Summary: The distribution and physiology of in its natural habitat in acidic hot springs and hot soils have been studied. This eucaryotic alga was the sole photosynthetic organism in habitats with pH less than 5 and temperatures greater than 40°. The upper temperature limit of the alga was 55° to 56° and the optimum temperature for growth was 45°. Temperature strains such as are found in blue-green algae of alkaline thermal habitats were not found for . In aquatic habitats the lower temperature limit was about 35° to 36°, the organism apparently being unable to compete at temperatures below this with other algae. In soils the alga was found at temperatures as low as 10°, apparently because in terrestrial habitats competition with other algae was less significant. The pH range at which the alga has been found in nature was from 0·05 to 5·0 and growth in culture occurred over this whole range. The optimum pH for growth was between 2 and 3. In nature the alga was found in habitats of widely varying light intensity, up to 7000 ft-candles. The alga became adapted to reduced light intensity by increasing its photopigment concentrations. Photosynthesis in populations adapted to reduced light intensities was inhibited by high light intensities. The alga grew well on glucose in the dark, and the concentration of photosynthetic pigments was reduced. When such bleached cells were transferred to the light in the presence of glucose, pigments were not synthesized and heterotrophic growth continued; when glucose was omitted, pigment synthesis occurred and photosynthetic growth resumed. Glucose did not inhibit pigment synthesis when added to cells growing in the light.


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