This study was designed to assess the importance of dissolved free amino acids (DFAA) as a nitrogen source for the scleractinian coral Stylophora pistillata. For this purpose, experiments were performed using (15)N-enriched DFAAs, and %(15)N enrichment was measured both in animal tissue and zooxanthellae at different DFAA concentrations, incubation time and light levels. As previously observed for urea, which is another source of organic nitrogen, DFAA uptake exhibited a biphasic mode consisting of an active carrier-mediated transport for concentrations below 3 micromol l(-1) and a linear uptake for higher concentrations. The value of the carrier affinity (K(m)=1.23 micromol l(-1) DFAA) indicated good adaptation of the corals to the low levels of DFAA concentrations measured in most oligotrophic waters. DFAA uptake was also correlated with light. The DFAA contribution to the nitrogen requirements for tissue growth was compared to the contribution of ammonia, nitrate and urea, for which uptake was also measured in S. pistillata. Inorganic sources (NH(4)(+) and NO(3)(-)) contributed 75% of the daily nitrogen needs against 24% for organic sources. Taken altogether, dissolved organic and inorganic nitrogen can supply almost 100% of the nitrogen needs for tissue growth.