Global change effects on biogeochemical mercury cycling

Abstract

Past and present anthropogenic mercury (Hg) release to ecosystems causes neurotoxicity and cardiovascular disease in humans with an estimated economic cost of $117 billion USD annually. Humans are primarily exposed to Hg via the consumption of contaminated freshwater and marine fish. The UNEP Minamata Convention on Hg aims to curb Hg release to the environment and is accompanied by global Hg monitoring efforts to track its success. The biogeochemical Hg cycle is a complex cascade of release, dispersal, transformation and bio-uptake processes that link Hg sources to Hg exposure. Global change interacts with the Hg cycle by impacting the physical, biogeochemical and ecological factors that control these processes. In this review we examine how global change such as biome shifts, deforestation, permafrost thaw or ocean stratification will alter Hg cycling and exposure. Based on past declines in Hg release and environmental levels, we expect that future policy impacts should be distinguishable from global change effects at the regional and global scales.

Keywords: Climate change; Environment; Exposure; Fish consumption; Minamata Convention; Toxicity.

Fig. 1

Modern day global Earth surface Hg cycling budget. Important differences from previous budgets are the revised volcanic emissions (Li et al. 2020), river transport and coastal sedimentation (Liu et al. 2021), deep sea sedimentation (Hayes et al. 2021), and enhanced vegetation and soil Hg⁰ uptake of 2850 Mg year⁻¹. Important uncertainties remain for atmosphere–ocean exchange of Hg. All-time anthropogenic Hg release to land and water is estimated to be 1 070 000 Mg, of which 390 000 Mg are still sequestered at contaminated sites and supplied annually by 7300 Mg year⁻¹ of new Hg releases (Streets et al. 2017). Terrestrial remote soil and discarded Hg pools both drive modern river Hg flux and re-emission to air. Major reservoir Hg inventories were also updated: atmosphere (Shah et al. 2021), soil (Lim et al. 2020) and ocean (Zhang et al. 2020). All other fluxes are from Shah et al. (2021)

Fig. 2

a Historical primary global anthropogenic Hg emissions since 1850 according to different emission inventories: AMAP/UNEP (UNEP 2018), EDGARv4.tox2 (Muntean et al. 2018), and Streets (Streets et al. 2019a, 2019b) inventories are shown in blue, green, and grey, respectively. b Atmospheric concentrations from EMEP monitoring stations (annual medians; in black) and reconstructed atmospheric concentrations from the Estibere peat core (Enrico et al. ; in grey) and Greenland firn air (Fain et al. ; in blue). The shaded region represents one standard deviation confidence interval. c Standardized biota tissue Hg concentrations trends for > 38 year long time-series from Fennoscandian freshwater fish (μg g⁻¹ wet weight; Braaten et al. 2017), US east coast marine bluefish (μg g⁻¹ wet weight; Cross et al. 2015), Svalbard polar bear dental tissue (ng g⁻¹ dry weight; Aubail et al. 2012), and murre egg Hg (μg g⁻¹ dw) from North-Canada (Braune et al. 2016) and herring gull eggs from Lake Superior (Blukacz-Richards et al. 2017). d Hg accumulation rates (normalized Hg flux to pre-industrial levels) in peat cores (n = 30; Enrico 2015) and remote lake sediment cores (n = 68; Zhang et al. 2014). The shaded region represents the standard deviation

Fig. 3

Paradigm shifts relevant to global change effects on Hg cycling. a The wealth of monitoring data since the 1990s on Hg wet deposition has fueled a paradigm where wet and dry Hg^II deposition are the main routes of terrestrial and marine ecosystem loading (UNEP 2003, 2018). Deposition of gaseous Hg⁰ (and minor Hg^II) forms is far more difficult to quantify. Only since 2005 have new methods, including Hg⁰ concentration gradient methods (Obrist et al. 2017), Hg⁰ micrometeorology (Skov et al. ; Osterwalder et al. 2016), and Hg stable isotopes provided a different picture (Demers et al. ; Enrico et al. 2016). These methods now confirm, across all biomes, that direct foliar and soil Hg⁰ uptake ‘deposits’ far more atmospheric Hg to soils (75%), and to lakes and coasts, than Hg^II wet and dry deposition alone (25%) (Zheng et al. ; Obrist et al. 2018). Important global Hg⁰ deposition to land and oceans implies different climate change sensitivity where factors such as (de-)forestation need to be considered (Feinberg et al. 2022). b Hg release to air, land and water. Much emphasis has been put on the volatility of Hg, in terms of its emissions and atmospheric cycling and dispersal. Only since the 2010s have robust estimates of global Hg release to land and freshwater ecosystems been made (Horowitz et al. ; Streets et al. 2017), and indicate that these have been much larger both in the past (1070 Gg to land and water since 1510, vs. 470 Gg to air) and in modern times (7.3 Gg year⁻¹ to land and water in 2010, vs. 2.3 Gg year⁻¹ to air) than Hg emissions to air. Historically, release to land and water was dominated by Hg mining and production, Ag mining, and chemical manufacturing (Streets et al. 2017). It is estimated that on the order of 390 Gg of legacy Hg release to land and water is sequestered at contaminated sites (Fig. 1; Streets et al. 2019a), while the remaining 680 Gg is dispersed in the soil–river–wetland–coastal sediment continuum. Extreme events associated with climate change, or land-use change and deforestation may therefore mobilize substantial amounts of sequestered legacy Hg from contaminated sites into aquatic ecosystems. c Microbial Hg methylation studies identified sulfate and iron reducing bacteria, and methanogenic archaea as key methylating species, leading to a paradigm where MeHg was mainly produced in anoxic sediments with dissolved Hg^II-sulfide species as substrate. The discovery of the hgcA and hgcB genes (Parks et al. 2013) and recent methodological developments in metagenomics are shaping a new paradigm, characteristic of a far larger diversity of methylating microbes across the full redox spectrum, and methylating a diverse set of Hg substrates (Regnell and Watras 2019)

Fig. 4

Projected global anthropogenic emissions and Hg levels in the environment in 2050 based on four scenarios. A1B and A2 refer to business-as-usual scenario or to a more fragmented economic growth and technological development, respectively. CP (current policy) and MFR (maximum feasible reduction) scenarios assume near-constant emissions or the application of the best available technologies, respectively. Adapted from Zhang et al. (2021a, b)

Fig. 5

Schematic overview of different Hg modeling strategies (see also SI text). Box models (left) provide simple, flexible simulations of specific environments or the global Hg cycle, in particular over long, millennial time scales. Multi-dimensional chemistry, transport and exposure models (middle) provide more realistic simulations of regional or global Hg cycling that incorporate gridded emissions, 3D meteorology, biodiversity and ocean currents. Earth system models (ESMs) are comprehensive, coupled 3D models that allow simultaneous simulation of Hg emission, climate change and associated global change trajectories, including human and societal control factors

Fig. 6

A diagram demonstrating the metabolic repertoire of Hg methylating microbes relating to the biogeochemical cycling of carbon, nitrogen, and sulfur compounds, with an emphasis on catabolic processes. This analysis revealed several potential coupling points between Hg methylation and microbial nutrient processing that are likely to be affected by global changes but with unclear direction and magnitude, and deserving further research. Notably, the net result of production vs. destruction of one-carbon compounds (e.g., CH₄), or how alterations in nitrogen microbial metabolism (e.g., increased N₂ fixation, or response to anoxia) will affect Hg methylation. For more information see the Supplementary Information