Growth, photosynthetic rate, and induction of two low CO2 inducible genes of Chlamydomonas reinhardtii strain CC125 were quantified in a range of physiologically relevant CO2 and O2 concentrations (5% to 0.005% and 20% or 2% O2) using airlift bioreactors to facilitate the simultaneous measurement of both growth and in situ photosynthesis measurements. Within these CO2 concentration ranges, O2 concentrations (20% versus 2%) had no significant affect on growth, photosynthetic rate, or induction of the periplasmic carbonic anhydrase (Cah1) and glycolate dehydrogenase (Gdh) genes in wild type C. reinhardtii. It has been suggested that the CO2/O2 ratio might act as a signal for the up-regulation of limiting CO2 induced genes and/or of the CCM, but these results failed to support the hypothesis that the CO2/O2 ratio plays any role in signaling. The mRNA abundance of the Cah1 and Gdh genes appeared to be regulated in concert, suggesting co-regulation by the same signaling pathway, which, due to a lack of an O2 affect, seems unlikely to involve photorespiration or photorespiratory metabolites. Instead, it appeared that the CO2 concentration alone was responsible for regulation of limiting CO2 acclimation responses. Based on growth, photosynthesis and gene expression characteristics, three distinct CO2-regulated physiological states can be recognized within the studied parameters, a High CO2 (5% to 0.5%) state, a Low-CO2 (0.4% to 0.03%) state, and a Very Low-CO2 (0.01% to 0.005%) state. The concentration range where the external CO2 induced Cah1 expression and Gdh up-regulation was between 0.5% and 0.4% CO2, differentiating the High from the Low CO2 states. Photosynthetic characteristics monitored in cultures acclimated to a CO2 concentration for up to 52 hours were distinct in the three CO2-regulated physiological states. For example, the estimated K0[subscript .]5(CO2) of the Low CO2 state was 10 [mu]M CO2 and that of the Very Low CO2 state was 0.9 [mu]M CO2. In addition to a greater photosynthetic CO2 affinity, the Very Low CO2 state could be distinguished from the Low CO2 state by an increased cell doubling time and a smaller cell size.