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

Photoreduction characteristics of divalent inorganic mercury (Hg) 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 under various light conditions. The experimental results showed that percentage reduction of Hg was significantly influenced by radiation wavelengths, and dramatically decreased with the presence of algae. The highest percentage reduction of Hg 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 photoreduction with the presence of algae under different light conditions. The percentage reduction of Hg decreased from 15% to 11% when living and dead AF concentrations increased by 10 times (from 10 to 10 cells/mL), and decreased from11% to ~9% in the case of living and dead MA increased. Algae can adsorb Hg and decrease the concentration of free Hg, thus inhibiting Hg photoreduction, especially under the conditions with high concentrations of algae. No significant differences were found in percentage reduction of Hg between living and dead treatments of algae species. The results are of great importance for understanding the role of algae in Hg photoreduction.