Photoperiod mediates the effects of elevated CO2 on the growth and physiological performance in the green tide alga Ulva prolifera

Ulva spp., an increasingly important food, are the dominant species of the large-scale green tides. In this study, both the growth and the physiological responses of the Ulva prolifera were studied after cultured in three different light and dark regimes (12:12, 14:10 and 16:8-h light/dark) in combination with current (420 $μ$atm; LC) and increased (1000 $μ$atm; HC) levels of atmospheric CO2. Grown rate of U. prolifera was significantly enhanced by increased CO2 under the three light:dark regimes, especially under 16:8 h-light:dark, indicating that growth was C-unsaturated at present CO2 levels. U. prolifera showed a significantly higher growth rate and lower dark respiration rate (Rd) at 16:8 h-light:dark treatment than at 12:12 h-light/dark treatment, regardless of the CO2 treatment. The photochemical performance was largely unaffected by elevated CO2 and daylength. These results suggest that U. prolifera in a future CO2 enriched coastal water, seems to be resilient to higher CO2 concentrations, and this could be enhanced by longer daylength.