Tolerance of the forest soil microbiome to increasing mercury concentrations

•Mercury caused severe diversity loss and shifts in the soil microbiome.•Fungal communities were generally less affected than the bacterial ones.•A wide range of mercury-responsive taxa were found.•Mercuric reductase genes were enhanced in soils with high Hg.

Mercury (Hg) occurs naturally in soil and can reach high concentrations in the environment due to anthropogenic activities; however, little is known about its impact on the soil microbiome. Here, we tested the impact of different concentrations of Hg on soil bacterial and fungal communities by carrying out microcosm experiments with seven different forest soils. The highest concentration of Hg (32 μg Hg g−1 dry soil) caused severe diversity loss and shifts in the bacterial and fungal community structures and composition. Bioavailable Hg was reduced in soils with the highest proportion of clay, but the impact of Hg on microbial community structures was still evident in these soils. Lower concentrations of Hg (≤3.2 μg Hg g−1 dry soil) had only a limited effect on the soil microbiome. Fungal communities were generally less affected than the bacterial communities. The bacterial Hg-detoxification capacity, as assessed by mercuric reductase gene abundance, was reduced in soils with the lowest amount bioavailable Hg. We found a wide range of Hg-responsive taxa in soils spiked with high amounts of Hg, although they were generally not specific to any soil type or taxonomic group. Overall, our data show that the impacts of Hg on the soil microbiome and its detoxification responses depend on soil characteristics.