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 μatm; LC) and increased (1000 μatm; 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 (R_d) 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.