Hg-accumulation in above-ground fern biomass and spore malformations around industrial sites (Czechia): Implications for mass extinction scenarios

We investigated mercury uptake by the polypod fern Dryopteris filix-mas from two mercury emission sites in the Czech Republic along transects of high to low soil Hg concentrations, with the aim to determine whether ferns can accumulate Hg in above-ground biomass where it could disrupt meiotic processes. We focus on Hg because it has been hypothesized to have caused widespread malformation of spores in ferns proliferating in conjunction with one of the Big Five mass extinction events at the end of the Triassic (201.5 Ma). Here, we show that Hg-concentrations in the roots of Dryopteris filix-mas reflect Hg-contamination in the topsoils, while Hg-concentrations in above-ground foliage appears not directly related to topsoil contamination. We employed thermal desorption to determine that fern root systems mainly absorb Hg from soils while above-ground biomass may also have received Hg via gaseous atmospheric uptake. Further detailed Hg-concentration analyses of various fern parts led to the new insight that the highest levels of Hg reside in the spore-producing sori where it could in theory disrupt sporogenesis. To further examine the role of Hg in driving malformations, we isolated Dryopteris spores for microscopic inspection. However, no clear correlation was established between the relative abundance of aberrant spores and Hg-contamination in ferns or soils. Hence, other factors such as temperature, moisture availability, hybridization and other toxic metals likely also influenced fern reproduction. Nevertheless, the high bioaccumulation of Hg in the spore-producing sori should stimulate further research into the binding and influence of Hg by ferns and its role as a driver of plant sterility across mass extinction events.