Microbiome: A forgotten target of environmental micro(nano)plastics?

Microplastics (MPs) and nanoplastics (NPs) are emerging pollutants in different environmental compartments (air, soil and water) and that may induce several ecotoxicological effects on organisms and their microbiota. A considerable number of studies has been addressing and highlighting the effects of MPs/NPs on biochemical, molecular and behavior effects of aquatic organisms. However, less attention has been focused on microbiota. Here, a critical overview of published studies focusing on microorganisms affected by MPs and NPs after in vitro or in vivo exposure is provided. Available studies regarding the properties of MPs/NPs, microbial phyla, experimental conditions, techniques employed, and effects are summarized. The link between microbiota disruption and other effects on other hosts (e.g., crustaceans, fish, and mammals) as also analyzed. Overall, the literature review shows that most studies with microorganisms were performed in vitro (MPs: 44.11%; NPs: 23.52%) in comparison with in vivo tests (MPs: 32.35%; NPs: 11.76%). The most studied MP/NPs were polystyrene particles, generally spheres, with sizes <50 μm and concentrations ranged between 100 and 1000 mg L^-1. The most studied main phyla were Proteobacteria, Bacteroidetes, Firmicutes, and Actinobacteria. MPs/NPs induced microbiome composition disruption, immune response (i.e., immune modulator release, immune cells activation and inflammatory response), enzyme activity changes (i.e., catalase, urease, dehydrogenase, alkaline phosphatase, and fluorescein diacetate hydrolase) and gene expression changes. The immune responses changes were related to microbiome disruption. Research gaps are highlighted and recommendations for future research indicated that microbiome is sensitive to MP/NPs and microbiome disruption can be a valuable tool to assess the risk of plastic particles to human and environmental health.