cells with constant and homogenous refractive index, which are not true for
phytoplankton cells with cell membranes and inclusions such as nuclei and chloroplasts.
Most studies using flow cytometry to measure cell size have utilized either direct
measurements of Coulter volume (e.g., Yentsch et al. 1986; Li and Wood 1988; Demers et
al. 1989), or empirically derived relationships between cell size and forward light scatter
(FLS). The relatively tight correlations obtained in empirical calibrations (Robertson and
Button 1988; Olson et al. 1988) suggest that for many cell types, deviations from sphericity
and homogeneity may not be of major importance, and that quantitative interpretations of light
wavelengths might reduce this source of error, as well as allow some information to be gained
about the accessory pigment composition of the individual cells (e.g., Olson et al. 1989).
Measurements of single-cell fluorescence should be useful in modeling photosynthetic rates
in the sea and in studies of bulk water absorption. Effects of nutrient limitation and absorption
by photoprotective pigments in cells near the surface remain to be investigated.
Light Scatter: The combination of forward and side scatter signals normally measured in flow
cytometry can yield useful information about the size and refractive index of individual
particles (see chapter by G. Salzman, this volume). Using polystyrene particles (spherical,
homogenous particles with constant refractive index), Ackleson and Spinrad (1988) have
shown that measured forward and side scatter values agreed well with differential scattering
cross-sections computed from Mie theory. The ratio of forward to side scatter can be used as
an indicator of refractive index (Spinrad and Brown 1986), as demonstrated by analyzing
droplets of oils with different refractive indices. Ackleson and Spinrad (1988) combined these
approaches by plotting forward against side scatter, and comparing the data for measurements
of several oil suspensions against Mie computation results assuming either constant particle
size and varying refractive index or vice versa (Fig. 14). The measured data for each
suspension of oil droplets (which covered a range of sizes) fell on a line parallel to the
computed curves for constant refractive index but not to those for constant size. This approach
has been used to evaluate the size and refractive index of both cultures (Ackleson and Spinrad
1988) and natural populations of phytoplankton (Ackleson et al. 1988), with the calculated
results close to expected values. This is encouraging, considering that the calculations assumed
spherical cells with constant and homogenous refractive index, which are not true for
phytoplankton cells with cell membranes and inclusions such as nuclei and chloroplasts.
Most studies using flow cytometry to measure cell size have utilized either direct
measurements of Coulter volume (e.g., Yentsch et al. 1986; Li and Wood 1988; Demers et
al. 1989), or empirically derived relationships between cell size and forward light scatter
(FLS). The relatively tight correlations obtained in empirical calibrations (Robertson and
Button 1988; Olson et al. 1988) suggest that for many cell types, deviations from sphericity
and homogeneity may not be of major importance, and that quantitative interpretations of light