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In this study, bacterial mercury (Hg) methylation was investigated under the influence of red-tide algae of Skeletonema costatum (S. costatum). The distribution and speciation of total mercury (THg) and methylmercury (MeHg) were profiled by employing Geobacter metallireducens (G. metallireducens GS-15) as the methylating bacteria. G. metallireducens GS-15 showed different capabilities in methylating different inorganic forms of Hg(II) (HgCl 2) and Hg(II)-Algae (HgCl 2 captured by S. costatum) to MeHg. In the absence of S. costatum , a maximum methylation efficiency of 4.31 ± 0.47% was achieved with Hg(II) of 1–100 μg L−1, while accelerated MeHg formation rate was detected at a higher initial Hg(II) concentration. In the presence of S. costatum , there were distinct changes in the distribution of THg and MeHg by altering the bioavailability of Hg(II) and Hg(II)-Algae. A larger proportion of THg tended to be retained by a higher algal biomass, resulting in decreased methylation efficiencies. The methylation efficiency of Hg(II) decreased from 3.01 ± 0.10% to 1.01 ± 0.01% with 10-mL and 250-mL algal media, and that of Hg(II)-Algae decreased from 0.83 ± 0.13% to 0.22 ± 0.01% with 10-mL and 250-mL Hg(II)-Algae media. Fourier transform infrared spectrometry, surface charge properties and elemental compositions of S. costatum were used to infer that amine, carboxyl and sulfonate functional groups were most likely to interact with Hg(II) through complexation and/or electrostatic attraction. These results suggest that red-tide algae may be an influencing factor on bacterial Hg methylation in eutrophic water bodies. Unlabelled Image • Bacterial Hg methylation was investigated in the Hg-algae-bacteria system. • Bacterial MeHg yield and methylation efficiency depended on algal biomass. • Higher algal biomass reduced the potential risk of bacterial Hg methylation. • S. costatum exerted significant influences on Hg distribution and speciation. • Various functional groups in algal cells were involved in Hg-algae interactions. [ABSTRACT FROM AUTHOR]

Changes in the biogeochemical cycling of mercury (Hg) and Hg species were investigated in a typical marine aquaculture area located at Zhoushan island, Zhejiang province, east China. Mercury species were analyzed in different environmental samples collected during a field survey and a simulation batch experiment. The field work comprised both summer and winter collection of water and sediment samples from marine aquaculture sites (MAS) in a field survey and from corresponding reference sites (CRS) located 2500 m from the MAS. THg concentrations in water were 91.3 ± 70.3 and 115 ± 22.6 pmol L−1 in summer and winter, respectively. Particulate Hg accounting for ˃60% of THg and positively correlated with total suspended solid content in water. Dissolved organic carbon in water was positively correlated with dissolved Hg. Significantly higher (p < 0.001, F = 102) total methylmercury (TMeHg) contents were observed in MAS (0.31 ± 0.26 ng g−1) than in CRS (0.06 ± 0.03 ng g−1) in the sediment solid phase. Moreover, MeHg formation rate in MAS was clearly higher than in CRS in the simulation experiment. Both the field survey and the simulation experiment highlighted the readier formation of MeHg in MAS than in CRS. TMeHg levels in blackhead seabream (Acanthopagrus schlegelii), red snapper (Lutjanus campechanus) and perch (Perca fluviatilis) were only 52.7 ± 5.74, 23.7 ± 2.51 and 24.3 ± 3.86 ng g−1, values significantly lower than the safety guideline (1000 ng g−1) established by the World Health Organization. Unlabelled Image • THg concentration in water was 91.3 pmol·L−1 and PHg account for 63.4% of THg. • Fish farming increased THg accumulation and MeHg formation in sediment. • Suspended solid influence the Hg distribution and sedimentation in water. • MeHg concentration in fish tissues was significantly lower than WHO guidelines. [ABSTRACT FROM AUTHOR]