Magnesium dihydrogen disulphite

Bioaccumulation: terrestrial

Description of key information

Taking into account (i) the rapid dissociation of magnesium dihydrogen disulfite and decomposition of sulfites upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural magnesium and sulfur cycle, (ii) ubiquitousness of magnesium and inorganic sulfur substances in soil and (iii) essentiality of magnesium and sulfur in terrestrial organisms, magnesium dihydrogen disulfite is expected to have a low potential for bioaccumulation in terrestrial organisms.

Additional information

(1) Environmental fate and stability in soil: Magnesium dihydrogen disulfite dissociates into sulfite anions and magnesium cations in environmental solutions, including soil porewater.

(a)Magnesium compounds are very soluble and magnesium is highly mobile after its release by weathering, under all environmental conditions. Under the pH, redox and conductivity regimes typically found in streams, Mg is likely to be present almost exclusively as Mg²⁺(Salminen et al. 2005 and references therein).Conclusively, magnesium cations become part of the global magnesium cycle.

(b)Sulfites are unstable in the environment, including in topsoil, and become part of the natural sulfur cycle. Under oxygen-rich conditions, sulfites are rapidly oxidized catalytically by (air) oxygen or by microbial action to sulfate. Microbial oxidation of reduced sulfur species including elemental sulfur (S), sulfide (HS^-), sulfite (SO₃^2-) and thiosulfate (S₂O₃^2-) is an energetically favorable reaction carried out by a wide range of organisms, i.e. sulfur oxidizing microorganisms (SOM) resulting in ultimate transformation into sulfate (SO₄^2-, Simon and Kroneck, 2013).

Observed SO₃^2-oxidation rates in soils are dependent on soil characteristics, i.e. are decreasing with increasing soil pH. In a study performed by Lee et al. (2007), soils collected from the surface horizon (0 to 20 cm) were amended with 0.3 % w/w CaSO₃. Based on the analysis of soils leachates initial SO₃^2-oxidation rates were dependent on soil pH – however final recovered sulfate concentrations were similar among all tested soils (pH range 4.0 - 7.8) irrespective of pH, yielding > 75% recovery of the total added sulfur.

In highly reduced (water-logged) soils, reduction to sulfides may take place with subsequent formation of solid-phase minerals and metal sulfides of very low bioavailability/solubility, including FeS, ZnS, PbS and CdS (Lindsay, 1979, Finster et al., 1998). Thus, under anoxic conditions, sulfate is readily reduced to sulfide by sulfate-reducing bacteria (SRM) that are common in anaerobic environments. Other sulfur-containing microbial substrates such as dithionite (S₂O₄^2-), thiosulfates (S₂O₃^2-) or sulfite (SO₃^2-) may also be anaerobically utilised, ultimately resulting in the reduction to sulfide (H₂S).

A significant set of microbial populations grows by disproportionation of sulfite, thiosulfate or elemental sulfur, ultimately yielding sulfate or sulfide (Simon and Kroneck 2013 and references therein; Janssen et al. 1996, Bak and Cypionka, 1987).

In sum, sulfites may reasonably be considered chemically unstable under most environmental conditions, are rapidly transformed into other sulfur species and ultimately become part of the global sulfur cycle.

(2) Ubiquity and natural/ambient background:

(a)Magnesium is the seventh most abundant element in the Earth’s crust with a quoted average of 2.76%, and the Mg²⁺ion is the second most abundant cation in sea water, after Na⁺. (Salminen et al. 2005 and references therein). Magnesium is ubiquitous and a constituent of soil minerals. The FOREGS dataset reports magnesium/magnesium oxide concentrations for 845 topsoil samples (dried, grinded, sieved to < 2 mm, pulverised to < 0.063 mm, and analysed by XRF, LOQ: 0.01 %) sampled on a grid across Europe. Magnesium oxide data were converted into magnesium concentrations. Based on 833 paired samples for EU-27, UK and Norway, baseline magnesium levels in topsoilfrom 30 mg/kg to 148467 mg/kg magnesium with 5^thand 95^th percentiles of 603 mg/kg and 18055 mg/kg magnesium, respectively, and a median concentration of4583mg/kg magnesium. Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 18055 mg/kg can be regarded as representative background concentration for magnesium in European topsoils.

