Lysosomal enzymes in extracellular digestion in the unicellular eukaryote Tetrahymena

The ciliate Tetrahymena thermophila was starved for orthophosphate in a synthetic medium at pH 7.5. These cells did not utilize phosphorylcholine, final concentration 1 mM, as a phosphate source for cell growth and multiplication. If the phosphorylcholine solution, however, was incubated for 24 h at pH 5.5 with extracellular, "spent" medium from a culture in early stationary phase of growth, then it promoted culture growth readily at pH 7.5. It was shown that the spent medium in the same concentration did not stimulate growth in itself. It is concluded that extracellular digestion of phosphorylcholine enabled the cells to grow and multiply in a nutrient medium having organic phosphate compounds as the only phosphate source. It is argued that the phosphatases in the spent medium are of lysosomal origin.