Assessing the effect of mercury pollution on cultured benthic foraminifera using DNA metabarcoding and morphological approaches

Heavy metals such as mercury (Hg) occur in ecosystems from both natural sources and human activities, with large variations in concentrations and pose a significant health hazard through bioaccumulation and biomagnification. The aim of this experimental study is to assess the impact of different concentrations of Hg through time on the diversity of cultured benthic foraminifera. Tanks with concentrations of Hg up to 100 ppm were considered. Mesocosms containing 1-cm-thick sediment from each tank were subsampled at pre-established time intervals for geochemical (Hg content), morphological and molecular analyses. Morphological analyses were based on both Rose Bengal and the CellTracker Green (CTG) techniques. Molecular analyses comprised high-throughput sequencing (HTS) of foraminiferal DNA. A total of 6 datasets were generated: two from Rose Bengal morphological analysis (fraction> 63 μm and fraction >125 μm), one from CTG morphological analysis (fraction>125 μm), three from HTS run (one for each considered filter: Filter1, Filter10, Filter100). Species richness, Shannon and Fisher α alpha indices were all negatively correlated with the Hg concentrations both in water and sediment regardless the considered datasets. Since the strongest correlations were observed among diversity indices and normalized fourth-root transformed HTS data (Filter1), the multivariate analyses (nMDS and Cluster Analysis) were based on this dataset only. The nMDS highlights the negative effect of Hg pollution on the benthic foraminiferal assemblages at the highest concentrations, namely 10 ppm after three weeks and 100 ppm after two weeks of treatment. This experiment demonstrates the huge potential of metabarcoding for unraveling the impact of pollution on benthic foraminiferal assemblages.