1Pedro M Antunes*, 2Sidney L. Stürmer*, 3James D. Bever,4Pierre-Luc Chagnon, 5V. Bala Chaudhary, 10Coline Deveautour,1Catherine Fahey, 6Vasilis Kokkoris,7Ylva Lekberg, 8Jeff R. Powell,9Carlos A. Aguilar-Trigueros,8Haiyang Zhang1Biology Department, Algoma University, Sault Ste. Marie, Ontario, Canada, P6A 2G42Departamento de Ciências Naturais, Universidade Regional de Blumenau, Blumenau, SC, 89030-903, Brazil3Kansas Biological Survey and Center for Ecological Research and Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas, USA4Institut de Recherche en Biologie Vegetale, Universite de Montreal, 4101 Sherbrooke Est, Montreal, QC, H1X2B2, Canada5Department of Environmental Studies, Dartmouth College, Hanover, New Hampshire, USA6Amsterdam Institute for Life and Environment (A-LIFE), Faculty of Science, Section Systems Ecology, Vrije Universiteit Amsterdam, De Boelelaan 1085, 1081 HV Amsterdam, the Netherlands7MPG Ranch & Department of Ecosystem and Conservation Sciences, W.A. Franke College of Forestry and Conservation, University of Montana, Missoula, Montana, USA8Hawkesbury Institute for the Environment, Western Sydney University, Penrith, New South Wales, Australia9Institute of Biology, Freie Universität Berlin, Altensteinstrasse 6, 14195 Berlin, Germany.10UniLaSalle, Rouen, France*Equal contribution

Microplastics (MPs) can now be found in all the Earth's biomes, thereby representing a global change phenomenon with largely unknown consequences for biodiversity and ecosystem functioning. Soil protists are eukaryotic, primarily single celled organisms that play important roles in the soil food web. Microplastics have been shown to affect protist populations in freshwater and marine environments, yet the interactions between soil protists and MPs remain largely unknown. Here we examined whether phagotrophic protists present in a forest soil can ingest MPs and experience declines in abundance. We exposed protists to soil treatments with different concentrations of MPs using commercial polymer fluorescent microspheres and used fluorescence microscopy to find evidence of MP ingestion. In addition, we quantified the total number of active phagotrophic protists over time. We show that most soil protists (>75% individuals) can readily ingest and keep MP within their food vacuoles, even at relatively small MP concentrations (0.1% w/w). There was a trend for higher prevalence of ingestion and for declines in protist abundance at the highest concentration of MPs (1% w/w). However, more data are necessary to further ascertain cause-effect relationships. This is the first report indicating that most soil protists present in a forest soil can ingest MPs, which may have important consequences for the transport and uptake of MPs in the soil food web.

Plant diversity has often been linked to increased productivity. However, this apparent diversity-productivity relationship for plants may rely on interactions with microbes. Furthermore, these relationships are likely to be context dependent in response to resource availability. We used a biodiversity-ecosystem function experiment with trees exposed to high and low water availability treatments to determine if soil fungal communities mediate the diversity-productivity relationship. We found that richness and community composition of soil fungi and the interaction between plant pathogen and ectomycorrhizal richness were significant drivers of above-ground productivity and supported indirect, positive effects of tree species richness and functional diversity on productivity. Soil fungi also mediated tree water relations, reducing the effect of water limitation on productivity. Fungal communities were important drivers of positive net diversity and complementarity effects on productivity. Our study provides evidence that soil fungi play an important role in diversity-productivity relationships under variable water availability.

1. Invasive plants are considered major threats to biodiversity globally; however, our understanding of the long-term dynamics of invasion remains limited. 2. Over time, invasive plants can accumulate pathogens capable of causing population declines because invaders have a greater chance of encountering such pathogens as they spread and native pathogens adapt to use invasive plants as a resource over time. However, reports typically focus on individual species and ecologists lack a synthesis approach capable of predicting pathogen susceptibility in plant invaders. 3. Pathogen resistance and tolerance are tightly coupled to plant traits, which we suggest can provide a framework for understanding and predicting novel pathogen accumulation. 4. We reviewed the literature to synthesize plant traits associated with pathogen susceptibility and to determine the prevalence of novel pathogen accumulation on invasive plants. We then used these data and applied a multivariate model to associate plant traits with pathogen effects to predict pathogen susceptibility of invasive plants. Finally, we provide directions for future research. 5. Considering the emergence of trait-based approaches, comprehensive databases, and new data on individual invasions, advances in our understanding of invasive plant-pathogen interactions can lead to breakthroughs both at fundamental and management decision-making levels.