Responses of Heterotrophic Cultures of Chlorella vulgaris Beyerinck to Darkness and Light. II. Action Spectrum for and Mechanism of the Light Requirement for Heterotrophic Growth

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Abstract
C. vulgaris Beyerinck (Emerson''s strain) failed to grow in the dark even when sugars were provided. Growth rate in darkness on a glucose medium remained constant for 2 days and then declined to almost zero. Pigment concentrations also declined in darkness. Changes in flow rate of 1% CO2-in-air from 0 to 7 ml/minute caused a progressive increase in the dark growth rate over a 5-day period, but did not maintain growth in the dark. Rates above 7 ml/minute produced no changes in growth rates. White light intensities below the compensation point of the alga maintained heterotrophic growth. This response saturated at 0.8 [mu]w/cm2. Light in the 425 m[mu] region was 4 times as effective as white light in stimulating heterotrophic growth. The action spectrum showed a secondary peak of growth stimulation in the 575 mu region. Respiration of glucose was stimulated by low intensities of white light. Malonate and cyanide inhibited the growth of the alga on inorganic and glucose medium under 300 ft.-c. of white light, suggesting that the succinic dehydrogenase and cytochrome oxidase systems were present. Formic and acetic acids, substances inhibitory to growth, were excreted into the medium in the dark cultures. The evidence suggests that respiration of glucose cannot proceed for an extended length of time in darkness. The reason for this is postulated to be the lack of a cytochrome or cytochrome precursor.