Growth rate and quantum yield time response for a diatom to changing irradiances (energy and color)1

The growth rate of a diatom (Chaetoceros protubcram) was followed during transfers from moderate to higher or lower irradianccs (PAR) with differing spectral compositions (“white,” “blue,” and “green” light). Chlorophyll-specific and carbon-specific absorption coefficients of the algae were monitored, allowing changes in the quantum yield for growth (@),) to be assessed. All the parameters appear to be practically unaffected by chromatic conditions, provided that the photosynthetically usable radiation (PUR) is made equal. Diatoms respond to the energy level whatever the color. Nevertheless, due to opposite influences of the modifications in the “package” eflt thereafter it remains low and steady. Conversely the enhanced value observed just after transfer to low irradiance is not maintained beyond 2 or 3 d; after that, a’, is restored practically to its initial value.