Fig. 1. Uptake of dissolved inorganic nitrogen from sea water by the Haliclona—Ceratodictyon symbiosis. (A) Uptake of ammonium from sea water spiked with 20 µmol l^-1 NH₄Cl, when branches were incubated in the light (N=3). (B) Uptake of nitrate from `normal', unspiked sea water, when branches were incubated in the light (N=5). Values are means ± S.D. Ammonium and nitrate concentrations in sea water without the association did not change significantly over time (one-way analysis of variance, P>0.2, N=3 for both nutrients), with ammonium concentrations being 20.84±0.83 µmol l^-1 and 18.84±2.89 µmol l^-1 and nitrate concentrations being 2.10±0.27 µmol l^-1 and 1.36±0.43 µmol l^-1 at the start and end of the experiments, respectively.