The growing application of nanocellulose in various industrial sectors with potential release into the natural environment demands for safety assessment and thus ecotoxicity. Here we tested two types of cellulose nanofibers, non-oxidized (CNF) and TEMPO-oxidized (TOCNF), by means of acute in vivo studies with the marine bivalve Mytilus galloprovincialis at 1 µg L−1 and 1 mgL−1 as respectively resembling realistic and acute exposure scenarios. Uptake and sub-lethal biological responses (lysosomal membrane stability, neurotoxicity, oxidative stress and biotransformation) were investigated along with changes in fibers with water salinity . TOCNF resulted more dispersed than CNF in natural sea water (NSW), probably due to higher repulsion among fibers driven by their negative surface charges and colloidal organic material. Both CNF and TOCNF were found in mussel’s tissues (gills and hemolymph) using labelled stocks . Destabilization of lysosomal membranes of hemocytes was observed and likewise an inhibition of P-gp efflux activities in gills being stronger for CNF at the highest concentration (1 mg L−1). Cholinergic enzymes (ASCh-ChE activities) were inhibited in hemocytes, gills and digestive glands regardless of CNFs oxidation and concentration tested. By contrast, neither oxidative stress nor biotransformation were affected in the mussel's digestive glands and gills. Overall findings showed CNF uptake by marine mussels and disruption in gills functionality and immune cells by mechanical interaction even at environmentally realistic exposure scenarios. The paucity of data on hazard posed by CNF for aquatic species demands for safety aspects to be considered in future risk assessment scenarios.

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