Abstract: Sargassum horneri(Turner) C. Agardh， one of the common macroalgae along the coast of Zhejiang Province， plays an important role in the marine ecosystem. Because of the environmental deterioration within decade years， S. horneri beds along the Nanji Archipelago had been degenerated. Therefore， to study the degenerated mechanism of S. horneri is the key point to keep littoral ecosystem balance and restore the macroalgae beds. In this study， we investigated the effect of two irradiance levels (low light， LL: 40-50 μmol·m-2·s-1; high light， HL: 100-110 μmol·m-2·s-1) and two CO2 concentrations (low CO2， LC: 076 μg·L-1; high CO2， HC: 196 μg·L-1) on the growth of S. horneri， in order to detect the response of this species to HL and HC. The results showed that the relative growth rate (RGR) of S. horneri in HL was 180%·d-1， 21 times of that grown in LC (085%·d-1). The RGRs were 289%·d-1 in HC and 249%·d-1 in LC， respectively， and the former was 116 times of the latter. Both the height of thalli and the number of leaves in treatments of HC and HL was higher than LC and LL， respectively， but the Chla contents were higher in LC and LL than in HC and HL， respectively．

Key words: CO2, irradiance, growth, Sargassum horneri