In this study, Nitzschia closterium was incubated in seawater at different pH values (8.10, 7.71, and 7.45) and using different nitrogen forms (NO3-N and NH4-N) in the laboratory. The results showed that the growth of N. closterium was inhibited by ocean acidification, with individuals under lower pH levels showing lower growth rates and lower nitrogen uptake rates for both nitrogen forms. The Vmax/Ks ratio decreased with decreasing pH, indicating the inhibition of nitrogen uptake, whereas the ratios for NH4-N cultures were higher than those for NO3-N cultures, implying the highly competitive position of NH4-N. Acidification might induce reactive oxygen species based on the result that the maximum enzyme activities of SuperOxide Dismutase (SOD) and CATalase (CAT) increased under lower pH levels. The SOD and CAT activities for the NO3-N cultures were higher than those for NH4-N cultures at the low pH level, indicating that acidification might cause more oxidative stress for NO3-N cultures than for NH4-N cultures. Thus, ocean acidification might have a more detrimental effect on the growth of N. closterium under NO3-N conditions than NH4-N conditions, with a lower ratio (γ) of the maximum growth rate to the maximum nutrient uptake rate, and a drop in nitrate reductase activity under lower pH levels.