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

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Abstract
The growth rate of a diatom (Chaetoceros protuberans) was followed during transfers from moderate to higher or lower irradiances (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 (Φµ) 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” effect and in the C : Chl a ratio (Θ), the light‐harvesting capabilities of the living carbon pool vary only weakly. Therefore the growth rate change (from 0.3 to 1.2 d−1) is essentially governed by the change in Φµ (from 0.01 to 0.052 mol C retained mol−1 quanta absorbed). The kinetics of the Chl‐a‐per‐cell modification and of the Φµ response is rather fast (within approximately one generation). Quantum yield for growth is halved after a transfer to high irradiance; 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, Φµ is restored practically to its initial value.

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