Algae: A considerable regulator of methylmercury in mercury-contaminated paddy fields

Algae play a significant role in regulating the biogeochemical cycle of mercury (Hg), particularly its highly toxic form, methylmercury (MeHg), within wetlands and lakes. However, despite their widespread presence in paddy fields, their specific effects on MeHg dynamics are not yet fully understood. This study investigated the algae-mediated distribution and accumulation of MeHg in Hg-contaminated paddy fields, utilizing field investigation and microcosm experiment. The results indicated that algae demonstrated a multi-source uptake pattern and had a high capacity to accumulate both total Hg (926 ± 1.69 ng/g) and MeHg (83.6 ± 1.12 ng/g). This accumulation was predominantly characterized by inorganic Hg species, accounting for 88.4 %. Algal capsular extracellular polymeric substance (EPS) exhibited higher levels of labile organic fractions, such as cysteine (0.59 ± 0.01 µg/g), and increased MeHg concentration (84.2 ± 4.51 ng/kg), which contrasted with the levels found in colloidal EPS (0.06 ± 0.01 ng/g and 15.0 ± 1.61 ng/kg). Meanwhile, a significant correlation (P < 0.05) was observed between thiol-containing cysteine and MeHg concentrations in algal EPS. Algal incubation further revealed the formation of mineral-EPS-algae aggregates on algal surfaces, accompanied by the colonization of Hg-methylating bacteria. Therefore, the surface of algae may form a favorable matrix for microbial Hg methylation and the accumulation of MeHg. These findings emphasize the crucial roles of algae in regulating MeHg dynamics within algae-water system in Hg-contaminated paddy fields.