Effects of Ca2+ and K+ on motility and photomotility of the marine ciliate Fabrea salina

Fabrea salina is a motile ciliate marine protozoan belonging to the order Heterotrichida that shows photomotile reactions. We report here the results of a study on motility and phototaxis of F. salina exposed to different Ca2+ and K+ concentrations. Our findings show that concentrations of potassium ions greater than that of the growth medium and concentrations of calcium ions lower than that of the growth medium cause a significant increase in phototactic response, while the cell speed is not significantly affected. A more thorough analysis shows that the absolute concentrations of Ca2+ or K+ are not important and that the phototactic responsiveness depends mainly on the ratio of the K+ concentration to the square root of the Ca2+ (the so-called Ja value), in agreement with the results obtained in other protozoa. Our results are tentatively discussed in terms of altered membrane potential or ion channel conductances. (C) 1998 Elsevier Science S.A. All rights reserved.