Journal of Experimental Botany, Vol 49, 1157-1162, Copyright © 1998 by Oxford University Press
W Ullrich, J Lazarova, C Ullrich, F Witt and P Aparicio
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 pre-loaded 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 seems 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.
ARTICLES
Nitrate uptake and extracellular alkalinization by the green alga Hydrodictyon reticulatum in blue and red light
Institut fur Botanik, Technische Universitat, Schnitspahnstr. 10, D-64287 Darmstadt, Germany; Laboratory for Membrane Transport, Institute of Microbiology, Videnska 1083, CR-142 20 Prague 4, Czech Republic; Centro de Investigaciones Biologicas, CSIC, Velazquez 144, E-28006 Madrid, Spain; Corresponding author; e-mail: ullrich@bio1.bio-tu-darmstadt.de
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