NITROGEN UPTAKE KINETICS OF FRESHLY ISOLATED ZOOXANTHELLAE

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.