Effect of pH on Photosynthesis of Euglena mutabilis Schmitz, an Acidophilic Benthic Flagellate

The dependence of the photosynthetic rate of Euglena mutabilis Schmitz on pH across a range of 2.0–10.0 was investigated. Populations of E. mutabilis isolated from sediments of acidic mine drainage in Sensui (pH = 3.95) and a volcanic cold spring in Bougatsuru (pH = 5.32) were cultured in a pH-adjusted growth media for 96 h, and photosynthetic rate and dark respiration rate were measured. The maximum gross photosynthetic rate of E. mutabilis cells from the Sensui drainage population did not vary significantly over the pH range of 2.0–7.0, and their dark respiration rate showed high values at pH = 7.0. The maximum gross photosynthetic rate of E. mutabilis cells from the Bougatsuru spring population did not vary significantly within the pH range of 2.0–6.0, and their dark respiration rate tended to show high values at pH = 7.0. E. mutabilis can colonize under circumneutral conditions up to and including pH = 6.0.