Arsenic (As) is a ubiquitous toxic element which occurs naturally in the earth’s crust. During recent years, human activities have caused serious As pollution. Since aquatic microorganisms served as essential primary producers in the ecosystem, the study of the effects of As on aquatic microorganisms will provide important information for understanding the risk of As to the environment and ecosystem. However, the effects of As(III) on growth and photosynthesis of phytoplankton is still unclear.

Microcystis aeruginosa was considered as one of the most common cyanobacteria and an ideal material for studying the effects of contaminants on photosynthesis. Photosystem II (PSII) has been demonstrated to be very sensitive to various pollutants, which can be reliably probed by chlorophyll fluorescence. But the knowledge of the effects of As(III) on the photosynthetic capparatus of phytoplankton is limited.

Therefore, by using M. aeruginosa as material, WANG Shuzhi et al. investigated the effects of As(III) on growth, pigments content, oxygen evolution and PSII activity of cyanobacteria.

The results showed that various concentrations of As(III) did not show significant effects on growth and total carotenoids content within 24h of treatment. After 48h of treatment, 10mgL^-1 As(III) significantly inhibited the growth and synthesis of carotenoids of M. aeruginosa,  while As(III) at concentrations ranging from 0 to 1 mgL^-1 showed no significant inhibition.

Chlorophyll α synthesis, oxygen evolution and chlorophyll fluorescence were more sensitive to As(III) exposure than carotenoid synthesis and growth. Chlorophyll α content, fast fluorescence rise transients and fluorescence decay kinetics appeared to be affected after the cells were exposed to 1 and 10mgL^-1 As (III) for more than 24h. Treatment with 10mgL^-1 As(III) for 24h or longer led to flattening of the fluorescence transient and drastic decrease of amplitude of fast phase of Q_A^- reoxidation kinetics. Exposure to As(III) mainly inhibited the quantum yield for primary photochemistry, density of reaction centers and photosynthesis performance index, and increased the dissipated energy.

The decrease in amplitude of the fast and middle phases further revealed that once electron transfer from Q_A^- to Q_B was inhibited by As(III), more Q_A^- was reoxidized via S₂(Q_AQ_B)^- charge recombination. As(III) stress may result in an increased stability of the S₂QB^- and S₂QA^- recombination.

The result was published in Ecotoxicology and Environmental Safety in October, 2012.

CONTACT:
Prof. PAN Xiangliang
E-mail: panxl@ms.xjb.ac.cn