Additionally, magnesium concentrations in agricultural soils were determined in the GEMAS project. For the EU-27, UK and Norway, 1867 and 1781 samples of agricultural and grazing land soil, respectively, were analysed for magnesium. Magnesium concentrations of respective aqua regia extracts were measured by ICP-MS (LOQ: 50 mg/kg). Magnesium levels of agricultural soil rangefrom < 50.00 mg/kg to 116907.70 mg/kg magnesium with 5^thand 95^th percentiles of 383.90 mg/kg and 131560 mg/kg magnesium, and a median of2842mg/kg magnesium. In grazing land, soil concentrations of magnesium rangefrom 66.93 mg/kg to 124,782.68 mg/kg with 5^thand 95^thpercentiles of 341 mg/kg and 12811 mg/kg magnesium, and a median of2787mg/kg magnesium. Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 13160mg/kg can be regarded as representative background concentration for magnesium in European agricultural soils and the 95^thpercentile of 12811 mg/kg can be regarded as representative background concentration for magnesium in European grazing land soils.

(b) Sulfur is a ubiquitous natural component of soil. Most terrestrial environments have substantial sulfur levels whereas sulfur-deficient environments are rare. In soil, sulfur can be found as pure element, sulfide (salts containing S^2-) and sulfate (SO₄^2-) minerals and in various organic substances. In all but highly reduced soils, sulfate is the most stable species at environmentally relevant pH > 4. Other stable sulfur species such as SO(g), S^2-, S₂O₃^2-and S₂O₄^2-are, however, not prevalent in soils (Lindsay, 1979). Due to its ability to exist in a wide range of oxidation states, sulfur plays an important role in living organisms, both as a structural component and a redox-active element. Soluble states of sulfur such as sulfates and sulfites are common in their various elemental forms. The three most abundant forms of sulfur are elemental sulfur, sulfate (SO₄^2-) and sulfide (S^2-) and sulfur containing oxyanions, i.e. sulfite (SO₃^2-), dithionite (S₂O₄), thiosulfate (S₂O₃) and polythionates such as trithionate (S₃O₆^2-) and tetrathionate (S₄O₆^2-, Simon and Kroneck 2013). Due to its key importance for biological processes and unique metabolic versatility, i.e. its appearance in amino acids, iron-sulfur proteins, thioredoxins and sulfolipids, the major fraction of the sulfur in surface soil horizons is present in organic combinations, e.g. in plant litter, microbial biomass or stabilized in soil organic matter with the remainder occurring as inorganic sulfate (Maynard et al., 1998).

A total of 837 topsoil samples were processed in the FOREGS-program to determine sulfur background concentrations. Sulfur concentrations of respective aqua regia extracts were measured by ICP-AES (limit of quantification (LOQ): 50 mg/L). Based on 775 paired samples from the FOREGS dataset, the median sulfur content of European topsoil amounts to 222 mg/kg ranging from <50 to 6,518 mg/kg, and the 95^thpercentile of 645 mg/kg can be regarded as representative background in European topsoils (Salminen et al. 2005).

Additionally, sulfur concentrations in agricultural soils were determined in the GEMAS project. For the EU-27, UK and Norway, 1867 and 1781 samples of agricultural and grazing land soil, respectively, were analysed for sulfur. Sulfur concentrations of respective aqua regia extracts were measured by ICP-OES and/or ICP-MS. Sulfur levels of agricultural soil range from < 5 to 68,226 mg/kg sulfur with a median of 209 mg/kg and a 95^thpercentile of 783.91 mg/kg. In grazing land, soil concentrations of sulfur range from < 5.00 to 98,189 mg/kg with a median of 310 mg/kg and a 95^thpercentile of 645 mg/kg (Reimann et al. 2014). Taking into account the high quality and representativeness of the data set, the 95^thpercentile of 783.91 mg/kg can be regarded as representative background concentration for sulfur in European agricultural soils and the 95^thpercentile of 645 mg/kg can be regarded as representative background concentration for sulfur in European grazing land soils.

(3) Essentiality:

(a)The macro element magnesium, which is the most abundant divalent cation within living cells (Smith & Maguire 1998), is essential to living organisms as an essential component of bones (including crustacean exoskeletons) and cartilage, and an important activator of many key enzyme systems including kinases (i.e. transfer of the terminal phosphate of ATP to sugar or other acceptors), mutases (transphosphorylation reactions), muscle ATPases and the enzymes cholinesterase, alkaline phosphatase, enolase, isocitric dehydrogenase, arginase, deoxyribonuclease and glutaminase. Moreover, magnesium stimulates muscle and nerve irritability (contraction), is involved in the acid-base balance (pH regulation) and plays an important role in carbohydrate, protein and lipid metabolism (Tacon 1987). Magnesium deficiency causes characteristic syndromes, reflecting its specific functions in the metabolism of animal or plants (Soetan et al 2010). In general, magnesium is readily absorbed through the gastro-intestinal tract (also gills of aquatic organisms) and transported in the blood plasma, either in ionic form or bound to plasma proteins. Most of the magnesium is stored primarily in bone (e.g. ~60 % for common carp (ADCP, 1978)), but also in muscle and extracellular fluids (Tacon 1987). The process of magnesium homeostasis prevents any bioaccumulation (OECD SIDS 2011), even at extremely high concentrations which might be harmful to organisms. Magnesium can be excreted via urine (main route of excretion) and faeces, but also via breast feeding (mammals) (OECD SIDS 2011) or via the gills (fish), although the latter one has shown to be limited (ADCP 1978). In photosynthetically active organisms, magnesium is the central atom of the porphyrin ring structure of chlorophyll.

