Marine Biology (1999) 135: 219±222
Ó Springer-Verlag 1999
J. T. Wang á A. E. Douglas
Essential amino acid synthesis and nitrogen recycling in an alga±invertebrate symbiosis
Received: 26 October 1998 / Accepted: 28 June 1999
Abstract When aseptically-cultured sea anemones, Aiptasia pulchella, were incubated with 14C-labelled glucose, aspartate and glutamate, radioactivity was incorporated into animal protein. Radioactivity was recovered from all amino acids in the protein hydrolysates of A. pulchella bearing the symbiotic alga Symbiodinium sp., and from all but seven of the amino acids in A. pulchella experimentally deprived of their algae. These data suggest that these seven amino acids (histidine, isoleucine, leucine, lysine, phenylalanine, tyrosine and valine) may be synthesized by the symbiotic algae and translocated to the sea anemone's tissues; and that methionine and threonine, two amino acids traditionally considered as dietary essentials for animals, are synthesized by A. pulchella. Essential amino acid translocation from the symbiotic algae to the animal host is a core element in symbiotic nitrogen-recycling. Its nutritional value to the animal host is considered in the context of the amino acid biosynthetic capacity of the host.
Introduction Virtually all shallow-water benthic Cnidaria (corals, sea anemones etc.) at low latitudes bear dino¯agellate algae of the genus Symbiodinium. It has been proposed that these algae contribute to the nutrition of their animal hosts in two ways: (1) by providing photosynthesisderived carbon to the animal tissues (Muscatine 1990); (2) by recycling nitrogen, i.e. assimilating animal-derived nitrogenous waste-compounds into compounds, espeCommunicated by J.P. Thorpe, Port Erin J.T. Wang á A.E. Douglas (&) Department of Biology, University of York, P.O. Box 373, York YO1 5YW, England Fax: 0044 (0)1904 432-860 e-mail:
[email protected]
cially essential amino acids, which are translocated back to the animal tissues (Muscatine and Porter 1977; Falkowski et al. 1993). The essential amino acids which animals generally cannot synthesize are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (Morris 1991). The animal host can thus be considered to have gained access to two complex metabolic capabilities generally absent from the animal kingdom ± photosynthesis and essential amino acid synthesis ± through symbiosis with algae (Douglas 1994). The evidence for photosynthate release by algae to the animal is overwhelming (Trench 1993), but the incidence of nitrogen recycling and its signi®cance to animal nitrogen-nutrition are disputed. The diculties with nitrogen recycling are threefold. (1) substantial essential amino-acid translocation from algal cells to the animal has not been demonstrated; in all symbioses involving Symbiodinium sp., amino acids are undetectable or at low concentrations in the photosynthate translocated from algal cells to the animal tissues (e.g. Trench 1971; Sutton and Hoegh-Guldberg 1990; Wang and Douglas 1997). (2) Several scleractinian corals, including species that naturally host Symbiodinium sp., have been reported to synthesize essential amino acids (Fitzgerald and Szmant 1997), so undermining the nutritional value of nitrogen recycling by the algae. (3) The chief line of indirect evidence for nitrogen recycling, elevated ammonium production by animals deprived of their symbiotic algae, has been discredited. This eect has traditionally been interpreted as evidence that the algae represent a major sink for animal-derived waste ammonium (Wilkerson and Muscatine 1984; Falkowski et al. 1993), but there is now persuasive evidence that the lowered ammonium production in symbiotic animals is a consequence of enhanced ammonium assimilation by the animal, stimulated by the receipt of photosynthetic carbon from the algal cells (Wang and Douglas 1998). The present study arose from a consideration of the processes underlying nutrient release by photosynthetic
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algae. It is well-established that Symbiodinium sp. releases photosynthate within 1 min of ®xation (Trench 1993). We reasoned that experimental designs which explore photosynthate release would fail to detect amino acid transfer to the animal tissues if the amino acids were released many hours after carbon ®xation or were not synthesized from photosynthesis-derived carbon skeletons. We therefore examined the metabolic fate, over several days, of radiolabelled organic carbon compounds administered to the symbiosis.
Materials and methods Aiptasia pulchella cultures The experiments were conducted on a clonal culture of the sea anemone Aiptasia pulchella, derived originally from Tongkang, Taiwan, and maintained in aerated arti®cial sea water (ASW, Instant Ocean Salts) at 25 °C under a 12 h light:12 h dark regime at 50 lmol m)2 s)1 photosynthetically active radiation (PAR). The anemones contained Symbiodinium sp. of Group-b, sensu Rowan and Powers (1991) (Douglas unpublished data), at a density of 2 to 4 ´ 106 algal cells mg)1 protein; they are termed ``symbiotic'' individuals. A sample of anemones was bleached of its symbiotic algae by cold shock-treatment (Steen and Muscatine 1987), followed by incubation in darkness for many months. The bleached anemones bore very small numbers of algal cells (
