Assessment of mercury uptake routes at the soil-plant-atmosphere interface

Abstract

Mercury is easily transformed from the elemental state or numerous other mercury compounds to its gaseous form. The gaseous mercury species remain in the atmosphere for sufficient time to be carried long distances. In many cases, some of these differences produce a bias in experimental research, especially for the understanding of the soil-plant-atmosphere system in polluted sites. The main controversy concerns the preferential uptake route in plants and whether the process is reversible or not. The aims of the work described here were to acquire a better knowledge of the transfer routes in the soil-plant-atmosphere system and to ascertain whether plant mercury uptake is reversible. Factors such as the presence of mercury in the atmosphere and irrigation water were evaluated using two common species in Castilla-La Mancha: vines ( Vitis vinifera, L.) and olive trees ( Olea europea, L.). The results provide evidence of three important aspects: plant mercury uptake via roots does occur but it seems minor; plant mercury uptake from the atmosphere involves a continuous exchange at the leaf-atmosphere interface; and plant mercury bioaccumulation seems to be a reversible process. These findings have implications for the use of plants as biomonitors by restricting the exposure times, and in phytoremediation, particularly for polluted soils.

Experimental assessment of the daily exchange of atmospheric mercury in Epipremnum aureum

“…These data are consistent with the previous results reported by Naharro et al (2018), who stated that almost 37% of Hg previously taken up by olive trees can be desorbed in a pristine area-a proportion higher than the minor fraction described by Rutter et al (2011). These data limit the non-reversible character of the Hg uptake process, and this suggests that a fraction of the 'immobilized' Hg in the interior of the leaf could be emitted if TGM levels in the atmosphere decrease dramatically (Naharro et al 2018). This 'immobilized' Hg fraction in the interior of the leaf was not only in an oxidized state but was also metabolized Hg bound to biothiols or proteins (cysteine complexes; Carrasco-Gil et al 2013).…”

“…Rutter et al (2011) estimated that 96% of Hg uptake goes to the interior of the leaf and this involves both the exchangeable Hg fraction and the non-exchangeable fraction. These conclusions were confirmed by Naharro et al (2018), who estimated that nonreversible exchange is the minor component for olive trees in a Hg-contaminated site.…”

Biomonitoring of Hg0, Hg2 and Particulate Hg in a Mining Context Using Tree Barks+

“…The relationship between the data distributions of leaf THg ( Figure 2 B) and TGM levels using the same sampling sites ( Figure 2 C) is also clear, especially considering the relative location of maximum values on the main gaseous Hg source. This relationship was expected considering that the primary Hg uptake pathway for leaves is the atmospheric pathway and that the translocation of Hg captured by roots can be considered negligible [ 20 , 21 ]. On the other hand, the distribution of THg in bark ( Figure 2 A) shows important differences in terms of the maximum content, which was found about 500 m southeast of the main dump.…”

“…The use of Platanus hispanica leaves allowed us to identify isolated emission sources in urban environments (for example, waste-management industries treating Hg-containing residues under conditions that result in inadvertent release into the atmosphere), as well as monitoring the annual evolution of remediation works in contaminated areas, by taking advantage of the deciduous nature of the chosen tree. However, several factors must be taken into account that can limit its use or the interpretation of the data obtained, namely that Hg exchange at the tree-atmosphere interface is bidirectional [ 21 , 22 ]; the relation between exposure and uptake (or re-emission) may not be linear; there is no standardized method, and therefore the results are rarely comparable; the Hg cycle in the study area may be complex and may contain interactions between environmental compartments that do not allow correct interpretation of the data; and the fractionation of atmospheric Hg between the different species may lead to errors in interpretation [ 23 ]. Despite this, the application of this biomonitoring method in Almadén has allowed us to locate the main source of Hg gaseous emission in the area and to satisfactorily delimit its area of influence (orange in Figure 2 B).…”

Paleolimnological characterization of metal(loid) sources and transport processes in Canadian subarctic lakes

“…The biological uptake of metals occurs when plants and micro-organisms incorporate metals from the atmosphere (Luo et al, 2019;Naharro et al, 2018;Obrist et al, 2017;Wang et al, 2012), from soil or sediments (Klaminder et al, 2005;Naharro et al, 2018), or from solution (de Paiva Magalhães et al, 2015;Rieuwerts et al, 2015;Violante et al, 2010). Once metals enter living organisms, they bioaccumulate if the rate of intake exceeds the elimination rate in individual organisms.…”