Comparative assessment of microplastics and microalgae as vectors of mercury and chlorpyrifos in the copepod Acartia tonsa.

ABSTRACT

Microplastics (MPs) raise concerns not only as pollutants themselves, but also due to their ability to act as vectors of pollutants adsorbed from seawater, transferring them to marine organisms. However, the relevance of MPs as carriers of pollutants compared to microalgae needs further exploration. This study compared the role of MPs (2-10 µm non-oxidized and 10-15 µm oxidized high-density polyethylene) and natural organic particles (Rhodomonas lens microalgae, MA) as carriers of mercury (Hg, 2.3 µg Hg/L) and chlorpyrifos (CPF, 1.0 µg CPF/L) to adult Acartia tonsa copepods, after 24-48 h exposure. Dose-response experiments were first performed with adult female copepods exposed to oxidized MPs (0.25-4.0 mg/L), waterborne Hg (0.01-10.0 µg/L) and Ox MPs + Hg (0.25-4.0 mg oxidized MPs/L + 0.50-8.0 µg Hg/L) for 48 h, to complement previous studies that focused on the pesticide CPF. Effects were evaluated with four replicates for physiological and reproductive responses (6 females/replicate), biochemical techniques (40 individuals/replicate) and Hg/CPF bioaccumulation measurements (1000 individuals/replicate). Copepods accumulated Hg/CPF similarly from dissolved pollutants (6204 ± 2265 ng Hg/g and 1251 ± 646 ng CPF/g) and loaded MPs (3125 ± 1389 ng Hg/g and 1156 ± 266 ng CPF/g), but significantly less from loaded MA (21 ± 8 ng Hg/g and 173 ± 80 ng CPF/g). After 24-48 h, copepods exposed to MPs + Hg/CPF showed generally greater biological effects than those exposed to dissolved Hg/CPF or to MA + Hg/CPF, although differences were not statistically significant. MA + CPF had significantly lower AChE inhibition (1073.4 nmol min-1 mg-1) and MA + Hg lower GRx induction (48.8 nmol min-1 mg-1) compared to MPs + Hg/CPF and dissolved Hg/CPF (182.8-236.4 nmol min-1 mg-1 of AChE and 74.1-101.7 nmol min-1 mg-1 of GRx). Principal component analysis suggested different modes of action for Hg and CPF.