Abstract: Alexandrium tamarense, a typical species around China coastal water zone, was cultured under conditions of five N/P ratios (1∶1, 16∶1, 50∶1, 100∶1, 200∶1) for 14 days. The growth rates and pigment contents were measured at the end of the experiment. Then the cultures were exposed under three types of radiation (visible light, P; visible light + ultraviolet A, PA; visible light + ultraviolet A+B, PAB). The effective photochemical efficiency and exposure response curves (ERC) were detected after 60min exposure. The results showed the fastest growth rate and highest pigment contents (Chl-a: 3.06×10^-5 μg·cell^-1 and Caro: 2.10×10^-5 μg·cell^-1) were observed in the 16∶1 (N/P) treatment. However, the growth rates and pigment contents declined significantly when the N/P ratio in the media increased or decreased. The growth rate was a function of N/P ratio according to a twoorder equation (R²>0.98). The variation of effective photochemical efficiency was fitted well by a firstorder exponential equation (R²>0.97). The effective photochemical efficiency decreased under PA and PAB treatments compared to that under P treatment. There was a negative linear relationship between the decreased degree of the effective photochemical efficiency and the ratio of algal cell photosynthetic system II D₁ protein repair rate and damage rate (r/k) (R²>0.98). The dependency of UVinduced photoinhibition in A. tamarense on the N/P ratio was fitted well by a two-order equation (R²>0.95). The r/k ratio was the highest and the decreased degree of the effective photochemical efficiency and the photoinhibition rate were the lowest in the 16∶1 (N/P) treatment. Therefore, the N/P ratio of 16∶1 is the optimum condition for the algal cells to defense the UVinduced photoinhibition by the alteration of r/k ratio.

Key words: tea plant, soil microbial community, phospholipid fatty acid., microbial utilization rate of carbon source, ratooning continuous cropping