Responses of marine planktonic protists to amino acids: feeding inhibition and swimming behavior in the ciliate Favella sp

Feeding rates of the tintinnid Favella sp. on the dinoflagellate Heterocapsa triquetra were inhibited by a number of dissolved free amino acids (DFAAs), with inhibition inversely proportional to the size of the amino acid side chain. The most inhibitory compounds (valine, cysteine, proline, alanine, and serine) reduced feeding to <20% of the control rate at a concentration of 20 μM. Inhibition was dose-dependent, with a threshold of ca. 200 nM for proline, and did not depend on ciliate feeding history (well-fed versus starved). Inhibition occurred rapidly (<5 min after exposure) and was partially reversible upon removal of DFAAs. Detailed analysis of swimming did not reveal consistent changes in Favella sp. behavior upon exposure to inhibitory amino acids. In contrast to Favella sp., the heterotrophic dinoflagellate Gyrodinium dominans showed no feeding response to 20 μM DFAAs, while the tintinnid Coxliella sp. exhibited reduced feeding (to approximately 50% of control rates) in response to a subset of the amino acids active in Favella sp. Our findings, along with the prevalence of some inhibitory compounds at nM concentrations in natural waters, point to a signaling function for these amino acids. Feeding deterrence in Favella sp. is, however, contrary to the typical attractant or stimulatory role of DFAAs, which has been documented for organisms ranging from bacteria to metazoans. The information content of the signal remains unclear but may be related to detection of prey quality during suspension feeding by Favella sp.