Zooxanthellae freshly isolated from the coral host Pocillopora damicor
nis exhibited substrate-saturable uptake kinetics for ammonium, nitrat
e and urea. Maximum uptake velocity for ammonium [10. 1 nmol. (mug chl
-a).-1h-1] was greater than that for nitrate [2.8 nmol.(mug chl-a).-1h
-1] and urea [0.37 nmol. (mug chl-a).-1h-1]. Half-saturation constants
for uptake of the three nitrogen compounds were in the range of 10-15
mumol.l-1. Generally, uptake of any one nitrogen substrate appears to
be inhibited by the other 2 substrates when the latter are present in
higher concentrations. The observed interactions can be explained by
the uncompetitive model. These results are interpreted as indicative o
f a simultaneous uptake of all 3 nitrogen compounds by the zooxanthell
ae.