Abstract: In order to understand the heavy metals resistance mechanism of mangrove, a stress experiment with the seedlings of mangrove species Avicennia marina from Fujian Province was conducted to study the impacts of different concentration (1, 5, 10, 50, and 100 mg·L^-1) mercury (Hg²⁺) on the seedlings leaf photosynthesis and antioxydase activities. With the increase of added Hg²⁺ concentration and exposure time, the seedlings leaf net photosynthetic rate (P_n) and chlorophyll content decreased. Under the stress of high concentration Hg²⁺ (≥50 mg·L^-1), the decrement of P_n was higher than that under the stress of low concentration Hg²⁺, and the P_n decreased rapidly after 48 h. There was a significantly negative correlation between P_n and extracellular carbon dioxide concentration (C_i). The chlorophyll content decreased with increasing  Hg²⁺ concentration. The stomatal conductance (G_s) had different responses to the stress of different  Hg²⁺ concentration. At low  Hg²⁺ concentration, stomatal factors could limit the photosynthesis; while at high  Hg²⁺ concentration, non-stomatal factors could be the main causes inhibiting photosynthesis. Low  Hg²⁺ concentration promoted the activities of superoxide dismutase and peroxidase, while high  Hg²⁺ concentration inhibited the activities of the two enzymes, being the lowest under the stress of 100 mg Hg²⁺·L^-1. All the results implied that Hg²⁺ could inhibit the leaf photosynthesis of A. marina, and high concentration Hg²⁺ impaired the active oxygen elimination capability of A. marina, making the plants easily to be damaged.

Key words: Molecular microbial ecology, Molecular technique, Microbial ecology