Abstract: The bloom-forming marine diatom Skeletonema costatum (Greville)Cleve (strain No.2042)was cultured under different levels of pH and inorganic nitrogen(N)at 20℃ and 210 μmolm-2s-1 (12:12/L:D)and its extracellular carbonic anhydrase(CA)and photosynthetic O2 evolution were investigated in order to see its physiological response to changes in pH under high N(300μmol/L)and low N(10μmol/L)levels during S.costatum bloom.The extracellular CA activity was assayed by the potentiometerical method,and the photosynthetic O2 evolution was determined by a Clark-type Oxygen Electrode.When pH in seawater rose from 8.2 to 8.7,the extracellular CA activity was induced and photosynthetic CO2 affinity(1/Km) increased,and,at pH8.7,the extracellular CA activity and photosynthetic CO2 affinity at the high N level were 2-times and 80% higher than those at the low N level,respectively.The chla-specific light-saturated photosynthetic rates(Pma)were no significant differences under different pH and N levels,but the cell-specific light-saturated photosynthetic rates(Pmc) was 100% higher at the high N level than at the low N level;By contrast,the cell-specific dark respiratory rates (Rdc) were insignificant differences among the different pH and N levels,but the chla-specific dark respiratory rates (Rdc) were l-fold higher at the low N level than at the high N level.The apparent photosynthetic efficiencies (α) were not significantly different for the alga grown among the different pH and N levels.The cellular chla content was l-fold higher at high-N-grown cells than at low-N-grown cells,but it was not affected by the pH changes.It was concluded that the alga increased the activities of extracellular CA and photosynthetic CO2 affinity with the rise of pH in seawater (or the decrease of CO2 in seawater),but their activites were higher at high N level than at low N level,suggesting that,during occurance of the bloom, S.costatum could develop the inorganic carbon concentration mechanism (CCM) to elevate the efficiency of inorganic carbon utilization,and to maintain the photosynthetic activity under low CO2 condition.The repleted N was benefited to improve the efficiency of CCM and then enhance the capacity of photosynthetic CO2 fixation under low CO2 (high pH) condition.