The increase in the input of nutrients, mainly nitrogen (N) and phosphorus (P), to marine water-with eutrophication as consequence-is a worldwide phenomenon. The supply of these nutrients would usually result in the occurrence of the red tide. Nitzschia closterium f. minutissima, a red tide alga, is distributed in coastal marine water in China. In this paper, impacts of different nitrogen and phosphorus concentrations on inorganic carbon utilization and carbonic anhydrase activity in N. closterium f. minutissima were investigated. The results showed that the algal growth was significantly stimulated by high levels of nitrogen and phosphorus. The specific growth rate was increased by 60% and 128% in the highest nitrogen（880μmol?L-1）and phosphorus （18.4μmol?L-1）concentrations, respectively, compared with the ambient level in natural seawater. Extracellular carbonic anhydrase activity in N. closterium f. minutissima was not detected potentiometrically, but intracellular carbonic anhydrase activity still existed under lower nitrogen conditions (2-14 μ mol?L-1). Both extracellular and intracellular carbonic anhydrase activity were evidently increased by the elevation of nitrogen levels (220-880 μmol?L-1). The activity of intracellular carbonic anhydrase was significantly enhanced under high phosphorus concentrations (9.2-18.4 μmol?L-1), but the extracellular carbonic anhydrase activity was insignificantly affected by phosphorus enrichment. The values of maximal photosynthetic rate (Vmax) and CO2 concentration supporting half of Vmax (K1/2 CO2) appeared to be higher values when the algae were grown under high nitrogen and phosphorus concentrations. The relationship between photosynthetic O2 evolution rate and dissolved inorganic carbon (DIC) concentrations showed that the photosynthesis was fully saturated in the ambient DIC levels in natural seawater, regardless of nitrogen and phosphorus concentrations. The maximal photochemical efficiency (Fv/Fm) was also markedly increased with nitrogen and phosphorus enrichment, and was higher by 36.8% and 19.4% in the highest nitrogen (880 μmol?L-1) and phosphorus (18.4 μmol?L-1) concentration than the ambient level in natural seawater. The above results suggested that the alga could adapt to the different nitrogen and phosphorus circumstances through the change of intracellular and extracellular carbonic anhydrase activity to regulate inorganic carbon utilization.