Phylogeny of prokaryotes involved in mercury speciation in free-living and particulate-attached fractions in Minamata Bay, Japan

Marine prokaryotes are involved in mercury (Hg) speciation in marine environments. However, information regarding the specific microbial lineages contributing to Hg speciation (namely, methylation, demethylation, and reduction) in seawater remains limited. In this study, we investigated the genes involved in methylmercury (MeHg) demethylation (merB), Hg reduction (merA), and Hg methylation (hgcA and hgcB) using a metagenome analysis encompassing free-living (FL; 0.2–3 µm) and particle-attached (PA; > 3 µm) fractions in April and June in Minamata Bay, Japan. We analyzed the total Hg (THg) and MeHg concentrations in dissolved, small particulate (0.2–3 µm), and large particulate (> 3 µm) fractions. Hg analysis showed no significant differences in THg and MeHg concentrations among the three fractions. However, THg and MeHg in the pooled particulate (fractions were higher than those in the dissolved fractions) in June. As for the Hg-related genes, a high merB abundance rather than merA was observed when compared using all samples, suggesting that microbial MeHg demethylation could be prominent rather than Hg reduction in the seawater column. Phylogenetic analyses revealed that Alphaproteobacteria and Actinobacteria were dominant in the total merA and merB-like sequences, respectively. For both merA and merB genes, some lineages appeared in either or both FL and PA fractions. A deltaproteobacteria-like hgcA was also detected in the PA fraction in April. Therefore, the link between distinct phylogenetic lineages within the PA and FL fractions could be significant for Hg speciation in Minamata Bay.