Mercury
Mercury occurs in the environment in metallic, ionic and organic forms. Hg released from aquatic and terrestrial environments into the atmosphere is mostly in its gaseous elemental state, Hg0. In the atmosphere, Hg0 is slowly oxidized to the inorganic mercuric state, Hg2+, which is subsequently returned to land and water through wet and dry deposition (Raj and Maiti, 2019, ATSDR, 1999). Thus, the quantity of Hg deposited onto the soil surfaces or in the water increases with every new wave of aerial deposition. In addition, wet or dry aerial deposition recirculates the air-born Hg gas previously emitted into the atmosphere as a by-product of diverse industrial emissions (Gworek et al., 2020, Vallee and Ulmer, 1972). In particular, organic mercuric compounds such as methyl-, ethyl-, dimethyl- and diethyl-mercury, which are generated both through abiotic and biotic processes, are highly toxic. Hg is not metabolised, nor is it excreted, causing its bioconcentration and accumulation via food chains. For example, many instances are known of methylmercury bioconcentration via aquatic food webs particularly those based on sea food (Kumari et al., 2020, Leterme et al., 2014, Silver and Hobman, 2007). Human poisoning through inhalation, oral ingestion or uptake through the skin can manifest itself at intake levels as low as 3 µg kg-1 day-1 (ATDSR, 2019).
Processes in the rhizosphere related to mercury toxicity Within soils Hg is found in multiple forms depending on organic acids, pH, bedrock composition and biotic factors. Hg is often bound to negative, reactive groups such as sulfhydryls, carboxyls, phenols and alcohols that form a major component of humic or fulvic acids. This promotes formation of HgS (mercuric sulfide, cinnabar), HgCl2(mercuric chloride) and Hg(OH), the sulfide being the least soluble (USEPA; United States Environmental Protection Agency, 2007). Mercury ions can also be integrated into carbon skeletons in the form of organomercurials. Soil microbes interfere with Hg forms and bioavailability; microorganisms can enable Hg2+release from complexed forms in the soil through protonation mediated by H+-ATPases, chelation via organic acid secretion, siderophore production, and chemical transformation and volatilization, commonly achieved by mercuric reductase, (e.g. (Artz et al., 2015). Biological conversion of the Hg2+ into methyl-Hg, mainly carried out by anaerobic bacteria, leads to integration into the food chain (ATSDR, 1999, Cao et al., 2021, Ma et al., 2019).
Mercury uptake and distribution Plant root cells take up Hg as a linear function of the ambient concentration and specific membrane transporter proteins to catalyse the transmembrane movement of Hg have not been identified. The lipophilic nature of elemental and organic mercurials implies that these forms may simply diffuse across the lipid bilayer. It is feasible that ionic Hg2+ is not taken up as such but may rely on (microbial) reduction to Hg0 before it can enter the root symplast. Within the symplast, the high affinity of Hg for thiol groups directs it towards cysteine residues.
Only a small fraction of total Hg is translocated to the leaves (< 0.3% for Hg2+ and < 3% for MeHg) and most of the accumulated Hg remains in the roots for two main reasons (Schwesig and Krebs, 2003): Firstly, Hg2+ can be ligated to sulfhydryl, hydroxyl and carboxyl groups of compounds such as organic acids (Riddle et al., 2002) and sulphur-rich structural proteins of the cell wall such as extensins and expansins (Carrasco-Gil et al., 2013) that are present in the root apoplast. Secondly, Hg can electrostatically interact with anionic compounds such as phosphates, carbonates and sulfates to form insoluble precipitates that limit symplastic mobility.
Cellular detoxification of mercury Mercury detoxification largely depends on vacuolar sequestration, with phytochelatins as intermediates and members of the ABCC family that catalyse vacuolar deposition. In the vacuolar lumen Hg-PC complexes can be converted into crystals, largely neutralising intracellular toxicity (Carrasco-Gil et al., 2011).
Although more problematic with transition elements such as Pb and Cd, Hg does generate oxidative stress. This notion is supported by multiple transcrptomics studies that report increased activity of enzymatic and non-enzymatic antioxidants. An example is a study on Medicago sativa exposed to 30 µM Hg (Ortega-Villasante et al., 2005); plants showed abrupt cessation of growth and cell necrosis within the first 24 h, while the subsequent depletion of glutathione and lipid and protein oxidation were accompanied by strong induction of antioxidant enzymes such as SOD, POX and APX (Zhou et al., 2007).