3:45 PM - 4:00 PM
[AOS17-08] Phylogeny and distribution of prokaryotes involved in mercury speciation in the western North Pacific
Keywords:Western North Pacific, Marine prokaryotes, Mercury, Methylated mercury
Mercury (Hg) is one of the heavy metals with high toxicity, and exists universally in various chemical forms in the natural environment and circulates globally. In the ocean, Hg mainly exists as elemental mercury (Hg(0), divalent mercury (Hg(II)), and methylmercury (MeHg). MeHg is known to be neurotoxic and accumulates in marine organisms through the food chain. Consequently, elucidating the MeHg formation and degradation mechanisms in seawater is important for understanding MeHg dynamics in the ocean. Microbial activity in the ocean mediated Hg chemical transformations (i.e., Hg methylation, MeHg demethylation, and the Hg(II) reduction). However, information regarding the specific microbial taxa involved in these processes and their spatial distribution across oceanic basins remains limited. This study aimed to elucidate the phylogeny and distribution of microbial genes associated with Hg methylation, demethylation, and reduction (hgcAB, merB, and merA) using metagenomic and quantitative PCR (qPCR) analyses.
Seawater samples were collected during the Mirai MR21-06 Leg 1 and Hakuho-Maru KH-22-7 cruises, targeting the western North Pacific, and analyzed for total Hg (THg) and MeHg concentrations, alongside metagenomic sequencing. The results of Hg analysis for the MR21-06 cruise revealed an increasing trend in MeHg concentrations in the bathypelagic zone (1000–1500 m), ranging from 0.27 to 0.66 pM. Additionally, a significant positive correlation between MeHg concentrations and apparent oxygen utilization (AOU)—a proxy for microbial respiration—suggests that MeHg production is closely linked to microbial metabolic activity. Metagenomic analysis of the same samples revealed an enrichment of hgcAB, merB, and merA genes in the mesopelagic zone with relatively high MeHg concentration. Phylogenetic analysis represented that Nitrospina dominated the hgcA gene sequences (67% of total), while Alphaproteobacteria prevailed in the merB gene sequences (57% of total). Furthermore, metabolic function analysis of Metagenome-Assembled Genomes (MAGs) reconstructed from metagenomic data revealed that MAGs with hgcA genes exhibited nitrite reduction capabilities, whereas those carrying merA and merB genes were associated with methane production. These findings highlight the strong linkage between the Hg transformation processes and the nitrogen and methane metabolisms in the ocean.
Moreover, qPCR analysis using primers designed based on metagenome sequence was applied to quantify the Nitrospina-hgcA copy number on seawater samples from the KH-22-7 expedition (from surface to 5,000 m depth). Results showed an increasing trend in Nitrospina-hgcA at depths of 500–1500 m, accompanied by a positive correlation with methylmercury concentrations. These data suggest that the Nitrospina lineage can contribute to the large-scale distribution of methylmercury in the western North Pacific.
Seawater samples were collected during the Mirai MR21-06 Leg 1 and Hakuho-Maru KH-22-7 cruises, targeting the western North Pacific, and analyzed for total Hg (THg) and MeHg concentrations, alongside metagenomic sequencing. The results of Hg analysis for the MR21-06 cruise revealed an increasing trend in MeHg concentrations in the bathypelagic zone (1000–1500 m), ranging from 0.27 to 0.66 pM. Additionally, a significant positive correlation between MeHg concentrations and apparent oxygen utilization (AOU)—a proxy for microbial respiration—suggests that MeHg production is closely linked to microbial metabolic activity. Metagenomic analysis of the same samples revealed an enrichment of hgcAB, merB, and merA genes in the mesopelagic zone with relatively high MeHg concentration. Phylogenetic analysis represented that Nitrospina dominated the hgcA gene sequences (67% of total), while Alphaproteobacteria prevailed in the merB gene sequences (57% of total). Furthermore, metabolic function analysis of Metagenome-Assembled Genomes (MAGs) reconstructed from metagenomic data revealed that MAGs with hgcA genes exhibited nitrite reduction capabilities, whereas those carrying merA and merB genes were associated with methane production. These findings highlight the strong linkage between the Hg transformation processes and the nitrogen and methane metabolisms in the ocean.
Moreover, qPCR analysis using primers designed based on metagenome sequence was applied to quantify the Nitrospina-hgcA copy number on seawater samples from the KH-22-7 expedition (from surface to 5,000 m depth). Results showed an increasing trend in Nitrospina-hgcA at depths of 500–1500 m, accompanied by a positive correlation with methylmercury concentrations. These data suggest that the Nitrospina lineage can contribute to the large-scale distribution of methylmercury in the western North Pacific.