Mercury and Arctic Char Gill Microbiota Correlation in Canadian Arctic Communities

Abstract

Arctic char is a top predator in Arctic waters and is threatened by mercury pollution in the context of changing climate. Gill microbiota is directly exposed to environmental xenobiotics and play a central role in immunity and fitness. Surprisingly, there is a lack of literature studying the effect of mercury on gill microbiota. To fill this knowledge gap, our primary goal was to measure to what extent gill exposure to mercury may alter gill microbiota activity in Arctic char. Specifically, we calculated the correlation between the taxonomic distribution of gill-associated bacterial symbiont activity and total mercury concentration in livers and muscles in wild populations of Arctic char in the Canadian Arctic. Our results showed that total mercury concentrations in tissues were higher in Ekaluktutiak (Nunavut) than in the other sites in Nunavik. Proteobacteria was the main phylum correlated to mercury concentration in both tissues, followed by Bacteroidetes and Cyanobacteria. In the most contaminated sites, Aeromonas and Pseudomonas (Proteobacteria) were predominant, while mercury concentration negatively correlated with Photobacterium (Proteobacteria) or Cerasicoccus (Verrucomicrobia). In summary, we found that mercury contamination correlates with active gill microbiota composition, with potential implications of strains in modulating mercury toxicity, making them interesting for future biomarker studies.

Keywords: 16s rRNA gene transcript; Arctic char; Canadian Arctic; bacterial activity; gill microbiota; mercury contamination.

Figure 1

Maps of the four Inuit communities where Arctic char were collected. Ekaluktutiak, Salluit, Inukjuak, and Kangiqsualujjuaq were situated in four different hydrological basins in the Canadian Arctic: Cambridge Bay (Kitikmeot, Nunavut), Hudson Strait, Hudson Bay, and Ungava Bay (Kativik, Nunavik), respectively.

Figure 2

Boxplot of mercury concentrations (mg kg⁻¹) wet weight in Arctic char tissues. Mercury content in the liver (A) was measured in Ekaluktutiak, Salluit, Inukjuak, and Kangiqsualujjuaq, while mercury content in the dorsal muscle (B) was assessed only in Ekaluktutiak, Salluit, and Kangiqsualujjuaq. Statistical significance: “***” p < 0.001, “**” p < 0.01, “*” p < 0.05.

Figure 3

Relative abundance of the 100 most active ASVs at a genus rank in the gill microbiota of the Arctic char in the muscle dataset (n = 96, 4 communities) (A) and the liver dataset (n = 86, 3 communities) (B). Statistical significance: “***” p < 0.001, “**” p < 0.01, “*” p < 0.05.

Figure 4

NMDS plots relied on UniFrac weighted distances calculated in two different datasets: the muscle dataset with the communities Ekaluktutiak (green circle), Salluit (pink cross), and Kangiqsualujjuaq (turquoise blue square) (A) and the liver dataset with samples from the three same communities and Inukjuak (dark blue triangle) (B). Mercury concentrations in the liver and muscle and the Fulton index were fitted on the NMDS plots in red arrows.

Figure 5

Spearman correlations between bacterial genera abundance and mercury concentrations in the muscle (A) and liver (B) are represented in a network with a minimum coefficient of |0.4| and a p-value adjusted with Bonferroni of 0.05. Each node represents one genus; its size depends on the abundance, and its color changes according to its phylum. Green edges represent positive correlations, while red edges represent negative correlations. Thicker edges represent stronger Spearman correlations.