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Key words: Alkalinization, blue light, Hydrodictyon, nitrateuptake, stoichiometry nitrate/protons.Nitrate uptake and the medium alkalinization relatedto it were studied with nets of the coenocytic, giantcell, green alga Hydrodictyon reticulatum. A compar- Introduction ison of red, blue and white light irradiation showed nospecial control of nitrate uptake and of the corres- In several species of microalgae, blue light signals areponding alkalinization of the external medium by light involved in nitrate uptake and nitrate reduction, but soquality, but rather a response as expected for the far it has been unknown whether this is a general charac-photosynthetic apparatus. In the dark, nitrate uptake teristic of all algae or specically of microalgae. Bothrates amounted to one-fifth of those in saturating nitrate uptake and nitrate reduction depend on metabolicwhite light. This is in contrast to the chlorococcal energy (Ullrich, 1983, 1992). Nitrate uptake depends onmicroalga Monoraphidium braunii, where blue light ATP via the plasma membrane H+-ATPase in a secondaryspecifically switched on nitrate uptake-dependent active H+co-transport process. Nitrate reduction requiresalkalinization and where uptake and reduction of reducing equivalents. The subsequent ammonium assim-nitrate strongly depended on blue light; the rates in ilation via GS-GOGAT requires ATP, reducing equiva-pure red light and in the dark being very low. The lents and the supply of carbon skeletons. Enzymes ofstoichiometric ratio between nitrate taken up and nitrate assimilation, including the membrane carriers,extracellular alkalinization was close to 1 (0.86) in air have constitutive components to maintain a low level ofwith CO activity in the dark or in the absence of substrate. Nitrate,2but close to 2 (1.84) in N2for nitrate pre-loaded cells. In the absence of any carbon source, a as well as nitrite, usually induces high amounts andhigh proportion of the absorbed and reduced nitrogen activities of the related proteins, but in many casesis released, most of it as ammonium which causes the induction additionally requires light. In the microalga M.excess alkalinization and some as nitrite, which lowers braunii, short-term N starvation for a few hours in thethe ratio. Nitrite and ammonium release rates under light promotes the greatest increase in nitrate assimilationanaerobic, CO (Ullrich et al., 1981). However, algal families or even2-free conditions were also independentof red or blue light and continued for several hours individual species may behave diVerently in their responsewhen the medium was buffered at pH 6. The data indi- to light, particularly to light quality, whereas nitrate andcate that nitrate uptake, but less its reduction, is regu- nitrite always support the induction of their own meta-lated differently in vacuolate, coenocytic algae from bolic machinery, particularly in the absence of externalNH+4microalgae. In Hydrodictyon, nitrate uptake and reduc- . In these respects, pioneer studies were carried outtion seem to be controlled by energy supply; in various with the unicellular microalgae Chlorella (Calero et al.,microalgae, in addition, it is controlled specifically by 1980), Monoraphidium (Quin˜ones and Aparicio, 1990;blue light. Aparicio et al., 1994) and Chlamydomonas (Azuara and

Nitrate uptake and extracellular alkalinization by the green alga Hydrodictyon reticulatum in blue and red light