Phenotypic heterogeneity in a batch culture of Chlamydomonas reinhardtii with different light tolerances

Abstract

Phenotypic diversity is expected in natural microbial populations of genetically diverse individuals. However, phenotypic variation has also been observed in isogenic cultures. Using Percoll density gradients, I isolated two subpopulations of Chlamydomonas reinhardtii from two-day old batch cultures. The least dense subpopulation made up 86% of the culture; cells were larger, had greater growth, respiration, and photosynthetic rates, and greater chlorophyll and protein content per cell. Although the denser population comprised only 14% of the population and cells were not as metabolically active, it showed a greater capacity for dissipating excess light energy, was more tolerant of high-light stress, and had a greater resistance to accumulation of reactive oxygen species, suggesting a more global activation of light-stress pathways. The presence of these phenotypic variants may be part of a “bet-hedging” strategy, where distinct gene expression profiles in a population gives a selective advantage under different environmental conditions.