Interactive Effects of Increased Temperature and pCO2 Concentration on the Growth of a Brown Algae Ecklonia cava in the Sporophyte and Gametophyte Stages

To examine the effects of increased CO₂ concentration and seawater temperature on the photosynthesis and growth of forest forming Ecklonia cava (Laminariales, Phaeophyta), sporophytic discs and gametophytes were cultured under three pCO₂ concentrations (380, 750, 1000 ppm), four temperatures (5, 10, 15, 20∘C for sporophytes; 10, 15, 20, 25∘C for gametophytes), and two irradiance levels (40, 80μmol photons m⁻²s⁻¹ ) for 5 days. Photosynthetic parameter values ( ETRₘₐₓ , Eₖ , and α ) were generally higher as sporophytic discs were grown under low temperature and increased CO₂ concentration at 750 ppm. However, photosynthesis of Ecklonia sporophytes was severely inhibited under a combination of high temperature ( 20∘C ) and 1000 ppm CO₂ concentration at the two photon irradiance levels. The growth of gametophytes was maximal at the combination of 380 ppm (present seawater CO₂ concentration) and 25∘C . Minimal growth of gametophytes occurred at enriched pCO₂ concentration levels (750, 1000 ppm) and high temperature of 25∘C . The present results imply that climate change which is increasing seawater temperature and pCO₂ concentration might diminish Ecklonia cava kelp beds because of a reduction in recruitments caused by the growth inhibition of gametophytes at high pCO₂ concentration. In addition, the effects of increased temperature and pCO₂ concentration were different between generations - revealing an enhancement in the photosynthesis of sporophytes and a reduction in the growth of gametophytes.