Effets des fluctuations rapides de la lumière sur la photosynthèse du phytoplancton

Abstract

[ 14 C]-assimilation rates were measured on cultures of two unicellular green algae ( Chlamydomonas sp. and Oocystis sp.) as a function of light intensities (saturation curves), under steady light and also under rapidly alternating high and low light intensities. Assimilation rates vary according to the frequency of the intermittent light regime and it falls under two categories: (1) at 0.1 and 0.2 Hz, the assimilation rate is equal to the average of the rates observed at high and low light intensities under steady light, and (2) under 1.0, 1.6 and 10 Hz the assimilation rate is equal to the rate observed under a mean steady irradiance. Moreover, the range of assimilation rates at a given frequency depends on the difference between the high and the low intensities. Batch cultures of Oocystis sp. have been grown under intermittent light of 0.1, 1.0 and 10.0 Hz (same mean intensity). Growth rate under intermittent light of 0.1 Hz is −40% lower than the control under steady light. Photosynthetic potential (P Bmax )and efficiency (α) change with the growth stages of the cultures. At the end of the logarithmic growth phase, both photosynthetic parameters are maximum at 1.0 Hz and minimum at 0.1 Hz. Average cell concentrations of chlorophyll a increase as the frequency of the light regime decreases. During the log phase, concentration of carotenoids relative to chlorophyll a increases at 1.0 Hz, decreases at 0.1 Hz, and remains constant at 10.0 Hz. Under clear sky conditions, wave-induced light fluctuations in the photic layer may therefore enhance primary production, especially (1) in the lower part of the photic layer, where low frequency light changes might cause cell chlorophyll a to increase, and (2) at a depth of 1–4 m, where the main frequencies (of the order of 1.0 Hz), might cause a significant increase of both the photosynthetic potential (P Bmax )and efficiency (α).