SOIL MICROBIOLOGY Soil Testate Amoebae and Diatoms as Bioindicators of an Old Heavy Metal Contaminated Floodplain in Japan Manfred Wanner 1 & Klaus Birkhofer 1 & Thomas Fischer 2 & Miki Shimizu 1 & Satoshi Shimano 3 & Daniel Puppe 4 Received: 12 December 2018 /Accepted: 23 April 2019 # Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Soil protists are rarely included in ecotoxicological investigations, despite their fundamental role in ecological processes. Moreover, testate amoebae and diatoms contribute considerably to silicon fluxes in soils. We investigated the effects of heavy metals on testate amoebae (species and individual densities) and diatoms (individual densities) in aged soils of a floodplain (Watarase retarding basin, Japan) taking soil samples from two unpolluted reference sites and two polluted sites. The total concentrations of Cu, Pb, and Zn in soil were higher at the polluted sites as compared with the reference sites. The available concentrations of Co, Cu, and Zn in CaCl 2 extracts were higher at the polluted sites but available Pb was not detectable. Testate amoeba taxonomic richness was higher in the reference sites (45/38 taxa) than in the polluted sites (36/27 taxa). The reference sites had higher diatom and amoeba densities than the polluted sites. There was a significant negative correlation between total testate amoeba density and heavy metal concentration (available Co), while significant negative correlations were found between diatom density and Co, Cu, and Zn (available and total concentration). Densities of Cyclopyxis kahli cyclostoma, Centropyxis spp., and Trinema complanatum were negatively correlated to concentrations of available heavy metals. The observed decrease in individual numbers due to heavy metal pollution resulted in a considerable decline in protozoic (testate amoebae) and protophytic (pennate diatoms) silicon pools. Our data suggest that heavy metal pollution affects biogeochemical cycling in this system. Keywords Soil protists . Heavy metals . Silicon . Aged soils . Floodplain Introduction Soil pollution has been recognised as a severe problem to ecosystem functioning and human health since the second half of the twentieth century. However, for 1000 years, mining activities have been one of the most important sources of toxic elements, especially heavy metals (see [1]). Today, heavy metals are (at least locally) intensively monitored pollutants. They persist very long in the environment and have adverse effects on ecosystem functioning, as, e.g. primary production and nutrient cycling. Heavy metals accumulate in the food web, causing detrimental physiological effects on all kinds of organisms, human beings included (discussed in [2]). In this context, a toxicity-based monitoring approach rather than a chemically based approach appears more suitable since un- der field conditions, the environmental impact is associated with several pollutants, as well as transformation of the chemicals in soil [3, 4]. Moreover, total metal concentrations are poor predictors of toxicity. The longer the impact on the solid phase of the soil, the more heavy metals will bind to soil organic matter or mineral phases reducing the bioavailable amount in the pore water [2, 4]. Despite their good potential, soil protists are rarely includ- ed in ecotoxicological investigations [5]. For instance, terres- trial testate amoebae are sensitive bioindicators of soil abiotic and biotic conditions [6]. Their size ranges mostly between 20 and 150 μm and population density can reach 10 8 ind. m -2 [6, 7]. These unicellular eukaryotes consist of three main groups with about 2000 described species, from which about 300 occur in soils [8]. Arcellinida (Amoebozoa: Lobosa) have * Manfred Wanner wanner@b-tu.de 1 Department Ecology, Brandenburg University of Technology Cottbus-Senftenberg, 03013 Cottbus, Germany 2 Central Analytical Laboratory, Brandenburg University of Technology Cottbus-Senftenberg, 03013 Cottbus, Germany 3 Science Research Center, Hosei University, Tokyo 102-8160, Japan 4 Leibniz Centre for Agricultural Landscape Research (ZALF), 15374 Müncheberg, Germany Microbial Ecology https://doi.org/10.1007/s00248-019-01383-x