Griffiths, R.I.; Bass, D. 2016. Differences in soil micro-eukaryotic communities over soil pH gradients are strongly driven by parasites and saprotrophs. Environmental Microbiology, 18 (6). 10.1111/1462-2920.13220 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner.This article is protected by copyright. All rights reserved. Running title: Soil pH and protistan diversity.This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1111/1462-2920.13220 Accepted ArticleThis article is protected by copyright. All rights reserved. 2 INTRODUCTIONProtists are key components of microbial communities in both aquatic and terrestrial environments. They represent 10 4 -10 7 individuals per gram of dry soil (Adl and Coleman, 2005;Adl and Gupta, 2006;Bamforth, 2007) and over 50% of total aquatic biomass (Sherr and Sherr, 2002, 2007). The diversity of functional groups (trophic status, free-living vs symbiotic, etc.) makes them major participants of the microbial loop and regulators of biogeochemical flows (Calbet and Landry, 2004). With the continuing development and growing capability of molecular techniques, protist diversity is increasingly revealed as orders of magnitude greater than morphological or even earlier sequence-based assays suggested (Bates et al., 2012). The past ten years have seen extraordinary advances in our knowledge of microbial eukaryotic diversity, primarily through the adoption of molecular tools for phylogenetically-based classification which provides a coherent evolutionary framework to explore diversity.Additionally the routine use of environmental sequencing utilising next generation sequencing technologies has permitted the discovery of many new groups and novel eukaryotic lineages in many different biomes (e.g. Takishita et al., 2007;Jones et al., 2011; de Vargas et al., 2015). However, the challenge of overlaying ecological and biogeographical insight onto this diversity still remains, particularly in the complex and heterogeneous soil environment.Microscopy-based soil protist diversity studies far outnumber soil eDNA studies, and both lag far behind their marine and freshwater counterparts. Diversity assessments that rely on culturing and/or visual identification have revealed a large diversity of cell forms and taxa dominated by bacterivores, predators, and some autotrophs (inferred from e.g. Stout 1984;Bamforth, 2007; Domonell et al., 2013). These studies often rely on protists capable of growing in culture medium supplemented with bacteria and recognition of visually distinctive (and relatively large) forms, e.g. via liquid Accepted ArticleThis article is protected by copyright...

The phagotrophic flagellate Oxyrrhis marina shows a strong stoichiometric plasticity when fed differently grown Rhodomonas salina. We tested whether differently pre-conditioned O. marina displayed selective feeding behaviour from a mixture of nitrogen and phosphorus depleted R. salina. We observed selective feeding of O. marina, always selecting phosphorus rich R. salina independent of the pre-conditioning of the protists. In a second experiment, O. marina was again pre-conditioned either with nitrogen-or phosphorus-depleted R. salina and was refed with either of the differently limited R. salina in single food treatments (not in a mixture). The phagotrophic flagellate displayed compensatory feeding which means that O. marina feed more on the food source which they were not given before. Due to its stoichiometric plasticity, O. marina might handle bad quality food by following the stoichiometry of its prey and additionally by active selective feeding towards P-rich algae to enhance growth. Post-ingestion selection might as well be an important feature which means that ingested elements in excess are quickly excreted and scarce elements are ingested through accelerated food uptake.

Community dynamics are often studied in subsets of pairwise interactions. Scaling pairwise interactions back to the community level is, however, problematic because one given interaction might not reflect ecological and evolutionary outcomes of other functionally similar species interactions or capture the emergent eco-evolutionary dynamics arising only in more complex communities. Here we studied this experimentally by exposing Pseudomonas fluorescens SBW25 prey bacterium to four different protist predators (Tetrahymena pyriformis, Tetrahymena vorax, Chilomonas paramecium and Acanthamoeba polyphaga) in all possible single-predator, two-predator and four-predator communities for hundreds of prey generations covering both ecological and evolutionary timescales. We found that only T. pyriformis selected for prey defence in single-predator communities. Although T. pyriformis selection was constrained in the presence of the intraguild predator, T. vorax, T. pyriformis selection led to evolution of specialised prey defence strategies in the presence of C. paramecium or A. polyphaga. At the ecological level, adapted prey populations were phenotypically more diverse, less stable and less productive compared with non-adapted prey populations. These results suggest that predator community composition affects the relative importance of ecological and evolutionary processes and can crucially determine when rapid evolution has the potential to change ecological properties of microbial communities.

Mediterranean Posidonia oceanica meadows shelter an important biomass and biodiversity of amphipod crustaceans that graze on epiphytes. However, their actual significance for ecosystem functional processes is hard to estimate, due to the lack of adequate data. Here, a field microcosm-based inclusion experiment was used to test if three of the dominant taxa of the amphipod community (Apherusa chiereghinii, Dexamine spiniventris and Gammarus spp.) could exert top-down control on seagrass leaf epiphytes. Influence of amphipod activity on nutrient availability for the host species was also investigated. All grazer taxa significantly reduced biomasses of erect macroalgae and erect sessile animals present on leaves. None of them consumed encrusting epiflora or epifauna. This selective top-down control could have important implications for the structure of the epiphytic community on leaves of P. oceanica, which is one of the most diverse and abundant of all seagrass species. Grazing activity of all taxa caused higher N content of seagrass leaves, likely through amphipod excretion and/or sloppy feeding. Since P. oceanica meadows often grow in oligotrophic zones where plant growth can be nutrient-limited, this N enrichment could enhance seagrass production. Overall, the ecological interaction between P. oceanica and amphipods could be seen as a facultative mutualistic relationship. Our results suggest that amphipod mesograzers are key-elements in some of the functional processes regulating these complex and yet endangered ecosystems, which are essential components of Mediterranean coastal zones.