Limiting CO2 levels induce a blue light-dependent HCO3- uptake system in Monoraphidium braunii

The in situ photoactivation of an HCO3- uptake system in the green alga Monoraphidium braunii requires the irradiation of the cell suspensions with short wavelength radiation (blue, UVA and/or UVC). Plasma membrane ATPase inhibitors block the uptake of this monovalent anion at pH 9. M. braunii cells grown in high CO2 lack an HCO3- uptake system in their plasma membrane, but those grown in low CO2 can take up this anion at high rates. Cells grown in high CO2, transferred to CO2-limiting conditions in the light, start taking up HCO3- in 30 min, although they take 90 min to reach maximum rates of HCO3- transport. Therefore, this induction process seems to be triggered by low external CO2 concentration. In fact, increasing or decreasing the external HCO3- concentration does not induce the uptake system and only a decrease in CO2 concentration in the medium triggers the induction process. The appearance of the HCO3- transport activity is sensitive to cycloheximide, indicating that cytoplasmic protein biosynthesis is necessary for the induction of the uptake system. Photosynthetically active radiation, but not particularly blue light, is essential for induction of the uptake system to occur and the inhibition of photosynthesis by DCMU blocks it. From these results it can be inferred that when M. braunii cells detect a drop in CO2 concentration, they induce a blue light-dependent HCO3- uptake system.