Finally, fluctuations in external CO2 concentrations have been a popular line of investigation in marine and freshwater algae with CCMs (Matsuda, Bozzo and Colman 1998; Woods 1999; Colman 2000)
It has been recently demonstrated that in cells of Chlamydomonas reinhardii, external CO2 concentrations can affect the active uptake of CO2 and HCO3- and has been show to be suppressed in Chlamydomonas reinhardii grown in high external CO2 after about 8 days in 5% CO2 (pers com Colman 2000). After this period the new generation of cells lost the CCM capacity. However when transferred to a low CO2 external environment, active transport of CO2 and HCO3- within 2hrs of acclimation, and CA ext activity increased 10 fold after 6 hours after acclimation to 0.035% Co2. It has been proposed that the CCM is induced when the CO2 concentration in the medium is reduced to a critical level.
Matsuda, Bozzo and Colman (1998) have suggested the possibility of a CO2 sensor at the green algal cell surface, which under high CO2 growth conditions would cause repression of the CCM, whereas under CO2 depletion the sensing mechanism would initiate a signaling cascade culminating in the derepression of the CCM. Woods (1999) also was able to down regulate the CCMs in Chlorella spp. and Trebuxia spp .with a 2 day 5% CO2 treatment and then upregulated it with low CO2 treatment only after 2 hours.