Light-Dependent Ethanol Assimilation By Model Red Microaga Galdieria sulphuraria

The effect of ethanol on the red thermoacidophilic microalga Galdieria sulphuraria , capable of hemo- and photoheterotrophic growth, was studied. It was found that G. sulphuraria failed to grow and assimilate ethanol when kept in the dark, in contrast to a wide range of organic compounds known to this alga. Light conditions, on the other hand, activated ethanol uptake from the growth medium and intensify microalgal growth. The known stress effect of ethyl alcohol can be seen in the short-term increase in cellular respiration in the dark and its sustained increase throughout the growth period of G. sulphuraria in the light. Ethanol-induced increase in cellular respiration precedes light activation of photosynthesis. The growth enhancement of ethanol-supplemented culture of G. sulphuraria compared to photoautotrophic culture is most likely due to the complete oxidation of ethanol during respiration to carbon dioxide and its use by chloroplasts as an additional intracellular substrate for photosynthesis. The absence of ethanol consumption in the dark and its metabolic uptake in the light indicate photoactivation of alcohol dehydrogenase and acetaldehyde dehydrogenase, two key sequential enzymes in the ethanol oxidation pathway.