Journal Article

Nitrate uptake and extracellular alkalinization by the green alga <i>Hydrodictyon reticulatum</i> in blue and red light

W.R. Ullrich, J. Lazarová, C.I. Ullrich, F.G. Witt and P.J. Aparicio

in Journal of Experimental Botany

Published on behalf of Society for Experimental Biology

Volume 49, issue 324, pages 1157-1162
Published in print July 1998 | ISSN: 0022-0957
Published online July 1998 | e-ISSN: 1460-2431 | DOI:
Nitrate uptake and extracellular alkalinization by the green alga Hydrodictyon reticulatum in blue and red light

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Nitrate uptake and the medium alkalinization related to it were studied with nets of the coenocytic, giant cell, green alga Hydrodictyon reticulatum. A comparison of red, blue and white light irradiation showed no special control of nitrate uptake and of the corresponding alkalinization of the external medium by light quality, but rather a response as expected for the photosynthetic apparatus. In the dark, nitrate uptake rates amounted to one-fifth of those in saturating white light. This is in contrast to the chlorococcal microalga Monoraphidium braunii, where blue light specifically switched on nitrate uptake-dependent alkalinization and where uptake and reduction of nitrate strongly depended on blue light; the rates in pure red light and in the dark being very low. The stoichiometric ratio between nitrate taken up and extracellular alkalinization was close to 1 (0.86) in air with CO2 but close to 2 (1.84) in N2 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 alkalinization and some as nitrite, which lowers the ratio. Nitrite and ammonium release rates under anaerobic, CO2-free conditions were also independent of red or blue light and continued for several hours when the medium was buffered at pH 6. The data indicate that nitrate uptake, but less its reduction, is regulated differently in vacuolate, coenocytic algae from microalgae. In Hydrodictyon, nitrate uptake and reduction seem to be controlled by energy supply; in various microalgae, in addition, it is controlled specifically by blue light.

Keywords: Alkalinization; blue light; Hydrodictyon; nitrate uptake; stoichiometry nitrate/ protons

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Subjects: Plant Sciences and Forestry

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