title: Interaction of germanium (Ge) with biosilicification in the freshwater sponge Ephydatia mülleri: evidence of localized membrane domains in the silicalemma.
abstract: In the presence of germanium (Ge) the needle-shaped silica spicules of the freshwater sponge Ephydatia m ulleri are very short and thin and possess bulbs with large spines. SEM-coupled X-ray analyses confirm the incorporation of Ge into the silica. A small number of bulbs are susceptible to erosion by HNO3 and hypochlorite and although the chemical basis of such erosion is presently unknown it suggests the presence of an organic matrix within the bulbs and/or an incomplete polymerization of the silica. Addition of Ge to control media in which silicification is newly initiated increases the incidence of erosion and results in centrally located eroded areas of the silica and discontinuities in its deposition. Removal of Ge from such newly forming structures results in a partial recovery of normal morphology (spine development and thickening of the silica) but only in the central region surrounding the bulbs. Both results establish the presence of a central, active region for silicification and further support the view that there is a distal spreading, away from this center, of transported forms of silica. Secondary centers may also be present. The newly assembled organic core of control structures is associated with tubular elements possibly derived from the surrounding membrane. In such newly silicifying structures the spicule tips contain oriented material in the form of "rays." Both of these new observations increase the likelihood of the presence of an organic matrix within the silica.
mesh_terms: D000818:Animals; D005857:Germanium / Q000378:metabolism*; D008566:Membranes / Q000378:metabolism; D008855:Microscopy, Electron, Scanning; D011161:Porifera / Q000378:metabolism* / Q000648:ultrastructure; D012822:Silicon Dioxide / Q000378:metabolism*
chemical_list: D005857:Germanium; D012822:Silicon Dioxide