Chemoreception in the green alga Dunaliella tertiolecta

A capillary assay was employed to quantify positive chemotactic responses in the motile, unicellular, marine alga Dunaliella tertiolecta. Among a wide range of inorganic and organic compounds tested, only ammonium ion, l-tyrosine, l-tryptophan, and l-phenylalanine were found to be major atractants for the chlorophyte. l-Methionine and l-cysteine weakly attracted the alga at 10^-3 M. The minimum concentration of the major attractants needed to elicit an observable chemotactic response was approximately 10^-6 M. The maximum response occurred when the capillaries contained 10^-5 M l-tyrosine or l-tryptophan, 10^-4 M l-phenylalanine, and 10^-3 M ammonium chloride. The other amino acids, carbohydrates, B-vitamins, urea, and nitrate were among the chemicals that failed to attract D. tertiolecta. The alga apparently possesses one chemoreceptor that binds ammonium ion only, and another chemoreceptor that binds the three aromatic amino acids.