High and diurnally fluctuating carbon dioxide exposure produces lower mercury toxicity in a marine copepod

Coastal waters have experienced fluctuations in partial pressure of carbon dioxide (pCO2) and mercury (Hg) pollution, yet little is known concerning how natural pCO2 fluctuations affect Hg biotoxicity. Here, a marine copepod Tigriopus japonicus was interactively exposed to different seawater pCO2 (ambient 400, steady elevated 1000, and fluctuating elevated 1000 +/- 600 mu atm) scenarios and Hg (control, 2 mu g/L) treatments for 7 d. The results showed that elevated pCO2 decreased Hg bioaccumulation, and it was even more under fluctuating elevated pCO2 condition. We found energy depletion and oxidative stress under Hg-treated copepods, while combined exposure initiated compensatory response to alleviate Hg toxicity. Intriguingly, fluctuating acidification presented more immune defense related genes/processes in Hg-treated copepods when compared to steady acidification, probably linking with the greater decrease in Hg bioaccumulation. Collectively, understanding how fluctuating acidification interacts with Hg contaminant will become more crucial in predicting their risks to coastal biota and ecosystems.

Automated summary

The study investigated the interactive effects of fluctuating acidification and mercury exposure on a marine copepod. The results showed that elevated pCO2 decreased the bioaccumulation of mercury, and this effect was even more pronounced under fluctuating acidification. Combined exposure to acidification and mercury initiated compensatory responses to alleviate mercury toxicity. Fluctuating acidification also induced more immune defense related genes/processes in mercury-treated copepods compared to steady acidification, which may be linked to the greater decrease in mercury bioaccumulation.