(b)Sulfur is essential in two different ways: as a structural component and on a metabolic level. Sulfur forms a foundation of life itself: it is needed in the synthesis of membrane lipids, is part of the amino acids cysteine and methionine and consequently part of proteins and enzymes (Sekowska et al. 2000). It is found in iron-sulfur centers of Ferredoxin (which functions as electron carrier in anaerobic respiration) and of Glutathione (which plays an important role as antioxidant at the prevention of ROS-induced cell-damage). Further, it is present in the cofactors Thiamine and Lipoic acid (which is linked to many dehydrogenases), and biotin.

Sulfur is taken up by invertebrates via the diet and most insects seem to require to take up methionine with their diet to cover their sulfur demands. Lack of sulfur-containing amino acids or organic sulfur in the diet leads to detrimental effects, as observed in the aphidMyzus persicae.Omission of methionine or cysteine leads to a strong reduction of growth and a greatly reduced number of viable offspring, with effects more pronounced in the absence of methionine. However, the demand for cysteine can be met by provision of other sulfur substances so that cysteine may not be an essential amino acid but might serve as sulfur carrier instead (Dadd & Krieger, 1968). Retnakan and Beck (1967) reported that provision of inorganic sulfur mitigated negative effects of the lack of methionine or cysteine and concluded that aphids could synthesize either of these amino acids, possibly with the help of symbionts. Supply of inorganic sulfur mitigated the need for cysteine and methionine uptake in the aphidsAcyrthosiphon pisumandNeomyzus circumflexus.However, the synthesis of the amino acids is also attributed to symbionts(Dadd, 1973 and references therein). Thus, the lack of sulfur-containing amino acids in the diet affects aphids but symbionts seem to counteract and to contribute to the diet by synthesizing these amino acids from inorganic sulfur (Douglas, 1988). In food-choice experiments, it was demonstrated that methionine had the strongest positive effect on selection of diet from an array of served diets, underlining the demand for methionine (Dadd,1973 and references therein).

In spiders, the sulfur-containing amino acid taurine is furthermore a component of spider silk and venom (Wiesenborn 2012 and references therein). In earthworms, S-containing dialkylfuransulfonates allow consumption of leaf litter by preventing adverse effects of polyphenols to the earthworms. Dialkylfuransulfonates comprise up to 20% of the total sulfur found in earthworms and play a vital role for detritus consumption in ecosystems all over the world (Liebeke et al. 2015).

Conclusion: (a) Magnesium is ubiquitous in the environment, occurring in minerals, soil, sediments and natural waters, and is present as an essential and actively regulated element in biota. According to the OECD SIDS (2011) on “magnesium chloride (CAS# 7786-30-3)”, a potential for bioaccumulation/bioconcentration cannot be identified. Thus, magnesium as essential element is actively regulated and does not bioaccumulate.

(b) Sulfur is ubiquitous in the environment, is actively regulated and fulfils essential roles in all cells, determining the structure and activity of a number of molecules and modulating a myriad of metabolic and catalytic processes. Accordingly, the bioaccumulation potential of sulfite is expected to be low.

Taking into account (i) the rapid dissociation of magnesium dihydrogen disulfite and decomposition of sulfites upon dissolution in environmental solutions, including soil porewater, and respective participation in the natural magnesium and sulfur cycle, (ii) ubiquitousness of magnesium and inorganic sulfur substances in soil, and (iii) essentiality of magnesium and sulfur in terrestrial organisms,magnesium dihydrogen disulfite is expected to have a low potential for bioaccumulation in terrestrial organisms.

References:

OECD SIDS (2011). Magnesium chloride. CAS 7786-30-3. SIDS Initial Assessment Report for SIAM 32, National Institute of Environmental Research in Korea, Paris, France.