The unicellular, asexual thermo-acidophilic algae of the class Cyanidiophyceae, order Cyanidiales (the ‘cyanidia’) include only three genera, walled Cyanidium and Galdieria, and ‘naked’ Cyanidioschyzon, names based on morphological and cytological characters. Most species and strains of this class live in acid hot springs or acid soils or steam vents associated with these springs at pH 0.5 to ∼ 4.0 at temperatures of ∼ 38–56°C. No other phototrophs live in this combination of factors in these habitats, except for a small overlap with other acidophilic algae at the highest pH and the lowest temperature. The optimum pH for growth of the ‘cyanidia’ in this study was ∼ 2.3. Galdieria-like walled cells of Cyanidioschyzon and naked Cyanidioschyzon cells were exposed in culture to higher pH conditions of 6.0, 5.5 and 5.0 in confined, illuminated environments (cotton plugged flasks). The subsequent acidification of the medium towards or to 2.3 occurred as growth and biomass increased. There was a direct correlation with final biomass (Chl a) and lower pH. All eight strains isolated from Yellowstone acidic conditions were able to lower the supra-optimal pH of their medium, while only two from other continents and none of the three from Japan were competent. It is probable that the ability to lower pH to an optimal level has survival value in some niches in natural habitats.