Membrane fluxes and comparative toxicities of aluminium, scandium and gallium

Measurements were made of the membrane fluxes and toxicities of three cations with trivalent forms, Al, Ga and Sc, in internodal cells of the giant alga Chara corallina. With this species it was possible to separate the cell wall from the cell contents to obtain membrane fluxes which were not complicated by adsorption of cations to the cell wall. Net uptake of Al was low, approximately 1.5 pmol m^-2 s^-1, compared to the influxes of the divalent cation ⁴⁵Ca of 82 pmol m^-2 s^-1 and the monovalent cation ²²Ha of 1100 pmol m^-2 s^-1 at the same external concentration. Traditional desorption methods for removing cell wall cations were found to be relatively ineffective in the case of trivalent cations and, consequently, influx measured without separating the cell wall component would greatly overestimate the true membrane flux, possibly by several orders of magnitude. Al, Ga and Sc all inhibited growth at 20 mmol m^-3 at pH 4.4. Toxicity decreased in the order Sc>Al>Ga. Sc and Al were also toxic to mature non- growing cells. Influx of ⁴⁶Sc increased with increasing pH, consistent with membrane permeation by hydroxy Sc rather than Sc³⁺. However, Sc was more toxic at low pH where Sc³⁺ was the dominant species and where influx was low and binding to cell walls was high. These results argue against Sc acting intracellularly and favour a toxicity mechanism which is initiated extracellularly.