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

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Abstract
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.