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Methylmercury (MeHg) is a hazardous neurotoxin, predominantly formed by microbial transformation of inorganic mercury in oxygen-depleted aquatic and terrestrial ecosystems. The ongoing deoxygenation of aquatic ecosystems due to global warming is likely to expand microbial niches for MeHg production. Although mercury methylators have also been reported to thrive in oxyge-deficients conditions in a few marine and freshwater ecosystems, there is a lack of comprehensive understanding of how they are distributed in freshwater systems. In this study, we retrieved hgcA genes, genomic marker for mercury methylation potential, from 586 metagenomes from the water column of 186 freshwater systems. Overall, hgcA genes were detected in the water column of 30 lakes, with the highest richness and abundance being detected in anoxic (0 mg O2L^-1) and hypoxic (>0–2 mg O2L^-1) compared to oxic conditions (>2 mg O2L^-1). Although Desulfobacterota had the highest hgcA gene richness across most freshwater systems, certain systems were dominated by hgcA genes from Bacteroidales and Kiritimatiellales, implying metabolic and ecological versatility of mercury methylators as a group. Our findings suggest that projected expanding deoxygenation may lead to new niches for mercury methylators in inland waters.

Mercury (Hg) is a global environmental concern due to its microbial conversion to methylmercury (MeHg), a potent neurotoxin that bioaccumulates in food webs and poses risks to ecosystems and human health. Thiol functional groups (RSH) play an important role in controlling Hg(II) speciation and bio-uptake in methylating bacteria, yet the spatial distribution and density of these thiols within cells remain largely unknown. We isolated subcellular fractions of the Hg methylating bacterium Geobacter sulfurreducens in the exponential growth phase, and used Hg LIII-edge EXAFS (Extended X-ray Absorption Fine Structure) to quantify thiols in the extracellular medium, inner and outer membranes, periplasm and cytoplasm. The whole-cell thiol content was determined to be 1.3 × 10−10 μmol cell−1. The inner membrane contributed 7.1 × 10−11 (53%), the outer membrane 1.2 × 10−11 (9%), the periplasm 3.6 × 10−11 (27%) and the cytoplasm 1.5 × 10−11 μmol cell−1 (11%). The extracellular fraction contributed an additional 5.7 × 10−11 μmol cell−1, corresponding to 30% of the thiols of the cell culture. Local thiol density (thiols normalized to TOC in individual compartment, RSH/TOC, μmol g−1 C) was 36, 450, 140, 600 and 29 μmol g−1 C in the cytoplasm, inner membrane, periplasm, outer membrane and extracellular fractions, respectively. EXAFS analyses demonstrate Hg-thiolate coordination across all compartments, with Hg-O/N bonding and elemental Hg0 formed at higher Hg loadings. In the periplasm, Hg-disulfide and traces of β-HgS were detected. The high thiol density at the membranes, relative to other compartments, may imply they have an important role in the retention and internalization of Hg(II). Periplasmic thiols may modulate Hg(II) transfer between membranes, and cytoplasmic thiols may regulate the intracellular availability of Hg(II) for methylation. This work provides the first compartment-resolved quantification of thiol abundances and densities in a model Hg-methylating bacterium at subcellular level, offering a mechanistic framework for understanding the speciation, bioavailability, and subcellular transformation of Hg(II) with relevance for other soft metals (e.g., Cd, Pb, Zn, Ag, and Cu).

Methylmercury (MeHg) accumulation in aquatic food webs is a key pathway for human exposure. We examined how terrestrially derived organic matter (tOM) influences MeHg bioaccumulation in coastal planktonic food webs by measuring MeHg bioaccumulation factors (BAFs) for a MeHg isotope tracer added to mesocosms receiving four tOM levels. Combined with two previous studies, the results showed that tOM alters MeHg bioaccumulation through opposing processes. All treatments produced highly heterotrophic food webs, with bacterial production contributing 68–87% of basal production and supporting high MeHg accumulation (log BAFs up to 6) due to complex trophic structure. However, average BAFs declined with increasing tOM among individual treatments, attributed to higher concentrations of thiol compounds associated with the dissolved organic matter (DOM-RSH) that suppress MeHg bioavailability. Together, these findings demonstrate that tOM inputs exert dual effects on MeHg bioaccumulation: enhancing it by promoting heterotrophy while simultaneously reducing it via increased DOM-RSH. Understanding the net outcome of these processes is essential for predicting MeHg dynamics under changing tOM inputs. (Figure presented.)

