Changes of cytoplasmic free Ca(2+) in the green alga Mougeotia scalaris as monitored with indo-1, and their effect on the velocity of chloroplast movements.

The fluorescent calcium-sensitive dye 1-[2-amino-5-(6-carboxyindol-2-yl)-phenoxy]-2-(2'-amino-5'-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid (indo-1) was loaded by a transplasmalemma pH gradient into filamentous cells and protoplasts of Mougeotia scalaris, such that most of the indo-1 fluorescence originated from the cytoplasm. Incubation of M. scalaris filaments in ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA)-buffered media (-log [Ca(2+)] (=pCa) 8 versus pCa 3) caused a consistent and significant decrease in the cytoplasmic free [Ca(2+)]. Pulses of the fluorescence excitation light (UV-A 365 nm, 0.7 s) caused an increase in cytoplasmic free [Ca(2+)] in M. scalaris that was nearly independent of the external [Ca(2+)] and of chloroplast dislocation by centrifugation. This calcium flux, highest in UV-A light, compared with blue or red light, probably resulted from a release of Ca(2+) from intracellular stores. Increased cytoplasmic [Ca(2+)] may affect the velocity of chloroplast rotation since UV-A-light-mediated chloroplast movement was faster than in blue or red light. Consistently, the calcium ionophore A23187 and the calcium-channel agonist Bay-K8644 both increased the velocity of the red-light-mediated chloroplast rotation. Based on these and other observations, a Ca(2+)-induced decrease in cytoplasmic viscosity in Mougeotia is presumed to occur.