Paper Reviewed
Li, Y., Wang, W.-X. and Wang, M. 2017. Alleviation of mercury toxicity to a marine copepod under multigenerational exposure by ocean acidification. Scientific Reports 7: 324, DOI:10.1038/s41598-017-00423-1.

Introducing their work, Li et al. (2017) write that ocean acidification "may potentially modify the responses of aquatic organisms to other environmental stressors including metals." And in light of that possibility, the team of three scientists designed an experiment to investigate the combined effects of so-called ocean acidification (OA) and mercury (Hg) toxicity on the development and reproduction of a marine copepod, differing from other studies of the subject by performing their analysis over multiple generations.

In accomplishing their objective, Li et al. selected the marine copepod Tigriopus japonicus, a harpacticoid species that inhabits tidal pools on rocky shores along coastal waters of the Western Pacific. Then, in a laboratory setting, they subjected T. japonicus specimens to a combination of ambient (400 ppm, pH 8.10) or elevated pCO2 (1000 ppm, 7.70 pH) and ambient (no Hg addition) or elevated (ambient plus 1.0 µg/L) Hg pollution for a period of four generations. For each generation they measured the Hg accumulation rates and seven life history traits, including survival rate, sex ratio, development time from Nauplius to copepodite, development time from Nauplius to adult, number of clutches, number of naupliee per clutch and fecundity.

The results of their experiment revealed that elevated CO2 (i.e., ocean acidification) alone had "small impacts on the seven life history traits," which "high resilience of this copepod to elevated pCO2," according to Li et al., "may be explained by their adaptability to their habitats such as tide pool and sea bottom where the pCO2 concentration often becomes high." They also report that exposure to Hg pollution alone led to increased Hg accumulation in the copepod with each new generation, as well as a reduction in the number of nauplii per clutch, which reduction was more obvious in the final two generations.

The "most interesting finding" in their work, according to the three researchers, was noted in the combined elevated CO2 and Hg pollution treatment, where "ocean acidification significantly reduced Hg accumulation in copepods at each generation." What is more, Li et al. note that the number of nauplii per clutch and the fecundity of the copepods were "significantly enhanced by the combined OA + Hg exposure, thus reversing the trend observed under Hg pollution alone. Consequently, given all of their findings, the scientists conclude that "OA alleviated Hg toxicity to reproductive performance in marine copepods," which mitigation bodes well for the future of this species.