Uptake of neutral red by the vacuoles of a green alga, Micrasterias pinnatifida

Neutral red (NR) in the culture medium entered the vacuoles of a green alga, Micrasterias pinnatifida, at a higher rate at pH 8 than at pH 5. NR remained soluble in vacuoles of cells cultured at pH 5, while it precipitated and formed granules in cells cultured at pH 8. The vacuoles of cells cultured at pH 8 contained fibrils, but those of cells cultured at pH 5 did not. The amount of NR that entered the cells was markedly reduced by the addition to the medium of nigericin at 10(-5) M, monensin at 10(-5) M, bafilomycin A(1) at 10(-5) M, or ammonium chloride at 50 mM. The formation of NR granules in vacuoles were strongly inhibited and the disorganization of NR granules were accelerated by the addition of nigericin at 10(-5) M, or bafilomycin A(1) at 10(-5) M to the culture medium. The possibility is discussed that NR which enters vacuoles might become positively charged (NRH(+)) by protons brought into vacuoles by proton pumps and that NRH(+) might combine with some negatively charged macromolecules to form aggregates or granules.