Photoreduction characteristics of divalent inorganic mercury (Hg2+) in the presence of specific algae species are still not well known.Laboratory experiments were conducted in the present study to identify the effects of different concentrations of living/dead algae species,including Aphanizomenon flosaquae (AF) and Microcystis aeruginosa (MA),on the photoreduction rate of Hg2+ under various light conditions.The experimental results showed that percentage reduction of Hg2+ was significantly influenced by radiation wavelengths,and dramatically decreased with the presence of algae.The highest percentage reduction of Hg2+ was induced by UV-A,followed by UV-B,visible light and dark for both living and dead AF,and the order was dark ＞ UV-A ＞ UV-B ＞ visible light for both living and dead MA.There were two aspects,i.e.,energy and attenuation rate of light radiation and excrementitious generated from algae metabolisms,were involved in the processes of Hg2+ photoreduction with the presence of algae under different light conditions.The percentage reduction of Hg2+ decreased from 15％ to 11％ when living and dead AF concentrations increased by 10 times (from 106 to 105 cells/mL),and decreased from11％ to ～9％ in the case of living and dead MA increased.Algae can adsorb Hg2+ and decrease the concentration of free Hg2+,thus inhibiting Hg2+ photoreduction,especially under the conditions with high concentrations of algae.No significant differences were found in percentage reduction of Hg2+ between living and dead treatments of algae species.The results are of great importance for understanding the role of algae in Hg2+ photoreduction.