Adverse effects of methylmercury (MeHg) on life parameters, antioxidant systems, and MAPK signaling pathways in the rotifer Brachionus koreanus and the copepod Paracyclopina nana

To evaluate the adverse effects of MeHg on the rotifer Brachionus koreanus and the copepod Paracyclopina nana, we assessed the effects of MeHg toxicity on life parameters (e.g. growth retardation and fecundity), antioxidant systems, and mitogen-activated protein kinase (MAPK) signaling pathways at various concentrations (1 ng/L, 10 ng/L, 100 ng/L, 500 ng/L, and 1000 ng/L). MeHg exposure resulted in the growth retardation with the increased ROS levels but decreased glutathione (GSH) levels in a dose-dependent manner in both B. koreanus and P. nana. Antioxidant enzymatic activities (e.g. glutathione S-transferase [GST], glutathione reductase [GR], and glutathione peroxidase [GPx]) in B. koreanus showed more positive responses compared the control but in P. nana, those antioxidant enzymatic activities showed subtle changes due to different no observed effect concentration (NOEC) values among the two species. Expression of antioxidant genes (e.g. superoxide dismutase [SOD], GSTs, glutathione peroxidase [GPx], and catalase [CAT]) also demonstrated similar effects as shown in antioxidant enzymatic activities. In B. koreanus, the level of p-ERK was decreased in the presence of 1000 ng/L MeHg, while the levels of p-ERK and p-p38 in P. nana were reduced in the presence of 10 ng/L MeHg. However, p-JNK levels were not altered by MeHg in B. koreanus and P. nana, compared to the corresponding controls. In summary, life parameters (e.g. reduced fecundity and survival rate) were closely associated with effects on the antioxidant system in response to MeHg. These observations provide a better understanding on the adverse effects of MeHg on in vivo life parameters and molecular defense mechanisms in B. koreanus and P. nana.