Studies On The Effects Of Elevated CO₂ Concentrations On The Growth And Photosynthesis Of Microalgae

Three species of microscopic green algae, Chlamydomonas reinhardtii, Chloreila pyrenoidosa and Scenedesmus obliquus were cultured at varied CO2 levels to investigate the impacts of increased atmospheric CO2 concentrations on their growth and photosynthesis. Increased CO2 concentrations enhanced the growth of the species. Specific growth rates under optimum conditions, increased with elevated CO2 concentrations, and reached maximal values at 30, 100 and 60 ii M in C. reinhardtii, C. pyrenoidosa and S. obliquus, respectively. This indicates that growth of the microalgae can be C02-limited in fresh waters. Elevated CO2 concentrations stimulated the growth of the microalgae at varied levels of nutrients (inorganic N and P). When CO2 concentration increased from 10 to 51 and 99~tM, specific growth rate in low or high N media increased by 8i16% or 13~44% in C. reinhardtii; by 13梸20% or 12?0% in S. obliquus; by 35挆81% or 29~79% in C. pyrenoidosa, respectively. On the other hand, specific growth rate in low or high P media, when CO2 concentration increased from 10 to 51 and 99~tM, increased by 13-46% or 10-44% in C. reinhardtii; by 15梸20% or 8-42% in S. obliquus; by 20-~-44% or 26-411% in C. pyrenoidosa, respectively. It was concluded that the effects of elevated CO2 concentrations on growth were more profound at low N in C. pyrenoidosa, at low P in C. reinhardtii and S. obliquu.s. Increased CO2(aq) concentrations in fresh waters associated with atmospheric CO2 rise would enhance the growth of these algae. Elevated CO2 concentrations enhanced photosynthesis of the species. The maximal net photosynthesis and light-saturating point increased with increased CO2 concentrations. Cells grown at different CO2 levels showed differed characteristics of photoinhibition when exposed to high irradiance. The photochemical effciency of high-C02-grown cells decreased faster than that of low-CO2-grown cells in C. reinhardtii, the contrary was observed in C. pyrenoidosa and S. obliquus. The photoinhibited cells could recover to some extent when placed under dim light or in 4 darkness. Recovery was faster in dim light than in darkness. The treatment of chloramphenicol had little effects on the recovery of C reinliardtii and C pyrenoidosa, but inhibited the recovery of S. obliquus. In addition, high-C02-grown cells showed better recovery in C. pyrenoidosa and S. obliquus.