Triclosan (TCS) is an emerging contaminant used as a preservative for its antifungal and bactericidal properties. Previous studies have showed different capacity of TCS sorption depending on the MP type, which was related to their physicochemical properties. As a part of the general objective to evaluate the capacity of polyethylene(PE) and polystyrene (PS) microplastics as TCS vectors in marine environments, the marine bivalve Mytilus galloprovincialis was used as a model organism to evaluate effects on immune function after in vivo co-exposure. Acute TCS spill scenario in the presence/absence of microplastics in the surrounding water was simulated under laboratory conditions. Mussels were simultaneously exposed to one type of weathered microplastics (PE or PS) and a high TCS concentration during 72 hours. In general, a decrease in lysosomal membrane stability was observed with all treatments compared to the control after three days, with most treatments resulting in moderate cell stress in mussels. A overall decrease in phagocytic efficiency was also observed in all treated mussels compared to the control mussels, the only exception being mussels individually exposed to PE, where a slight but not statistically significant increase was detected. No differences were observed between treatments in extracellular lysozyme activity and oxyradicals production. In general, mussels co-exposed to TCS+PE and TCS+PS showed effects on immune function that were lower than or comparable to those observed with individual exposures to TCS, PE and PS. Based on the results of this study, an additive effect on immune function derived from the co-exposure of TCS+PE and TCS+PS microplastics can be discarded. However, a weak antagonistic effect is suggested, potentially due to a reduction in their bioreactivity as a consequence of the adsorption of TCS to the surface of the ingested microplastics. The comparison of the results of bioaccumulation of TCS in mussels between the individual treatments and treatments with co-exposure to TCS, which are currently being analyzed, will help to strengthen or refute this hypothesis
