Testate amoebae (Protozoa) in lakes of the Qinghai-Tibet Plateau: Biodiversity, community structures, and protozoic biosilicification in relation to environmental properties and climate warming.

The Qinghai-Tibet Plateau (QTP) is characterized by a vast number of frozen and unfrozen freshwater reservoirs, which is why it is also called "the third pole" of the Earth or "Asian Water Tower". We analyzed testate amoeba (TA) biodiversity and corresponding protozoic biosilicification in lake sediments of the QTP in relation to environmental properties (freshwater conditions, elevation, and climate). As TA are known as excellent bio-indicators, our results allowed us to derive conclusions about the influence of climate warming on TA communities and microbial biogeochemical silicon (Si) cycling. We found a total of 113 TA taxa including some rare and one unknown species in the analyzed lake sediments of the QTP highlighting the potential of this remote region for TA biodiversity. >1/3 of the identified TA taxa were relatively small (<30 µm) reflecting the relatively harsh environmental conditions in the examined lakes. TA communities were strongly affected by physico-chemical properties of the lakes, especially water temperature and pH, but also elevation and climate conditions (temperature, precipitation). Our study reveals climate-related changes in TA biodiversity with consequences for protozoic biosilicification. As the warming trend in the QTP is two to three times faster compared to the global average, our results provide not only deeper insights into the relations between TA biodiversity and environmental properties, but also predictions of future developments in other regions of the world. Moreover, our results provide fundamental data for paleolimnological reconstructions. Thus, examining the QTP is helpful to understand microbial biogeochemical Si cycling in the past, present, and future.

Silicon in Plants: Alleviation of Metal(loid) Toxicity and Consequential Perspectives for Phytoremediation.

For the majority of higher plants, silicon (Si) is considered a beneficial element because of the various favorable effects of Si accumulation in plants that have been revealed, including the alleviation of metal(loid) toxicity. The accumulation of non-degradable metal(loid)s in the environment strongly increased in the last decades by intensified industrial and agricultural production with negative consequences for the environment and human health. Phytoremediation, i.e., the use of plants to extract and remove elemental pollutants from contaminated soils, has been commonly used for the restoration of metal(loid)-contaminated sites. In our viewpoint article, we briefly summarize the current knowledge of Si-mediated alleviation of metal(loid) toxicity in plants and the potential role of Si in the phytoremediation of soils contaminated with metal(loid)s. In this context, a special focus is on metal(loid) accumulation in (soil) phytoliths, i.e., relatively stable silica structures formed in plants. The accumulation of metal(loid)s in phytoliths might offer a promising pathway for the long-term sequestration of metal(loid)s in soils. As specific phytoliths might also represent an important carbon sink in soils, phytoliths might be a silver bullet in the mitigation of global change. Thus, the time is now to combine Si/phytolith and phytoremediation research. This will help us to merge the positive effects of Si accumulation in plants with the advantages of phytoremediation, which represents an economically feasible and environmentally friendly way to restore metal(loid)-contaminated sites.

Biodiversity and Ecosystem Functioning in Naturally and Experimentally Assembled Communities.

Numerous studies have proved that biodiversity and ecosystem functioning (BEF) are closely linked [...].

Silicon in the Soil-Plant Continuum: Intricate Feedback Mechanisms within Ecosystems.

Plants' ability to take up silicon from the soil, accumulate it within their tissues and then reincorporate it into the soil through litter creates an intricate network of feedback mechanisms in ecosystems. Here, we provide a concise review of silicon's roles in soil chemistry and physics and in plant physiology and ecology, focusing on the processes that form these feedback mechanisms. Through this review and analysis, we demonstrate how this feedback network drives ecosystem processes and affects ecosystem functioning. Consequently, we show that Si uptake and accumulation by plants is involved in several ecosystem services like soil appropriation, biomass supply, and carbon sequestration. Considering the demand for food of an increasing global population and the challenges of climate change, a detailed understanding of the underlying processes of these ecosystem services is of prime importance. Silicon and its role in ecosystem functioning and services thus should be the main focus of future research.

Silicon Cycling in Soils Revisited.

Silicon (Si) speciation and availability in soils is highly important for ecosystem functioning, because Si is a beneficial element for plant growth. Si chemistry is highly complex compared to other elements in soils, because Si reaction rates are relatively slow and dependent on Si species. Consequently, we review the occurrence of different Si species in soil solution and their changes by polymerization, depolymerization, and condensation in relation to important soil processes. We show that an argumentation based on thermodynamic endmembers of Si dependent processes, as currently done, is often difficult, because some reactions such as mineral crystallization require months to years (sometimes even centuries or millennia). Furthermore, we give an overview of Si reactions in soil solution and the predominance of certain solid compounds, which is a neglected but important parameter controlling the availability, reactivity, and function of Si in soils. We further discuss the drivers of soil Si cycling and how humans interfere with these processes. The soil Si cycle is of major importance for ecosystem functioning; therefore, a deeper understanding of drivers of Si cycling (e.g., predominant speciation), human disturbances and the implication for important soil properties (water storage, nutrient availability, and micro aggregate stability) is of fundamental relevance.

