The coupling effect promotes superoxide radical production in the microalgal-fungal symbiosis systems: Production, mechanisms and implication for Hg(II) reduction.

Redox transformation of mercury (Hg) is critical for Hg exchange at the air-water interface. However, the superoxide radicals (O) contribution of microalgal-fungal symbiotic systems in lake water to Hg(II) reduction is mainly unknown. Here, we studied the enhanced potential for O production by the coupling effect between microalgae and fungi. The relationships between microenvironment, microorganisms, and O production were also investigated. Furthermore, the implication of O for Hg(II) reduction was explored. The results showed that the coupling effect of microalgae and fungi enhanced O generation in the symbiotic systems, and the O generation peaked on day 4 in the lake water at 160.51 ± 13.06-173.28 ± 18.21 μmol/kg FW (fresh weight). In addition, O exhibited circadian fluctuations that correlated with changes in dissolved oxygen content and redox potential on the inter-spherical interface of microalgal-fungal consortia. Partial least squares path modeling (PLS-PM) indicates that O formation was primarily associated with microenvironmental factors and microbial metabolic processes. The experimental results suggest that O in the microalgal-fungal systems could mediate Hg(II) reduction, promoting Hg conversion and cycling. The findings highlight the importance of microalgae and fungal symbiotic systems in Hg transformation in aquatic environments.