Growth kinetics of algal populations exsymbiotic fromParamecium bursaria by flow cytometry measurements

Abstract

The ciliate Paramecium bursaria normally exists as a green paramecium system because each animal cell carries several hundred, unicellular, green, algal cells in its cytoplasm. One of the remarkable and poorly understood pecularities of this system is the steady state in the number of algae 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. Asynchronously growing cell populations of several algal strains (SA-1, SA-3, and SA-9) exsymbiotic from P. bursaria were characterized by flow cytometry. Algal endogenous chlorophyll and DNA contents were monitored to analyze cell growth kinetics at logarithmic and stationary culture phases. Cell sorting visualized the morphology of algae corresponding to the hyperhaploid (2C and 4C) DNA peaks. Cell-division cycle-dependent changes in chlorophyll and DNA content distributions were most dramatic in logarithmically growing algal populations (an increase in the number of S-phase cells and cells with more chlorophyll), which are thought to be associated with accelerated DNA and chlorophyll metabolism in log-phase algal cultures. Upon reaching the stationary phase of growth, algal populations distinctly showed, in addition to one haploid (1C) DNA peak, two hyperhaploid peaks (2C and 4C) corresponding mainly to cells with two and four nuclei, respectively. Growth characteristics of algae exsymbiotic from P. bursaria monitored 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.