Investigation of the biochemical controls on mercury uptake and mobility in trees

Spatio-temporal trends of mercury levels in alluvial gold mining spoils areas monitored between rainy and dry seasons in the Peruvian Amazon

Artisanal and small-scale gold mining (ASGM) in the Amazon has degraded tropical forests and escalated mercury (Hg) pollution, affecting biodiversity, ecological processes and rural livelihoods. In the Peruvian Amazon, ASGM annually releases some 181 tons of Hg into the environment. Despite some recent advances in understanding the spatial distribution of Hg within gold mine spoils and the surrounding landscape, temporal dynamics in Hg movement are not well understood. We aimed to reveal spatio-temporal trends of soil Hg in areas degraded by ASGM.,. We analyzed soil and sediment samples during the dry and rainy seasons across 14 ha of potentially contaminated sites and natural forests, in the vicinities of the Native community of San Jacinto in Madre de Dios, Peru. Soil Hg levels of areas impacted by ASGM (0.02 ± 0.02 mg kg-1) were generally below soil environmental quality standards (6.60 mg kg-1). However, they showed high variability, mainly explained by the type of natural cover vegetation, soil organic matter (SOM), clay and sand particles. Temporal trends in Hg levels in soils between seasons differed between landscape units distinguished in the mine spoils. During the rainy season, Hg levels decreased up to 45.5% in uncovered soils, while in artificial pond sediments Hg increased by up to 961%. During the dry season, uncovered degraded soils were more prone to lose Hg than sites covered by vegetation, mainly due to higher soil temperatures and concomitantly increasing volatilization. Soils from natural forests and degraded soil covered by regenerating vegetation showed a high capacity to retain Hg mainly due to the higher plant biomass, higher SOM, and increasing concentrations of clay particles. Disturbingly, our findings suggest high Hg mobility from gold mine spoil to close by sedimentary materials, mainly in artificial ponds through alluvial deposition and pluvial lixiviation. Thus, further research is needed on monitoring, and remediation of sediments in artificial to design sustainable land use strategies.

Mercury transformations in algae, plants, and animals: The occurrence, mechanisms, and gaps

Mercury (Hg) is a global pollutant showing potent toxicity to living organisms. The transformations of Hg are critical to global Hg cycling and Hg exposure risks, considering Hg mobilities and toxicities vary depending on Hg speciation. Though currently well understood in ambient environments, Hg transformations are inadequately explored in non-microbial organisms. The primary drivers of in vivo Hg transformations are far from clear, and the impacts of these processes on global Hg cycling and Hg associated health risks are not well understood. This hinders a comprehensive understanding of global Hg cycling and the effective mitigation of Hg exposure risks. Here, we focused on Hg transformations in non-microbial organisms, particularly algae, plants, and animals. The process of Hg oxidation/reduction and methylation/demethylation in organisms were reviewed since these processes are the key transformations between the dominant Hg species, i.e., elemental Hg (Hg0), divalent inorganic Hg (IHgII), and methylmercury (MeHg). By summarizing the current knowledge of Hg transformations in organisms, we proposed the potential yet overlooked drivers of these processes, along with potential challenges that hinder a full understanding of in vivo Hg transformations. Knowledge summarized in this review would help achieve a comprehensive understanding of the fate and toxicity of Hg in organisms, providing a basis for predicting Hg cycles and mitigating human exposure.

Tree rings as historical archives of atmospheric mercury: A critical review

Historical concentrations of atmospheric mercury (Hg) are uncertain, as monitoring only began a few decades ago. Tree rings can serve as historical archives of Hg, providing centennial trends. The vast majority of tree-ring Hg studies have been published in the last decade, demonstrating the growing use of tree rings for Hg dendrochemistry. Thus, there is a need for a systematic review on current knowledge of tree rings as archives of atmospheric Hg. In this review, the predominant pathways of Hg uptake to tree rings are discussed, including the initial Hg uptake from the surrounding environment, fixation, and subsequent translocation. Foliar uptake of Hg was found to be the most important uptake route for Hg in tree rings, the root and bark route being negligible. Our summary of the suitability of different tree species indicates that radial translocation is the biggest limiting factor for Hg dendrochemistry, shifting and blurring historical Hg trends. Based on the review findings, Picea (spruce) and Larix (larch) are the most promising genera for Hg dendrochemistry. Additionally, the use of tree-ring Hg archives in combination with other co-located archives, namely lake sediments, peat, and ice, is suggested as it enhances the viability of observed tree-ring historical Hg trends. Finally, we propose future directions and recommendations for research using tree-ring Hg, including sampling protocols, experimental designs, and tree selection.