Impacts of exposure to microplastics alone and with adsorbed benzo(a)pyrene on scope for growth and biomarker responses in the marine mussel Mytilus galloprovincialis (L.)

Microplastics (MPs, <5 mm) are an internationally recognized pollutant that are ubiquitous in the marine environment. They can derive from the mechanical, chemical and biological degradation of larger plastic debris or can be manufactured for specific purposes, such as personal care products. MPs can be ingested by invertebrates such as filter-feeding bivalve molluscs and thus, may damage them. Due to their hydrophobicity and relatively large surface areas, MPs can act as carriers of persistent organic pollutants (POPs) in the ocean and may therefore facilitate the transfer of these POPs to organisms (The Trojan-horse effect). Smaller MPs have been proposed to present greater risk to marine organisms due to their enhanced abilities to adsorb POPs and to pass through gut epithelial barriers in comparison to larger MPs. This study aimed to examine the effects of chronic exposure of two different sized PS microspheres (0.5um and 4.5um) alone and with sorped BaP on M. galloprovincialis. In order to investigate this a 26 day exposure study was carried out and responses of mussels were determined on early cellular level biomarkers (Catalayse activity, lysosomal membrane stability and DNA damage) and on whole organism responses (scope for growth and condition index). The results demonstrated that BaP transfers from MPs to mussels and bioaccumulates in mussel tissue with increased exposure time, and that smaller MPs pose an increased hazard in terms of the Trojan horse effect than larger MPs. Effects of MPs and their co-contaminants were observed at environmentally relevant concentrations, and were observed to increase with exposure time in the majority of biomarkers used. Increased effects of sorped BaP compared to MPs alone were demonstrated in general cellular stress responses (LMS and CAT) but there was no additional effect of sorped BaP on DNA damage despite its genotoxic potential. An apparent increased effect of smaller microplastics on DNA damage was also noted highlighting the importance of utilising a variety of MP sizes in studies investigating MP potential toxicity. Meanwhile at a whole organism level a general hormetic effect was demonstrated on energy budget as measured by SFG following exposure in all MP treatments. This appeared to be a compensatory effect whereby the mussels increased their AE in order to increase energy intake to make up for energy expended dealing with stress observed in the lower level biomarker responses. Thereby evidencing a link between MP effects at different levels of biological organisation. Effects on energy budget can have consequences for fitness, survival and reproduction thereby leading to population level effects or higher. However further work is still required to understand the effects of a variety of plastic type, size, shape combinations together with a wide variety of pollutants. Future investigations into the bioavailability and effects of MP co-contaminants should also include water or sediment borne exposure pathways in order to better understand the significance of the MP+contaminant exposure pathway.