Fig. 4. Nitrogen budget for the Haliclona—Ceratodictyon symbiosis. The budget incorporates the maximum ammonium release rate in darkness (0.110 mg Ng^-1 dry mass day^-1), the growth rate of the association (0.83% day^-1, as reported by Trautman et al., 2000), the alga:sponge biomass ratio (70%:30% alga:sponge) and the nitrogen content of the two partners (alga, 1.9%; sponge, 6.7%; both on a dry mass basis). Nitrogen that is surplus to the growth requirements of the alga is assumed to be translocated back to the sponge, and the budget is balanced by the presumed acquisition of nitrogen from the ambient environment. Abbreviations: dissolved inorganic nitrogen (DIN); dissolved organic nitrogen (DON); particulate organic nitrogen (PON). Biomass units are mg nitrogen g^-1 dry mass of the whole association. Flux units are mg nitrogen g^-1 dry mass of the whole association per day. Solid arrows are used for proven fluxes (quantified directly except where stated as `derived'), while broken arrows are used for possible fluxes.