Review on protozoic silica and its role in silicon cycling

•Overview of publications on protozoic silica in the context of silicon (Si) cycling.•Testate amoebae potentially play a key role in biogeochemical Si cycling.•Total protozoic Si pool quantities might be equal to phytogenic Si pool quantities.•Importance of single idiosomes and corresponding Si pools is emphasized. Biogenic silicon (BSi) has been found to play a fundamental role in the link between global Si and carbon cycles, because it represents a key factor in the control of Si fluxes from terrestrial to aquatic ecosystems. Furthermore, various beneficial effects of Si accumulation in plants have been revealed, i.e., increased plant growth and resistance against abiotic and biotic stresses. Due to intensified land use humans directly influence Si cycling on a global scale. For example, Si exports through harvested crops and increased erosion rates generally lead to a Si loss in agricultural systems with implications for Si bioavailability in agricultural soils, which is controlled by BSi to a great extent. However, while corresponding research on phytogenic BSi (i.e., BSi synthesized by plants) has been established for decades now, studies dealing with protozoic BSi (i.e., BSi synthesized by testate amoebae) have been conducted just recently and in the current review I summarized the findings of these field and laboratory studies. My review clearly highlights the potential of testate amoebae for Si cycling in terrestrial ecosystems and identifies knowledge gaps that have to be filled by future studies. In this context, especially the importance of single idiosomes (i.e., the building blocks of testate amoeba shells) is emphasized as there are no data on total protozoic Si pool quantities (represented by intact shells and single idiosomes) available yet. The filling of these knowledge gaps will be crucial for a detailed understanding of the role of testate amoebae in the biogeochemistry of terrestrial ecosystems.