Combined proteomic and gene expression analysis to investigate reduced performance in rainbow trout (Oncorhynchus mykiss) caused by environmentally relevant microplastic exposure

The pollution of the environment with microplastics (MPs) is affecting aquatic organisms worldwide, and yet intensive research, has thus far failed to deliver an adequate understanding of the detrimental effects of MP ingestion by fish. Investigations using established health and performance parameters are often insufficient to determine MP toxicity, especially when considering MPs in environmentally relevant concentrations. In the present study, label-free quantitative (LFQ) proteomics of liver tissue was combined with gene expression analysis in order to investigate the long-term effects of MP exposure on rainbow trout (Oncorhynchus mykiss). With the help of a specially designed diet, two groups of fish were exposed for 120 days to environmentally relevant concentrations of MPs (on average 13 particles per fish, every 2 days) and to slightly increased levels representing those expected in the near future (on average 73 MP particles per fish, every 2 days). Both groups were compared to a control. The results provide evidence that long-term exposure to MPs has a dose-dependent negative effect on the performance of rainbow trout. No differences in blood glucose level, hematocrit level or lipid peroxidation were observed between treatments. The proteomic analysis revealed 6071 unique proteins, but no significant change in hepatic protein concentrations compared to their matching controls, although certain proteins appear to have been up- or down-regulated multifold and should be considered in continuing experiments. When comparing highly regulated proteins with the levels of their respective mRNA transcripts, a good correlation was observed just for “differentially regulated trout protein 1”, encoded by drtp1. This may therefore be a suitable biomarker for future studies with trout. Several hypotheses were put forward to explain the observed differences in growth: nutrient dilution, caused by increased amounts of non-digestible material in the diet, and growth effects due to differences in diet quality could be excluded. Physical interference of MPs with the gastrointestinal tract are also unlikely, as fish are regularly exposed to particulate matter in natural environments and previous studies did not find evidence of such interferences. Instead, indirect detrimental effects of MPs, either due to their hydrophobic surface properties or the presence of certain additives, could cause allergic reactions, microbiota dysbiosis or general stress responses. Although no clear cause for the reduced growth was identified, the current study demonstrates the potential utility of omics approaches when dealing with such a complex question. Future studies should extend analyses to the gastrointestinal tract and associated tissues. It should be ensured that the MP exposure is realistic and that the duration of the experiments covers several months. Direct evidence of a significant negative influence of long-term exposure to realistic and near-future MP concentrations on fish highlight the importance of measures to prevent a further increase of MPs in the environment