Kinetics of ammonium assimilation in two seaweeds, Enteromorpha sp (Chlorophyceae) and Osmundaria colensoi (Rhodophyceae)

The kinetics of ammonium assimilation were investigated in two seaweeds fro m northeastern New Zealand, Enteromorpha sp. (Chlorophyceae, Ulvales) and O smundaria colensoi (Nook. f. et Harvey) R.E. Norris: (Rhodophyceae, Ceramia les), with the use of a recently developed method for measuring assimilatio n. In contrast to ammonium uptake, which was nonsaturable, ammonium assimil ation exhibited Michaelis-Menten kinetics in both species. Maximum rates of assimilation (V-max) were 27 and 12 mu mol.(g DW)(-1).h(-1) for Enteromorp ha sp. and O. colensoi, respectively, with half-saturation (K-m) constants for assimilation of 18 and 41 mu M At environmentally relevant concentratio ns, assimilation accounted for all of the ammonium taken up by both species . The maximum rate of assimilation in Enteromorpha sp. resembled very close ly that of the ammonium assimilatory enzyme, glutamine synthetase, when act ivities of the latter were measured in the presence of subsaturating substr ate (glutamate and ATP) concentrations. Moreover, the initial rate of gluta mine production (measured with HPLC) following ammonium enrichment was almo st identical to the rates determined above. The rate of ammonium assimilati on was therefore determined by three independent methods, two of which invo lve in vivo measurements, and it is suggested that the use of assimilation kinetics may be useful when examining the nutrient relations of seaweeds.