Heavy-metal pollution alters dissolved organic matter released by bloom-forming Microcystis aeruginosa

The risk of heavy metals to aquatic ecosystems has been paid much attention worldwide in recent years, however, the knowledge on effects of heavy metals on dissolved organic matter (DOM) released by Microcystis was quite poor, especially in eutrophic lakes. The aim of this study is to investigate effects of heavy metals on DOC release of Microcystis using EEM-PAFAFAC analysis. Microcystis aeruginosa was batch cultured for 18 days in BG-11 medium treated with a range of concentrations of Cu²⁺ (0.02, 0.05, 0.10 and 0.25 mg L⁻¹) and Zn²⁺ (0.10, 0.20, 0.50 and 1.00 mg L⁻¹), to investigate the effects of heavy metals on DOM release of Microcystis. The cultures without addition of Cu²⁺ or Zn²⁺ were set as controls. Cell density in the treatment of 0.25 mg L⁻¹ Cu²⁺ was 25% less than that in the control, but addition of 0.25 mg L⁻¹ Zn²⁺ had little effect on Microcystis growth. Moderate levels of Cu²⁺ (0.05 and 0.10 mg L⁻¹) and high level of Zn²⁺ (1.00 mg L⁻¹) stimulated DOC production of M. aeruginosa on day 10; moderate levels of Zn²⁺ (0.10–0.50 mg L⁻¹) stimulated DOC production on day 18 compared to the control. Four components of DOM: two humic-like components (C1 and C3) and two protein-like components (C2 and C4), were identified by fluorescence excitation – emission matrix spectroscopy combined with parallel factors (EEM-PAFAFAC) analysis. The fluorescence intensity of the four components increased when Microcystis was treated with moderate level of Cu²⁺ (0.05 and 0.10 mg L⁻¹) on day 10. In addition, the composition of DOM produced by Microcystis was not affected by heavy metals. Our results suggested that Microcystis increased DOM production in logarithmic phase by altering cellular secretion, but large biomass was the effective measure for M. aeruginosa to reduce toxicity of heavy metals in the stationary phase.