Variance within homogeneous phytoplankton populations, II: Analysis of clonal cultures.

In part I of this series of articles, a framework was presented for interpreting histograms of volume or fluorescence as measured by a flow cytometer on homogeneous phytoplankton populations. In this paper, the analytical framework is applied to flow cytometric histograms from laboratory experiments involving clonal phytoplankton cultures. The density function derived in part I was modified to include a third parameter representing a linear shift of the origin. This modified density function was fitted to chlorophyll fluorescence histograms for populations believed to be asynchronous (grown in continuous light) and also to histograms from populations grown on a 14:10 (h:h) light/dark cycle. Near-synchronous subpopulations sorted from an asynchronous population were also analyzed. In populations in which underlying assumptions (asynchronous divisions, constant growth) are valid, curve fits provide estimates of the inherent variability among cells at age 0. The implication of fitting the density function to populations in which these assumptions are not valid is discussed.