Corals are able to conserve and recycle inorganic nutrients in a generally nutrient poor environment due to their photosynthesizing symbionts. However it has been demonstrated that sponges are the main producers of nitrogenous nutrients in coral reefs contributing to a greater extent to the nutrient recycling than corals. The present study examined if sponges support corals in terms of nutrient supply. The study further investigated if sponges have an influence on the “well-being” of corals by releasing additional nutrients. Corals and sponges were incubated separately and together in either artificial seawater or artificial seawater enriched with cyanobacteria and nutrients (ammonium and nitrate). Changes in dissolved inorganic nutrients, in total dissolved nitrogen and dissolved organic nitrogen as well as in dissolved and total organic carbon were measured and compared between the different treatments. Furthermore, particulate matter on filters of the water samples was analysed for its carbon and nitrogen contents and ratios. According to the carbon and nitrogen measurements, sponges ingested cyanobacteria while the carbon and nitrogen ratios of the corals are not indicative of a consumption of cyanobaceria. An accumulation of nitrate was detected when sponges alone were incubated in water enriched with cyanobacteria and nutrients. This accumulation was not found in incubations of corals alone. When both organisms were incubated together a slight elevation in nitrate and nitrite concentrations occurred. Besides, the maximum electron transport rate of the coral symbionts was significantly higher in the presence of a sponge. These results indicate that sponges are able to convert the added cyanobacteria to nutrients and that corals may directly benefit from the additional nutrients released by the sponges.