Abstract:The dynamic responses of photosynthesis of Chaetoceros sp. to continuous high irradiance (1 000 μmol m-2s-1) for 12 h were demonstrated by measuring chlorophyll fluorescence. Both maximal photosynthetic rate (Pm, i.e. maximal relative electron transport rate) and maximal quantum yield ofphotosystem Ⅱ (Fv/Fm) decreased during 30 min of high light exposure, which showed obvious photoinhibition. Meanwhile, the initial slope α decreased while non-photochemical quenching (NPQ) increased, which means Chaetoceros sp. can reduce light absorption and dissipate more heat in order to protect their photosynthetic apparatus from damage. Chaetoceros sp. acclimated high light gradually and their photosynthetic activity recovered slowly during 1 000 μmol m-2s-1 exposure from 30 min to 8 h. α increased（although cannot reach the value before high light exposure）, Pm recovered to initial value at 4 h and even reached 2 times of the initial value at 8 h. Fv/Fm could only recover to half of the initial value during the acclimation. NPQ reached the maximum at 1 h, and then decreased. This result indicates that heat dissipation does not play major role in photoprotection after 1 h high light exposure. After exposure to high irradiance, there appeared transient post-illumination increase in chlorophyll fluorescence. The increase in chlorophyll fluorescence enhanced with increased duration of high irradiance, which reflects enhancement of cyclic electron transport around photosystem Ⅰ. Thus, after NPQ decreased, cyclic electron transport around photosystem Ⅰ may play important role in photoprotection.