Review on protozoic silica and its role in silicon cycling

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

“…[26]). However, the importance of other biogenic Si pools, especially the protozoic one (represented by a group of protists, i.e., testate amoebae), for Si cycling in some ecosystems has been revealed, and their significance for Si cycling in terrestrial ecosystems might be comparable to the role of protists (i.e., marine diatoms) for Si cycling in the oceans (see the review by Puppe [361]). Recent studies indicate that protozoic Si pools are strongly affected by land use [362,363], but we still do not know which effects protozoic Si pool changes have on the ecosystem scale (e.g., impacts on Si availability).…”

Silicon Cycling in Soils Revisited

“…These organisms use monomeric silicic acid (H 4 SiO 4 ) for the synthesis of hydrated amorphous silica (SiO 2 · n H 2 O), which is called biogenic amorphous silica (bASi). Based on their origin, bASi structures and residues in soils represent bASi pools that can be distinguished as follows: (i) bacterial bASi (formed in bacteria); (ii) fungal bASi (formed in fungi); (iii) phytogenic bASi (formed in plants); (iv) zoogenic bASi (formed in animals); and (v) protistic bASi (formed in protists) [ 81 ] ( Figure 2 ).…”

“…Regarding protozoic bASi, several studies have been conducted (reviewed by Puppe [ 81 ]). Protozoic silica in soils is mainly synthesized by testate amoebae, which form a polyphyletic group of unicellular eukaryotes (protists) with a shell (or test) ranging between about 5–300 µm.…”

“…Although this potential was recognized by some authors [ 80 , 84 , 85 , 149 , 150 ], it took some more years until the quantification of protozoic silica pools and annual bio-silicification of testate amoebae and implications for Si cycling became the focus of attention of several researchers [ 2 , 151 , 152 ]. These studies clearly showed that biosilicification by testate amoebae has to be considered in analyses of Si cycling in terrestrial biogeosystems, because Si fixation in testate amoeba shells is comparable to or even can exceed the amounts of Si absorbed by trees year by year [ 81 , 153 ].…”

“…Currently, we have no idea about global annual Si cycling rates of, e.g., protists (testate amoebae, diatoms) in terrestrial ecosystems, because estimations of global Si cycling by biota “only” consider Si cycling by vegetation. In his review of studies on protozoic silica, Puppe [ 81 ] emphasized that it is very likely that testate amoebae are potential key players in the Si cycle of terrestrial ecosystems because: (i) annual biosilicification rates of idiosomic testate amoebae are comparable to or even exceed annual Si uptake rates of trees; and (ii) it is most likely that total protozoic Si pools (considering not only intact shells but also single idiosomes) are much bigger than stated in publications, however, because it can be assumed that idiosomes most likely can be as stable as phytoliths, and thus are well preserved in soils. Thus, it would not be surprising if total protozoic Si pool quantities (shells plus single idiosomes) would be found to be equal phytogenic Si pool quantities in soils.…”