Acclimation to low or limiting CO2 in non-synchronous Chlamydomonas causes a transient synchronization of the cell division cycle

Acclimation of Chlamydomonas reinhardtii (hereafter, Chlamydomonas) to low or limiting CO2 or inorganic carbon (C-i) has been studied fairly extensively with regard to the mechanisms underlying the inducible C-i acquisition systems and the signal transduction pathway involved in recognizing and responding to decreased C-i availability. Investigation of low C-i acclimation responses typically is performed with non-synchronous cultures grown in continuous light to avoid any effects of the cell division cycle (CDC) confounding interpretation of acclimation responses. However, little is known about whether acclimation to low C-i might affect the distribution of cells among the various stages of the CDC. To investigate the effects of a limiting-C-i challenge on the CDC of Chlamydomonas, flow cytometry was used to monitor the distribution of cells among the CDC stages in both synchronous and non-synchronous cultures during acclimation to low or limiting C-i. When faced with C-i limitation, non-synchronous cultures of Chlamydomonas undergo transient synchronization as those cells past the Commitment point of the CDC undergo division, while the remainder of the cells pause their growth in early G-phase, with the result that the cells all accumulate in early G-phase, appearing transiently synchronized until acclimated sufficiently to the decreased C-i for growth to resume. This perturbation of the CDC by a limiting-C-i challenge has important implications for the interpretation of gene expression and other responses apparently induced by low or limiting C-i.