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 studiedwith nets of the coenocytic, giant cell, green alga Hydrodictyonreticulatum. A comparison of red, blue and white lightirradiation showed no special control of nitrate uptake and of thecorresponding alkalinization of the external medium by light quality, butrather a response as expected for the photosynthetic apparatus. In thedark, nitrate uptake rates amounted to one-fifth of those in saturatingwhite light. This is in contrast to the chlorococcal microalgaMonoraphidium braunii, where blue light specificallyswitched on nitrate uptake-dependent alkalinization and where uptake andreduction of nitrate strongly depended on blue light; the rates in pure redlight and in the dark being very low. The stoichiometric ratio betweennitrate taken up and extracellular alkalinization was close to 1 (0.86) inair with CO2 but close to 2 (1.84) in N2 for nitrate pre-loaded cells. Inthe absence of any carbon source, a high proportion of the absorbed andreduced nitrogen is released, most of it as ammonium which causes theexcess alkalinization and some as nitrite, which lowers the ratio. Nitriteand ammonium release rates under anaerobic, CO2-free conditions were alsoindependent of red or blue light and continued for several hours when themedium was buffered at pH 6. The data indicate that nitrate uptake, butless its reduction, is regulated differently in vacuolate, coenocytic algaefrom microalgae. In Hydrodictyon, nitrate uptake andreduction seems to be controlled by energy supply; in various microalgae,in addition, it is controlled specifically by bluelight.