The effects of metal nanoparticles on the microbiome and immune responses of earthworms

Metal nanomaterials are increasingly applied as an antimicrobial agent in consumer products, coatings and pesticides. Through environmental release, non-target microbes, including symbiotic microbiota associated with animals, may be at risk due to exposure to these nanomaterials. Here, the effects of biocidal nanomaterial exposures (i.e. copper oxide and silver) on the gut microbiome of three earthworm species (i.e. Eisenia fetida, Lumbricus terrestris and Aporrectodea calignosa) and associated soils are studied using a metabarcoding approach. Further, the consequences of a microbiome disruption by nanomaterials on the E. fetida immune responses and the resilience of E. fetida to an infection by the bacterium Bacillus subtilis are investigated. This thesis provides a unique and in-depth view of the gut bacterial microbiome of three environmentally relevant earthworm species and shows how metal pollutants can affect these host-associated bacterial communities. It is shown that the resident component of the earthworm gut microbiome is largely independent from the associated soil bacterial communities. Further, through high replication (within and across concentrations), this thesis shows that the earthworm resident gut microbiome is largely resilient to exposure to antimicrobial nanomaterials. However, a key earthworm symbiont (i.e. the Mollicutes ‘Candidatus Lumbricincola’) that likely plays a role in earthworm digestion is negatively affected by exposure to copper oxide nanomaterial. These adverse effects on this symbiont were recorded in both short-term lab exposures and long-term outdoor soil mesocosm studies. Despite the impact of copper oxide nanomaterial on the earthworm gut microbiome, no evidence for an effect of nanomaterial exposure on host immunity or host susceptibility to a bacterial infection was found. This thesis demonstrates that pollutants can adversely affect crucial earthworm microbes, highlighting the need for further testing for effects of pollutants on microbiomes. The methodological approach adopted in this thesis should guide such future studies