Anion sensitivity of motility and step-down photophobic responses of Euglena gracilis

The motility and step-down photophobic responses of Euglena are influenced by inorganic and organic anions. Persistent motility (with Ca²⁺, Mg²⁺ and K⁺ present) is supported with chloride or sulfate but not with acetate, nitrate or propionate as the only added anions. Cells in media containing acetate displayed a cell aggregation (clumping) behavior that was both red light sensitive and, under some conditions, was accompanied by suppression of the step-down photophobic response. Addition of sodium salts (Cl^-, SO ₄ ^2- , acetate or propionate) to cells in Cl^- or SO ₄ ^2- based media had differential effects on the duration of the step-down photophobic responses induced by blue light removal: anions alter the response. In addition, cells in all Cl^- containing media showed constant photophobic response duration following repeated stimulation. Cells in some SO₄ ^2- containing media, however, showed response summation to repeated stimulation. This latter effect was reversible and was overcome by the addition of chloride anions.