Abundance and expression ofhgcABgenes and mercury availability jointly explain methylmercury formation in stratified brackish waters

Abstract

Neurotoxic methylmercury (MeHg) is formed by microbial methylation of inorganic divalent Hg (HgII) and constitutes severe environmental and human health risks. The methylation is enabled by hgcA and hgcB genes, but it is not known if the associated molecular-level processes are rate-limiting or enable accurate prediction of MeHg formation in nature. In this study, we investigated the relationships between hgcA genes and MeHg across redox stratified water columns in the brackish Baltic Sea. We found that the abundance of hgcA genes and transcripts combined with the concentration of dissolved HgII-sulfide species were strong predictors of both HgII methylation rate and MeHg concentration, implying their roles as principal joint drivers of MeHg formation in these systems. In establishing relationships between hgcA genes and MeHg, we advance the fundamental understanding of mechanistic principles governing MeHg formation in nature and enable refined predictions of MeHg levels in coastal seas in response to the accelerating spread of oxygen deficient zones.

A consensus protocol for the recovery of mercury methylation genes from metagenomes

“…To date, all hgcA + microorganisms that have been experimentally tested have been found shown to produce MeHg (except for those with fused hgcAB -like sequences) (Gilmour et al, 2013; 2018), and protein modeling of novel hgcA sequences suggest they have comparable active sites to HgcA sequences in experimentally verified Hg methylators. Therefore, although recent findings revealed relationships between microbial expression of hgc transcripts and MeHg formation in the environment (Capo, Feng et al 2022 bioRxiv), and some putative hgcAB genes have been computationally modelled to possess functionality for methylation (Gionfriddo et al 2016, Lin et al 2021), we remain cautious about defining true hgc genes from environmental samples. As such, some studies have qualified hgc genes found in the environment as hgc genes (e.g., Gionfriddo et al, 2016; Bowman et al, 2020; Villar et al, 2020; Capo et al, 2020).…”

“…However, the recovery of MAGs from metagenomes is not always possible due to (i) the difficulty of obtaining MAGs from certain environments such as sediments and (ii) the low predominance of Hg methylators compared to other microorganisms in the environment, and therefore the lower probability of recovering hgc + MAGs. A recent work revealed the good congruence between the identification of hgc + MAGs and a hgc phylogeny based on Hg-MATE phylogeny (Capo, Feng et al, 2022 bioRxiv) highlighting that both approaches could be used to ensure the reliability in the identification of Hg methylators.…”

Anaerobic mercury methylators inhabit sinking particles of oxic water columns

“…Hg methylators are known to thrive in oxygen-deficient environments (e.g., rice paddies, wetlands, sediments, anoxic waters) in which redox conditions play an important role regulating both the activity of Hg II methylating microorganisms and the availability of Hg II for methylation (Bravo and Cosio, 2020). Recent works further highlighted the huge diversity of Hg methylators in aquatic water columnsmostly oxygen-deficient -and underlying sediments (Lin et al, 2021;Azaroff et al, 2020;Capo et al, 2022b).…”

A consensus protocol for the recovery of mercury methylation genes from metagenomes

“…bioinformatics, hg methylation, hgcAB genes, hg-MATE, marky-coco, mercury, metagenomics Capo, Feng, et al, 2022). Estimating the relative abundance of hgc sequences in a meta-genome/transcriptome requires normalization strategies that account for differences in sequencing depth and coverage to avoid over-or under-representing hgc + microorganisms and their functional importance.…”