Photodegradation in lakes is a major sink for the toxin methylmercury (MeHg) in forest-wetland-lake ecosystems. Previous attempts to estimate annual rates of MeHg photodegradation in lakes have followed a "black-box approach", where the process has been related to incident sunlight rather than photon absorption. Here we use experimental data from three contrasting dark and clear boreal lakes to develop the first apparent quantum yield (AQY) model for spectral MeHg photodegradation rates in lakes. The model was proven universal by its ability to predict experimental data from 22 lakes representing five global regions, covering wide ranges in dissolved organic carbon (DOC), specific UV absorbance at 254 nm, and pH (1.8–39.5 mg C L^–1, 1.7–5.7 L mg^–1 C m^–1 and 4.8–8.5, respectively). The AQY model manifests a dual role played by dissolved organic matter (DOM) as a sensitizer, by producing reactive transient species (RTS) upon photon absorbance, and as an inhibitor, by scavenging of RTS by antioxidants associated with aromatic structures in DOM. Using site-specific data on direct and diffuse solar irradiance, MeHg concentrations, and spectral light absorption properties in 1033 lakes, we estimate an annual MeHg photodegradation rate of 12.1 kg y^–1 in the total volume of Swedish lakes. This value corresponds to 24% of the estimated 51 kg of MeHg that annually is transported with runoff from soil into the same lakes. By the AQY model, we calculate the first regional estimates of MeHg photodegradation in lakes of boreal and temperate Europe, temperate North America, subtropical North America, and tropical South America, providing a basis for the establishment of regional MeHg cycling models.

Deoxygenation in aquatic ecosystems threatens biodiversity at all levels of functional and genetic diversity. Recent studies have shown the prevalence of microorganisms that transform mercury into neurotoxic methylmercury (mercury methylators—hgcA⁺ prokaryotes) in oxygen-deficient water columns. As climate warming expands coastal oxygen minimum zones, ongoing and near-future changes may ultimately lead to increased methylmercury formation. However, little is known about the presence of aquatic mercury methylators before the Industrial Revolution, marked by increased mercury emissions and deposition in the environment. Here we have detected hgcA genes in Black Sea sedimentary archives, with the highest abundance 9,000–5,500 years ago when anoxic conditions were documented in the water column. Historical sedimentary and modern water column data on mercury methylators provide valuable insights for projecting future methylmercury production in aquatic ecosystems impacted by ongoing deoxygenation. It also underscores the potential impacts of climate change on human exposure to methylmercury from mercury-contaminated seafood.

There is a societal demand to restore drained boreal peatlands for purposes of improving water quality and biodiversity and lowering emissions of greenhouse gases. Restoration measures are costly and neither the effects of drainage nor restoration on biogeochemical processes in the peat, and in downstream environments are well understood. This study assesses how 60–100 years of drainage followed by 6–9 years of restored conditions have changed the physical and chemical peat properties in restored boreal peatlands. Eight pairs of restored and natural peatlands were sampled down to 50 cm (n = 3 for each site). Each of the 50 cm peat cores was sliced into 25 two-centimetre discs, generating high-resolution records of the dry bulk density (BD), organic matter content (OM), C- and N- content, δ13C, and δ15N. Peat from the restored sites showed significantly higher BD and lower C:N ratio and OM content than the reference sites. Furthermore, peat from restored peatlands was systematically depleted in δ13C, and the OM was enriched in C and N. Long-term drainage could cause increased peat decomposition, leaving altered physical and chemical peat properties. For example, the C content in OM increases as the residual peat is enriched in aromatic and aliphatic moieties following decomposition. For the same reason, degraded peat can be δ13C depleted. Interestingly, differences between the restored and pristine sites were mainly found at 20–50 cm depth. Given the low peat formation rates in nutrient-poor peatlands, the superficial 20 cm peat was potentially recovering from drainage even before restoration.

Atmospheric mercury (Hg) uptake by vegetation and subsequent deposition via litterfall constitutes a major pathway in the global Hg cycle. However, the temporal dynamics of litterfall Hg deposition and its environmental controls remain poorly understood. Here, we present a detailed assessment of Hg concentrations and deposition fluxes for individual litter components in a Swedish boreal forest from 1987 to 2000. Atmospheric Hg concentrations declined 41 % during this period. Correspondingly, Hg concentrations in Scots pine and Norway spruce needles decreased significantly (∼22 % and ∼26 %, respectively). However, the total litterfall Hg deposition flux remained stable at 11.7 ± 1.8 μg m−2 yr−1, showing no clear temporal trend. Foliar litter (needles) contributed 44 % of total Hg deposition, while non-foliar litter (twigs, residual material, and cones) accounted for the remaining 56 % (32 %, 22 %, and 2 %, respectively). The importance of this non-foliar component in modulating litterfall Hg deposition is often overlooked. Litterfall Hg deposition was 1.7 times higher in the non-growing season than in the growing season, primarily due to greater litterfall biomass. The weak response of litterfall Hg deposition to declining atmospheric Hg concentrations highlights the importance of biological factors (e.g., litterfall compositions and productivity) in regulating Hg inputs to boreal forest. Our findings also underscore the need for long-term assessments of Hg deposition dynamics via different litterfall components for assessing the effectiveness of the Minamata Convention on Mercury.

