Heterotrophic Cultures of Galdieria phlegrea Shift to Autotrophy in the Presence or Absence of Glycerol

Despite being an autotrophic organism, Galdieria phlegrea (Galdieriaceae) has the ability to use glycerol thereby switching to heterotrophy in the dark and mixotrophy in the presence of light. To examine cellular changes during the switch to mixotrophic metabolism and finally to photoautotrophic metabolism, heterotrophic cells of G. phlegrea were exposed to light and split into two subcultures. Cells exposed only to light but cultivated in medium containing glycerol grew with a recovery time of at least 3 days. In a parallel culture, the simultaneous removal of glycerol from the culture medium and light exposure allowed Galdieria cells to rapidly recover their growth rate due to their ability to rapidly absorb ammonium from the medium. However, contrary to expectation, a higher content of total soluble protein was observed in light-exposed cells cultivated in medium containing glycerol compared to cells cultivated without glycerol. In addition, the level of Rubisco in cells exposed to light and cultivated without glycerol was higher than those in cells cultivated in medium containing glycerol, indicating full photosynthetic functionality after only 3 days of light treatment. The greater chlorophyll a content confirms that the photosynthetic activity of cells cultivated without glycerol recovers earlier than that of heterotrophic cells exposed to light but still cultivated in medium containing glycerol.