EFFECTS OF SHORT PULSES OF BLUE LIGHT ON THE ALKALINIZATION ASSOCIATED WITH THE UPTAKE OF NO3− AND CL− BY THE GREEN ALGA MONORAPHIDIUM BRAUNII AND RELATED ACTION SPECTRA

In Monoraphidium braunii, uptake of NO3−, NO2− and Cl− is associated with proton transport and triggered by blue light (BL). Only 10 s after cells able to reduce NO3− to NH4+ were irradiated with continuous, low‐fluence BL in the presence of NO3−, an alkalinization of the medium began and only became interrupted by switching off the BL with a 60–90 s time lag. With 30 s BL pulses, the NO3−‐dependent alkalinization lasted 3–5 min until it stopped. When the cells were exposed to continuous BL in the presence of Cl−, the alkalinization also started within 10 s but lasted only 3 min. After that, the pH remained constant and decreased when the BL was switched off. With 30 s BL pulses, the Cl−‐dependent alkalinization lasted 3 min and then decreased to its initial value. The NO3−‐dependent alkalinization shown by cells unable to reduce NO3− to NH4+ was similar to that observed in the presence of Cl−. These alkalinization rates fit the Bunsen‐Roscoe reciprocity law. With 2 s pulses of high‐fluence BL, the delay time of the NO3 ‐ or Cl−‐dependent alkalinizations was only 2 s, one of the fastest BL responses reported so far. The action spectra for Cl− and NO3− uptakes proved to be very similar and matched the absorption spectra of flavins, including the 267 nm peak.