Reef-building corals and other tropical anthozoans harbor endosymbiotic dinoflagellates. It is now recognized that the dinoflagellates are fundamental to the biology of their hosts, and their carbon and nitrogen metabolisms are linked in important ways. Unlike free living species, growth of symbiotic dinoflagellates is unbalanced and a substantial fraction of the carbon fixed daily by symbiont photosynthesis is released and used by the host for respiration and growth. Release of frxed carbon as low molecular weight compounds by freshly isolated symbiotic dinoflagellates is evoked by a factor (i.e., a chemical agent) present in a homogenate of host tissue. We have identified this "host factore in the Hawaiian coral Pocillopora damicornis as a set of free amino acids. Synthetic amino acid mixtures, based on the measured free amino acid pools of P. damicornis tissues, not only elicit the selective release of 14C-labeled photosynthetic products from isolated symbiotic dinoflagellates but also enhance total "4CO2 fixation.Dinoflagellates freshly isolated from the Hawaiian coral Pocillopora damicornis and the Pacific giant clam Tridacna crocea and incubated in sea water with a crude homogenate of their own host tissue fix 14C02 in the light and release a substantial fraction of the fixed carbon to the incubation medium, principally as [14C]glycerol, [14C]alanine, and [14C]glucose. In contrast, release of labeled organic carbon compounds by freshly isolated dinoflagellates incubated in sea water alone is always significantly less. Moreover, as a crude homogenate of animal tissue from the one host species also induces dinoflagellates from another host species to selectively release fixed carbon, the inducing factor would appear to be nonspecific (1).Release of fixed carbon by symbiotic dinoflagellates has been demonstrated in several cnidarian symbioses. Although a host factor (HF) has never been identified, its properties, depending on the host, have been described as heat labile (1-3) or heat stable (1, 4), enhancing carbon fixation (2) or not enhancing carbon fixation (3), absent from aposymbiotic hosts but inducible by reinfection with dinoflagellates, constitutive in aposymbiotic hosts (5), evoking release of fixed carbon from cultured symbiotic dinoflagellates and free-living nonsymbiotic dinoflagellates, not evoking the release of fixed carbon from cultured symbiotic dinoflagellates and free-living nonsymbiotic dinoflagellates (5, 6), and behaving like a substance with a molecular mass of <10 kDa (3) or -1 kDa (5). Here we identify HF in the Hawaiian reef coral P. damicornis as a set of free amino acids (FAAs) that not only evoke the selective release of photosynthate from symbiotic dinoflagellates in vitro but also enhance dinoflagellate carbon fixation. P. damicornis and the tropical sea anemone Aiptasia pulchella were collected from Checker Reef adjacent to the Hawaiian Institute of Marine Biology, Oahu, Hawaii, and the animals were transported by air to the University of California, Los Angeles. The c...