The physiological responses of Nitzschia palea Kützing,a freshwater diatom,to elevated CO2 were investigated and compared with those of a marine diatom,Chaetoceros muelleri Lemmermann previously reported.Elevated CO_2 concentration to 700μl/L increased the dissolved inorganic carbon (DIC) and lowered the pH in the cultures of N.palea,thus enhancing the growth by 4%-20% during the whole growth period.High CO2-grown N.palea cells showed lower levels of dark respiration rates and higher I_k values.Light-saturated photosynthetic rates and photosynthetic efficiencies decreased in N. palea with the doubling CO_2 concentration in airflow to the bottom of cultures,although the doubling CO_2 concentration in airflow to the surface cultures had few effects on these two photosynthetic parameters.N.palea cells were found to be capable of using HCO_3^-in addition to gaseous CO_2,and the CO_2 enrichment decreased their affinity for HCO_3^-and CO_2.Although doubled CO_2 level would enhance the biomass of N.palea and C.muelleri to different extents,compared with the marine diatom,it had a significant effect on the specific growth rates of N.palea.In addition,the responses of photosynthetic parameters of N.palea to doubled CO:concentration were almost opposite to those of C.muelleri.
