378
larger phytoplankton (which are difficult to sample quantitatively) were too noisy to detect any
pattern. It is worth noting that most eukaryotic phytoplankton species divide at night
(Chisholm 1981), so we would not expect the concentration of the larger cells to increase
during the day, when beam attenuation increases.
A simple explanation for the FLS patterns we observed is that they reflect cell growth during
the day as the cells photosynthesize; cell division and respiration at night would then cause
the mean FLS per cell to decrease. In a bottle incubation experiment in which Synechococcus
was monitored (Fig. 16), this explanation is supported by the cell concentration data: the cells
increased most in numbers after sunny days I and 3, and the FLS increases during the sunny
days were also larger than during the intervening cloudy day.
An alternate explanation for the observed patterns in light scatter could be that physiological
changes affect the cells' scattering properties without actually changing their biomass. There
is some evidence for this in the form of observations of abrupt changes in light scattering on
a short time scale when light intensity is changed or a culture is diluted with fresh medium
(Ackleson et al. 1988). The mechanisms underlying these changes, however, remain to be
resolved.
Physiology
Cell growth as reflected in cell size or cell cycle measurements represents an integration of
many physiological processes such as photosynthesis, photoacclimation, and nutrition. Several
of these processes have been studied using flow cytometric analysis. These can be as simple
as measurements of autofluorescence and cell size. For example, Sakshaug et al. (1987) used
flow cytometry to monitor subpopulations in cultures of the diatom Thalassiosira pseudonana
(Fig. 17). Upon transfer to very low light, cultures developed bimodal fluorescence
distributions, with the appearance of small, low-fluorescence cells. The average fluorescence
per cell of the whole population decreased with time in low light, but this was due to an
increase in the proportion of low-fluorescence cells; the average fluorescence of the cells in
each of the two populations was stable. The low-fluorescence cells were viable, since sorted
larger phytoplankton (which are difficult to sample quantitatively) were too noisy to detect any
pattern. It is worth noting that most eukaryotic phytoplankton species divide at night
(Chisholm 1981), so we would not expect the concentration of the larger cells to increase
during the day, when beam attenuation increases.
A simple explanation for the FLS patterns we observed is that they reflect cell growth during
the day as the cells photosynthesize; cell division and respiration at night would then cause
the mean FLS per cell to decrease. In a bottle incubation experiment in which Synechococcus
was monitored (Fig. 16), this explanation is supported by the cell concentration data: the cells
increased most in numbers after sunny days I and 3, and the FLS increases during the sunny
days were also larger than during the intervening cloudy day.
An alternate explanation for the observed patterns in light scatter could be that physiological
changes affect the cells' scattering properties without actually changing their biomass. There
is some evidence for this in the form of observations of abrupt changes in light scattering on
a short time scale when light intensity is changed or a culture is diluted with fresh medium
(Ackleson et al. 1988). The mechanisms underlying these changes, however, remain to be
resolved.
Physiology
Cell growth as reflected in cell size or cell cycle measurements represents an integration of
many physiological processes such as photosynthesis, photoacclimation, and nutrition. Several
of these processes have been studied using flow cytometric analysis. These can be as simple
as measurements of autofluorescence and cell size. For example, Sakshaug et al. (1987) used
flow cytometry to monitor subpopulations in cultures of the diatom Thalassiosira pseudonana
(Fig. 17). Upon transfer to very low light, cultures developed bimodal fluorescence
distributions, with the appearance of small, low-fluorescence cells. The average fluorescence
per cell of the whole population decreased with time in low light, but this was due to an
increase in the proportion of low-fluorescence cells; the average fluorescence of the cells in
each of the two populations was stable. The low-fluorescence cells were viable, since sorted