Adaptation of biochemical composition and cell size to irradiance in two microalgae: possible ecological implications

Two marine microalgae (Haptophyceae), Hymenomonas elongata and Isochrysis galbana, were grown in turbidostats in order to investigate the influence of light intensity on cell volume and biochemical composition. At irradiance levels that saturated growth (400 μE m⁻² s⁻¹), the algae contained a high level of carbon compounds, especially soluble sugars. At the irradiance that limited growth (25 μE m⁻² s⁻¹) sugar pools were lower for the 2 algae and H. elongata decreased its volume by 25 %. In the transitional phase that followed a shift from 400 μE m⁻² s⁻¹ to 25 μE m⁻² s⁻¹ rates at which biochemical ratios reached stability differed for the 2 algae. Re-establishing high light intensity after a period under low light intensity influenced algal volume in H. elongata; this event followed a particular kinetic pattern that we attempt to explain on theoretical grounds. Results are discussed with regard to durations of irradiance perturbations that can occur in the sea.