Fig. 1. Stimulation of luminescence by ionomycin. (A) Integrated luminescence over a total of 200 s (filled circles) and over the first 12 s (open circles) during and after addition of different concentrations of ionomycin or control solution to night-phase cells of Lingulodinium polyedrum in Hepes-buffered filtered seawater (HbFSW; N=3-4 for each stimulus tested). Values are means ± S.D. (B—D) Calcium dependence of luminescence stimulation by ionomycin when added to night-phase cells in Ca-free artificial seawater (ASW; total [Ca] = <40 µmol l^-1 and cheated by 1 mmol l^-1 EGTA). (B) Ionomycin treatment at a final concentration of 5 µmol l^-1 plus 2xCa to return extracellular Ca²⁺ to normal levels. The bar below the x-axis marks the approximate time of injection. (C) As in (B), but ionomycin was added without Ca²⁺. There was decreased stimulation and the time course of stimulation was slower. (D) Integrated luminescence produced by ionomycin or a control solution when added to Ca-free cells with or without simultaneous replacement of Ca²⁺ (N=11 for all stimuli except for 0.5% DMSO without Ca, for which N=5). Integrated luminescence over a total of 200 s (filled bars) and over the first 12 s (open bars) is shown. Values are means ± S.D.