NITRATE UPTAKE AND EXTRACELLULAR ALKALINIZATION BY THE GREEN-ALGA HYDRODICTYON RETICULATUM IN BLUE AND RED-LIGHT

Nitrate uptake and the medium alkalinization related to it were studie d with nets of the coenocytic, giant cell, green alga Hydrodictyon ret iculatum. A comparison of red, blue and white light irradiation showed no special control of nitrate uptake and of the corresponding alkalin ization of the external medium by light quality, but rather a response as expected for the photosynthetic apparatus. In the dark, nitrate up take rates amounted to one-fifth of those in saturating white light. T his is in contrast to the chlorococcal microalga Monoraphidium braunii , where blue light specifically switched on nitrate uptake-dependent a lkalinization and where uptake and reduction of nitrate strongly depen ded on blue light; the rates in pure red light and in the dark being v ery low. The stoichiometric ratio between nitrate taken up and extrace llular alkalinization was close to 1 (0.86) in air with CO2 but close to 2 (1.84) in N-2 for nitrate preloaded cells. In the absence of any carbon source, a high proportion of the absorbed and reduced nitrogen is released, most of it as ammonium which causes the excess alkaliniza tion and some as nitrite, which lowers the ratio, Nitrite and ammonium release rates under anaerobic, CO2-free conditions were also independ ent of red or blue light and continued for several hours when the medi um was buffered at pH 6, The data indicate that nitrate uptake, but le ss its reduction, is regulated differently in vacuolate, coenocytic al gae from microalgae, In Hydrodictyon, nitrate uptake and reduction see m to be controlled by energy supply; in various microalgae, in additio n, it is controlled specifically by blue light.