Interactions in the assimilation of nitrogen compounds by unicellular algae

• Main Author: Molloy, C. J.
• Published: Swansea University 1987
• Subjects: 571.29
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.638223

Interactions during the simultaneous uptake of ammonium and urea, and urea and nitrate, were studied in a variety of microalgae, especially the marine diatom <i>Phaeodactylum tricornutum</i> and the soil Chlorophycean <i>Chlorella fusca</i>. Inhibition of urea uptake by ammonium was often observed. The extent of this inhibition was dependent on the ammonium concentration used, the nitrogen status of the cells, and the species identity. High ammonium concentrations (relative to urea) strongly inhibited urea uptake, particularly in cultures which had previously been deprived of nitrogen for a prolonged period (48h). At low ammonium concentrations (350 μM), strong inhibition of urea uptake was observed with <i>Chlorella</i>, but not with <i>Phaeodactylum</i>, and further experiments revealed that <i>Chlorella</i> was more sensitive to ammonium inhibition of urea uptake than was <i>Phaeodactylum</i>. When urea and nitrate were added simultaneously to cultures of <i>Phaeodactylum</i> or <i>Chlorella</i>, both substances were assimilated, although the rate of uptake of either was lower than when the substance was presented as sole nitrogen source. Inhibition of nitrate uptake by urea was apparent in most experiments, and was strongest in non-illuminated cultures, previously exposed to a prolonged period of nitrogen deprivation. Experiments on the simultaneous assimilation of urea and NH_4^+ by the marine microflagellate <i>Mantoniella squamata</i> were complicated by an apparently high nickel requirement of this organism with relation to urea uptake. This was shwon to be an artefact: although nickel was found to be a necessary constituent of <i>Mantoniella</i> urease, the high nickel requirement was the result of strong chelation of free nickel ions in the medium, by the growth buffer 'bicine'.