Growth kinetics of algal populations exsymbiotic from Paramecium bursaria by flow cytometry measurements

Background: The ciliate Paramecium bursaria normally exists as a green para mecium system because each animal cell carries several hundred, unicellular , green, algal cells in its cytoplasm. One of the remarkable and poorly und erstood pecularities of this system is the steady state in the number of al gae per protozoan cell. A major point in the study of mechanisms governing the persistence of symbiont numbers is adequate understanding of the algal life cycle. Methods: Asynchronously growing cell populations of several algal strains ( SA-1, SA-3, and SA-3) exsymbiotic from P. bursaria were characterized by fl ow cytometry. Algal endogenous chlorophyll and DNA contents were monitored to analyze cell growth kinetics at logarithmic and stationary culture phase s. Cell. sorting visualized the morphology of algae corresponding to the hy perhaploid (2C and 4C) DNA peaks. Results: Cell-division cycle-dependent changes in chlorophyll and DNA conte nt distributions were most dramatic in logarithmically growing algal popula tions tan increase in the number of S-phase cells and cells with more chlor ophyll), which are thought to be associated with accelerated DNA and chloro phyll metabolism in log-phase algal cultures. Upon reaching the stationary phase of growth, algal populations distinctly showed, in addition to one ha ploid (1C) DNA peak, two hyperhaploid peaks (2C and 4C) corresponding mainl y to cells with two and four nuclei, respectively. Conclusions: Growth characteristics of algae exsymbiotic from P. bursaria m onitored by flow cytometry provide valuable information for the analysis of the algal life cycle, which is important for understanding the regulation mechanisms of symbiont numbers. (C) 2001 Wiley-Liss, Inc.