Nitrogen uptake kinetics of freshly isolated zooxanthellae

Abstract

Zooxanthellae freshly isolated from the coral host Pocillopora damicornis exhibited substrate-saturable uptake kinetics for ammonium, nitrate 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 of a simultaneous uptake of all 3 nitrogen compounds by the zooxanthellae.