The Effect of Acidification on the Ability of Euglena gracilis to Perform Positive Phototaxis

  • Ran Bi
  • Gabrielle Chan
  • Jasmine Hyun
  • Josie Duncan


The effect of acidification on the ability of Euglena gracilis to perform positive phototaxis, movement towards a light source, was observed. The importance of this study was to simulate and explore the effects ocean acidification due to climate change on the photosynthetic ability of this marine algae. The trials included pH 5, 6, and 7, with 7 being optimal and any pH outside this range being lethal to the organism. A stock solution of cultured E. gracilis was separated into three beakers and hydrochloric acid was used to decrease the pH for each respective treatment. In three separate petri dishes, there was a positive control, negative control, and test treatment. The positive control was completely exposed to light, the negative control was completely covered by dark, opaque plastic, and the test treatment was covered with a small cut-out window for light to enter. From a two-way ANOVA, the p-value for light was found to be 0.220, 0.119 for pH, and 0.162 for the interaction between light and pH. Since the p-values for all three are greater than the level of significance (0.05), we fail to reject the three null hypotheses, concluding that light has no effect on positive phototaxis, pH has no effect on positive phototaxis, and there is no interaction between light and pH. These findings do not support our prediction that a decrease in pH will reduce the ability of E. gracilis to perform positive phototaxis.