The difference in Km values is compatible with the hypothesis that Symbiodinium has a taurine signal transducer that is responsible for photosynthate release and is distinct from the taurine transporter. Taurine exists naturally in animals including mammals, birds, fish, and aquatic invertebrates such as oysters and mussels. Both taurine uptake and taurine-induced photosynthate release by Symbiodinium exhibited saturation kinetics, but with significantly different Km values of 68 and 21 M, respectively. Exogenous 14C-labeled taurine was taken up by the cells, and more than 95% of the internalized 14C was recovered as taurine, indicating that taurine-induced photosynthate release was not dependent on taurine metabolism. A complex mixture of amino acids at 75 mM also promoted photosynthate release, but the release rate was reduced by 34% after the omission of taurine (3 mM) from the mixture, suggesting that much of the effect of amino acids was largely attributable to taurine.
Photosynthate release, as induced by taurine and animal extract, was metabolically equivalent at both concentrations in that they (a) stimulated photosynthate release to the same extent and (b) induced the selective release of photosynthetically derived organic acids. Ito, et al. Exogenous concentrations of 10 M to 1 mM of the nonprotein amino acid taurine stimulated photosynthate release from the dinoflagellate alga Symbiodinium, which had been freshly isolated from the sea anemone Aiptasia pulchella. Froger, et al., Taurine: the comeback of a neutracéutico en la prevención de las degeneraciones retinianas, Prog Retin Eye Res, julio de 2014, 41: 44-63.
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