Abstract
The effect of external pH on two laboratory-cultured acid-intolerant species (Chlorella pyrenoidosa Chick and Scenedesmus quadricauda Turp. Bréb.) and one acid-tolerant species from a natural population (Euglena mutabilis Schmitz) was examined by measuring internal pH. These measurements were made with the weak acid (14)C-dimethyloxazolidine-2,4-dione after cells had been incubated for 2 and 6 hours at external pH levels from 3.0 to 8.0. Photosynthetic and respiration rates of the three species were also measured over the range of external pH levels.All three species regulated their internal pH levels over the 6-hour incubation time. C. pyrenoidosa and S. quadricauda had internal pH levels around 7.0, regardless of external pH. E. mutabilis had a wider internal pH range, from 5.0 at low external pH to 8.0 at high external pH. External pH had no effect on either photosynthetic or respiration rates. Statistical comparisons showed that there was a significant difference between the acid-intolerant and acid-tolerant species with regard to the level of internal pH maintained and the response of internal pH to external pH.