A differential effect of pH on cell growth and respiration

The pH of the growth medium has different effects on the growth and the respiration processes of a colorless strain of Euglena gracilis var. bacillaris when succinate, fumarate, malate, citrate and acetate are used as sole carbon sources. The responses of the cells were studied at pH 7.0, 6.0, 5.0, and 3.5 in a manner such that the conditions of the respiration studies approximated the conditions of the growth studies. Respiration, but not growth, was inhibited on succinate at pH 7.0. Growth, but not respiration, was inhibited on acetate at pH 5.0 as well as on malate at pH 3.5. The respiration rates of the cells on fumarate responded to changes in pH in a manner similar to that of the observed growth rates. No growth occurred on citrate at any of the pH's tested. The fact that succinate supported optimal growth at pH 7.0, where the substrate is essentially totally ionized and the growth rate of the cells on succinate is independent of pH, implies the presence of a transport mechanism for succinate ion in Euglena. The evidence is not sufficient to permit postulation of similar systems for the malate or fumarate substrates. Evidence is also presented to support the idea that the growth processes for Euglena utilizing acetate, malate, and possibly fumarate, are more sensitive to pH than are the respiration processes. Some implications of these findings on the relationship between energy production and growth, and between differential permeability of ions and free acids are discussed. The possible accumulation of weak acids within cells as a result of lower external pH values of the medium is also considered.