Photosensory transduction in the flagellated alga, Euglena gracilis I. Action of divalent cations, Ca 2+ antagonists and Ca 2+ ionophore on motility and photobehavior

Abstract

1. 1. The flagellated alga, Euglena gracilis, swins forward essentially in a straight path under constant light intensity. Strong motility of the cells can be supported by Mg 2+ alone but optimum motility is found in the presence of Mg 2+, Ca 2+ and K +. 2. 2. Ca 2+, Co 2+, Mn 2+ and Ba 2+ induced a concentration-dependent increase in the rate at which the cells change the direction of their swimming path (a klinokinesis). Ni 2+ immobilizes the flagellum. 3. 3. On perception of a reduction (‘step-down stimulus’) in blue light intensity in their environment, Euglena rotate in place (tumble) for a finite period (the step-down photophobic response). 4. 4. The duration of the tumbling is enhanced in the presence of divalent cations following the series Ca 2+ > Ba 2+ > Mn 2+ > CO 2+ > Mg 2+ = Ni 2+ = 0. 5. 5. Neither the tumbling response in the presence of low concentrations of Ca 2+ or the Ca 2+-stimulated response is altered by verapamil (a Ca 2+ conductance antagonist). The Ca 2+ conductance/active transport antagonist, ruthenium red, is also inactive. 6. 6. The Ca 2+ ionophrone, A23187, has little effect on flagellar activity in the absence of extracellular Ca 2+. However, in the presence of A23187, Ca 2+ induces a specific ligh-independent, concentration-dependent discontinuous tumbling response of the cells.