Seasonal fluctuation markedly impacts on Hg bioaccumulation and its toxic effects in aquatic life. This study explored influences of seasonal changes on bioaccumulation extent and toxic effects of Hg in Asian clams (Corbicula fluminea) and benthic microbiota. For this, we conducted clam-exposure experiments during various seasons in clean/control site (HS1) and contaminated sites (HS2, HS3, and HS4) of Hyeongsan River estuary, South Korea. The whole sediment analysis showed that the total mercury (THg) and methylmercury (MeHg) at HS4 site were highest during fall, remained similar concentration during winter, but lowered during summer. In opposite, porewater reveled highest concentration in summer, and gradually lowered in fall and winter seasons. The highest bioaccumulated levels of THg and MeHg were observed in clams collected from HS4 site during summer, accounting to 521.52 µg/kg and 161.04 µg/kg, respectively. In the identical season, the higher bioaccumulation potential in clams also correlated with the Hg levels in pore water. Furthermore, a significant higher values of biota-sediment-pore water accumulation factor (BSpAF) were observed in clams retrieved from contaminated sites (HS2~HS4). The detoxifying biomarker genes gsts1, cat, hsp70 and hsp90 were upregulated in clams harvested from HS2~HS4 sites. The higher concentrations of THg and MeHg in various sites showed negative impact on bacterial and metabolic diversities. In HS2~HS4 sites, the abundance of phylum Proteobacteria was increased. In contrast, the abundance of phylum Bacteroidetes was declined. Overall, these findings broaden the current state of knowledge of seasonal impact on Hg bioaccumulation and its toxicity. These outcomes may be supportive in monitoring and management practices of Hg contaminated sites.

Seasonality on mercury bioaccumulation and toxicity in Asian clams and microbiota of estuarine sediment

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