Blue Light Requirement for HC03 Uptake and Its Action Spectrum in Monoraphidium braunii

The uptake and assimilation of HCO₃ by the green unicellular alga Monoraphidium braunii can be monitored by the alkalinization of the external medium or by the O₂ evolution associated with the uptake and reduction of this anion. The activation of HCO₃ uptake in this microalga required the irradiation of the cell suspensions with low photon fluence rates of short wavelength radiation. Thus, when the cells were irradiated with strong red light in the presence of HCO₃, very little alkalinization of the external medium or O₂ evolution could be observed. The O₂ evolution rates measured under red light could be due to the assimilation of the CO₂ derived from the HCO₃ present in the medium. The blue light-dependent O₂ evolution rates were not diminished by a periplasmic carbonic anhydrase inhibitor, suggesting that HCO₃ -dependent O₂ evolution was due to the photoactivation of a selective HCO₃ uptake system at the plasma membrane. The action spectrum for HCO₃- uptake in M. braunii was very similar to those reported for NO₃- and CI^- suggested that a flavoprotein may be the photoreceptor for this response.