Mercury sorption and desorption in mosses: Molecular mechanisms and biomonitoring implications

Mercury is a globally distributed pollutant with significant ecological impacts. Terrestrial mosses have been widely used as Hg biomonitors, however, the mechanisms governing Hg accumulation, retention and release in these organisms remain poorly understood. This knowledge gap limits both the interpretation of biomonitoring data and the assessment of Hg ecotoxicological effects on mosses. This study investigates the deposition pathways, sources, and molecular mechanisms driving Hg accumulation in Pseudoscleropodium purum integrating isotopic, chemical, and molecular analyses. Our results showed that gaseous elemental Hg (Hg⁰) was the primary deposition pathway, accounting for 66% of total Hg inputs. However, following deposition, a substantial fraction of Hg⁰ appears to undergo oxidation within moss tissues. Together with post- depositional photoreduction, these processes reduce the remaining Hg⁰ fraction to only 5-35% of total Hg. Hg stable isotopes and chemical tracers provided no evidence for a major contribution from local sources such as forest fires and coal combustion, or AMDEs, suggesting that long-range atmospheric transport could be the prevailing source in the study area. Strong correlations between ∆²⁰¹Hg and ∆¹⁹⁹Hg (rho = 0.88) over a wide range of values is consistent with post-depositional photochemical reduction. Molecular analyses support that Hg accumulation in mosses is consistent with an increase in amide groups and a concurrent decrease in carbonyl groups within the extracellular space. The reverse process, amide hydrolysis, could lead to the release of previously bound Hg. These findings provide new mechanistic insights into Hg cycling in mosses, suggesting that functional groups in the cell wall and plasma membrane may influence Hg partitioning. These results challenge the assumption that mosses are reliable accumulative biomonitors, demonstrating instead that, at least for this species and under the studied environmental conditions, Hg content reflects a dynamic balance between deposition and photoreductive losses.