This report presents results from a project investigating the use of microbes as indicators of mercury (Hg) pollution in sediments. Microbes respond rapidly to environmental change, making them excellent bioindicators. However, to ensure reliable results, it is essential to analyze microbial communities soon after sampling to prevent degradation or loss of activity.

To address this, a portable molecular sequencing laboratory, OmiBox, was developed, enabling near in situ analysis of microbial taxonomic composition and gene expression. Additionally, microbial community composition was studied in Hg-contaminated fiber banks in the Gulf of Bothnia. The results revealed that certain taxonomic groups, such as bacteria from the phyla Campylobacterota and Desulfobacterota, were enriched in fiber-rich, Hg-polluted sediments.

The study also included tolerance experiments comparing bacteria from clean and contaminated sediments in response to Hg addition. Bacteria from polluted sites exhibited significantly greater tolerance, suggesting evolved resistance and retained microbial functionality. Nonetheless, Hg-contaminated sediments pose ecological risks, as methylmercury (MeHg) can bioaccumulate and magnify through the food web.

This project contributes a framework for understanding how bacterial community structure and function respond to Hg pollution in sediments, offering valuable tools for environmental monitoring and assessment.

Kemiska föroreningar orsakar problem världen över. Miljögifter som hamnat i akvatiska miljöer ackumuleras ofta till nivåer som har skadliga effekter på organismerna och hela ekosystemen. Inom EU-lagstiftningen används biomarkörer för att analysera statusen på naturliga miljöer. Mikroorganismer skulle kunna vara väl lämpade som bioindikator, eftersom de i stort sett finns överallt på jorden och snabbt svarar på miljöförändringar.

Målsättningen med detta projekt var att klarlägga om bakterier kan användas som indikator för kontaminerade sediment. Vi fokuserade på kvicksilver, som är ett vanligt förekommande miljögift i fiberrika sediment i Bottniska viken. Inom projektet utvecklades ett portabelt molekylärt sekvenseringslaboratorium (Omibox), experiment utfördes för att testa effekter av terrestra organiska ämnen samt mikroorganismers toleransnivåer för kvicksilverbelastning. Därutöver utfördes fältstudier i gradienter av kvicksilverbelastade områden i Bottniska viken för att hitta möjliga indikatorer i bakteriesamhället.

Methylmercury (MeHg) can form through the microbial transformation of divalent inorganic mercury (HgII). However, it remains unknown whether the total concentration of HgII is a main controlling factor for this methylation process in paddy soils unaffected by local Hg point sources. Here we study the occurrence and controlling factors for MeHg levels in non-contaminated rice paddy soil in Cambodia using 164 soil and 100 overlying water samples from different provinces in wet and dry seasons. Paddy soils were characterized with respect to particle size classes, nutrients, and biogeochemical parameters expected to influence Hg processes. Total mercury (THg) and MeHg concentrations in the soils were not related to geographical location or sampling season but to soil physical and chemical properties. We observed significant positive relationships between the concentrations of divalent inorganic Hg (HgII) and MeHg, suggesting that the concentration of HgII is the main factor determining the net formation of MeHg in non-contaminated rice paddy soils. The %MeHg of THg was used as a proxy of the potential for MeHg formation and was significantly, and inversely, correlated with the redox conditions of the soils, as approximated by the oxidation state of sulfur. The study elucidates critical factors driving MeHg levels in rice paddy soil, enhances the understanding of the MeHg formation process and provides a refined basis for soil quality regulation regarding Hg. The results suggest that reducing Hg inputs to paddies will be effective to lower MeHg concentrations in the soil, ultimately reducing its presence in rice grains.

Tree rings are an emerging atmospheric mercury (Hg) archive. Questions have arisen, though, regarding their mechanistic controls and reliability. Here, we report contrasting tree-ring Hg records in three collocated conifer species: Norway spruce (Picea abies), Scots pine (Pinus sylvestris), and European larch (Larix decidua), which are from a remote boreal forest. Centennial atmospheric Hg trends at the site, derived from varved lake sediments, peats, and atmospheric monitoring, indicated a steady rise from the 1800s, peaking in the 1970s, and then declining. Prior to ca. 2005, larch and spruce tree rings reproduced the peak in the atmospheric Hg trend, while pine tree rings peaked in the 1930s, likely due to the prolonged sapwood period and ambiguity in the heartwood-sapwood boundary of pine. Since ca. 2005, tree rings from all species showed increasing Hg concentrations in the physiologically active outer rings despite declining atmospheric Hg concentrations. The good agreement between Hg and nitrogen concentrations in active tree-ring cells indicates a similar transport mechanism and cautions against their applicability as atmospheric Hg archives. Our results suggest that tree-ring Hg records are controlled by atmospheric Hg and tree physiology. We provide recommendations for using tree-ring Hg archives that take tree physiology into account.