Effects of typical algae species (Aphanizomenon flosaquae and Microcystis aeruginosa) on photoreduction of Hg2+ in water body

Abstract Photoreduction characteristics of divalent inorganic mercury (Hg 2+ ) 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 Hg 2+ under various light conditions. The experimental results showed that percentage reduction of Hg 2+ was significantly influenced by radiation wavelengths, and dramatically decreased with the presence of algae. The highest percentage reduction of Hg 2+ 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 Hg 2+ photoreduction with the presence of algae under different light conditions. The percentage reduction of Hg 2+ decreased from ~ 15% to ~ 11% when living and dead AF concentrations increased by 10 times (from 10 6 to 10 5  cells/mL), and decreased from ~ 11% to ~ 9% in the case of living and dead MA increased. Algae can adsorb Hg 2+ and decrease the concentration of free Hg 2+ , thus inhibiting Hg 2+ photoreduction, especially under the conditions with high concentrations of algae. No significant differences were found in percentage reduction of Hg 2+ between living and dead treatments of algae species. The results are of great importance for understanding the role of algae in Hg 2+ photoreduction.