Carbon acquisition of marine phytoplankton: Effect of photoperiod length

We investigated the carbon acquisition of three marine microalgae, Skeletonema costatum , Phaeocystis globosa , and Emiliania huxleyi in response to different light regimes. Rates of photosynthetic O 2 evolution and CO 2 and HCO 3 − uptake were measured by membrane inlet mass spectrometry in cells acclimated to cycles of 16 : 8 light : dark (LD; h : h) and 12 : 12 LD and were compared with those obtained under continuous light. In addition, cellular leakage was estimated for different photoperiods and ambient CO 2 concentrations during growth. Maximum rates of photosynthesis more or less doubled under LD cycles compared with continuous light. In S. costatum and E. huxleyi , a remarkably higher contribution of HCO 3 − to the overall carbon uptake was observed under LD cycles. In contrast, P. globosa did not change its CO 2 :HCO 3 − uptake ratio in response to daylength. Half saturation concentrations (K 1/2 ) for O 2 evolution and inorganic carbon (Ci) uptake were also influenced by the photoperiod. Under LD cycles K 1/2 values for photosynthesis in S. costatum and P. globosa were similar or higher compared with continuous light, whereas they were much lower in E. huxleyi . With the exception of CO 2 uptake in E. huxleyi and P. globosa , affinities for Ci decreased under the LD cycles. Cellular leakage was highest for E. huxleyi and lowest for S. costatum and generally decreased with increasing CO 2 concentration. Although this study confirms species specific differences in the CO 2 ‐concentrating mechanisms (CCMs), the effect of daylength on CO 2 and HCO 3 − uptake has hitherto not been described. We put forward the idea that variations in light condition influence the cellular carbon demand, thereby imposing a stronger control on CCM regulation than the naturally occurring changes in CO 2 supply.