How Does Sphagnum Growing Affect Testate Amoeba Communities and Corresponding Protozoic Si Pools? Results from Field Analyses in SW China.

The policy and practice of ecological restoration and conservation in China obtained some remarkable results. For example, Sphagnum moss growing on abandoned farmland, which was peatland before agricultural use, has rapidly expanded the wetland area in SW China. Microorganisms such as testate amoebae are sensitive to environmental change and thus have been widely used as ecological indicators in various habitats. We analyzed differently aged Sphagnum growing plots on a Sphagnum growing farmland and natural Sphagnum plots in SW China to examine how Sphagnum-dwelling testate amoeba communities and corresponding protozoic silicon (Si) pools respond to ecological restoration practice. We found that abundance, taxon richness, and diversity of testate amoebae were higher in Sphagnum growing farmland plots compared to natural Sphagnum plots. Protozoic Si pools showed an increase with Sphagnum growing time representing increased Si accumulation by idiosomic testate amoeba shells. However, protozoic Si pools were negatively correlated with taxon richness and diversity of testate amoebae. Our results showed that (i) natural Sphagnum plots were not characterized by the expected higher biodiversity of testate amoebae compared to Sphagnum growing plots and (ii) consequently protozoic Si pool quantity in natural Sphagnum plots was less driven by biodiversity of testate amoebae than expected. We concluded our results to underline the value of (i) environmental restoration policy in general and (ii) testate amoeba communities and corresponding protozoic Si pools for Si cycling in restoration areas of peatlands in particular. Based on our results, we recommend a sustainable cultivation of Sphagnum moss and an additional establishment of protected areas, where no Sphagnum harvesting occurs. These protected Sphagnum areas might represent hot spots of undisturbed testate amoeba communities and corresponding protozoic Si pools and thus of microbial Si cycling.

Soil Testate Amoebae and Diatoms as Bioindicators of an Old Heavy Metal Contaminated Floodplain in Japan.

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 CaCl2 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.

Data on euglyphid testate amoeba densities, corresponding protozoic silicon pools, and selected soil parameters of initial and forested biogeosystems.

The dataset in the present article provides information on protozoic silicon (Si) pools represented by euglyphid testate amoebae (TA) in soils of initial and forested biogeosystems. Protozoic Si pools were calculated from densities of euglyphid TA shells and corresponding Si contents. The article also includes data on potential annual biosilicification rates of euglyphid TA at the examined sites. Furthermore, data on selected soil parameters (e.g., readily-available Si, soil pH) and site characteristics (e.g., soil groups, climate data) can be found. The data might be interesting for researchers focusing on biological processes in Si cycling in general and euglyphid TA and corresponding protozoic Si pools in particular.

Culture growth of testate amoebae under different silicon concentrations.

Testate amoebae with self-secreted siliceous shell platelets ("idiosomes") play an important role in terrestrial silicon (Si) cycles. In this context, Si-dependent culture growth dynamics of idiosomic testate amoebae are of interest. Clonal cultures of idiosomic testate amoebae were analyzed under three different Si concentrations: low (50µmolL-1), moderate/site-specific (150µmolL-1) and high Si supply (500µmolL-1). Food (Saccharomyces cerevisiae) was provided in surplus. (i) Shell size of four different clones of idiosomic testate amoebae either decreased (Trinema galeata, Euglypha filifera cf.), increased (E. rotunda cf.), or did not change (E. rotunda) under the lowest Si concentration (50µmolSiL-1). (ii) Culture growth of idiosomic Euglypha rotunda was dependent on Si concentration. The more Si available in the culture medium, the earlier the entry into exponential growth phase. (iii) Culture growth of idiosomic Euglypha rotunda was dependent on origin of inoculum. Amoebae previously cultured under a moderate Si concentration revealed highest sustainability in consecutive cultures. Amoebae derived from cultures with high Si concentrations showed rapid culture growth which finished early in consecutive cultures. (iv) Si (diluted in the culture medium) was absorbed by amoebae and fixed in the amoeba shells resulting in decreased Si concentrations.

Testate amoebae in 31 mature forest ecosystems--densities and micro-distribution in soils.

We studied testate amoebae and possible correlated abiotic factors in soils of 31 mature forest ecosystems using an easily applicable and spatially explicit method. Simple counting on soil thin-sections with a light microscope resulted in amoeba densities comparable to previously reported values, i.e. 0.1 × 10(8) to 11.5 × 10(8) individuals m(-2) (upper 3 cm of soil). Soil moisture and soil acidity seem to be correlated with amoeba densities. At sites of moderate soil moisture regimes (SMR 2-7) we found higher densities of testate amoebae at pH<4.5. At wetter sites (SMR ≥ 8) higher individual densities were recorded also at less acidic sites. The in situ description of amoebae, based on the analysis of a complete soil thin-section, showed a relatively uniform spatial micro-distribution throughout the organic and mineral soil horizons (no testate amoeba clusters). We discuss the pros and cons of the soil thin-section method and suggest it as an additional tool to improve knowledge of the spatial micro-distribution of testate amoebae.