Co-exposure to n-TiO2 and Cd2+ results in interactive effects on biomarker responses but not in increased toxicity in the marine bivalve M. galloprovincialis

The increasing production of nanoparticles (NPs) will lead to their release into the aquatic environment, where they could modify the bioavailability/bioconcentration and consequent biological impact of other contaminants. Interactive effects of n-TiO2, one of the most widespread NP type, and Cd(2+), a common heavy metal pollutant, have been described in freshwater species, whereas no information is available in marine organisms. In this work, the effects of co-exposure to n-TiO2 and Cd(2+) were investigated in the marine bivalve Mytilus galloprovincialis. Experimental conditions (100 μg/L, 96 h), were chosen in order to induce early but measurable stress responses (biomarkers) without toxicity. Several biomarkers, from molecular to tissue level, were measured in hemolymph and digestive gland; the effects on embryo development were also evaluated. In hemolymph, Cd(2+) abolished the increase in immune parameters induced by n-TiO2 (NO production and lysozyme activity). In the digestive gland, distinct interactive effects of n-TiO2 and Cd(2+) were observed on different lysosomal biomarkers (lysosomal membrane stability, lipid accumulation and lysosome/cytoplasm volume ratio) and transcription of the immune genes lysozyme and toll-like receptor (TLR). However, n-TiO2 did not affect specific metal-induced responses (metallothionein induction) and tissue metal accumulation. Cd(2+) alone, but not in combination with n-TiO2, affected embryo development. The interactive effects observed on different biomarkers were not apparently due to differences in bioavailability/bioaccumulation of Cd(2+) in the presence of n-TiO2 agglomerates; these effects may result from interactions of either contaminant with both common and distinct targets/mechanisms of action at different levels of biological organization. Overall, the results indicate that co-exposure to n-TiO2 and Cd(2+) did not result in increased adverse effects in M. galloprovincialis. These data underline the need for further knowledge on the potential interactions of NPs with existing contaminants in marine organisms.