Effect of Ethanol on Growth of the Red Microalga Galdieria sulphuraria

Polyextremophilic red microalgae of the genus Galdieria, which inhabit hot sulfur springs under conditions unusual for eukaryotes, are capable of heterotrophy. Among the dozens of exogenous organic substrates identified for Galdieria, ethanol is not mentioned as a possible energy source. It turned out that ethanol did not alter the growth of the model species Galdieria sulphuraria when grown in the dark. By contrast, the growth of microalgae is activated in the light, despite the known cell stressor effect of ethanol. The effect of ethanol as an oxidative stress factor has been indicated by the increase in cellular respiration observed in the dark and also in the light even before the activation of photosynthesis. The marked acceleration of growth of G. sulphuraria culture in the light is most likely due to the stimulation of respiration by ethanol with generation of CO2 and its use by chloroplasts as an additional carbon substrate during the photosynthetic process. Compared to the classical organic substrate glucose, the light-induced growth of G. sulphuraria culture in the presence of ethanol is less intense. It can be speculated that ethanol stress in light induces a system of two consecutive key enzymes in the primary alcohol metabolism chain (alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase), which then leads to the eventual complete oxidation of ethanol, resulting in accelerated growth of G. sulphuraria.