Silicification: The Processes by Which Organisms Capture and Mineralize Silica - Biomineralization - page 301

Figure 16 . Examples of spicules formed under different concentrations of silicic acid. (A) Control (3
micromolar), only one needle-like spicule type called a style. (B) (30 micromolar) Three spicule types:
small styles, Large styles and small C-shaped spicules called isochelae. (C) Larger version of an isochelae
showing teeth. (D) Aster-like desma with a developed centrum from samples grown in 100 micromolar
silicic acid. Scale bars for (A) and (B) are 100 microns, for (C) 10 microns, and for (D) 50 microns.
Modified from Maldonado et al. 1999 and published with permission of Nature Publishing Group.
sponges grown in higher concentrations of silicic acid produced an abundance of at least
three different spicule types although some of the spicules appeared deformed for the
sponges grown at the highest concentration of silicic acid, perhaps suggesting problems
with silicon transport under these conditions. The authors postulated that the changing
composition of sea water, in respect of silicon concentration, caused, in part by the rise of
the importance of diatoms in geological time around the Cretaceous-Tertiary boundary
(Lowenstam and Weiner 1989) may have been a factor in the loss of many varieties of
siliceous sponges from shallow waters where silicon concentrations were dramatically
reduced. It is also of note that the sponge Suberites domuncula grows best at a silicon
concentration of 60 micromolar (Simpson et al. 1985). Silicon concentration is also found
to affect expression of a gene encoding collagen (the structural protein) and a morphogen,
myotrophin (Krasko et al. 2000). Figure 17 shows the relative expression of both
collagen and silicatein in the model primmorph system.
The formation of the sponge skeleton is mediated by an Fe 2+ dependent enzyme
(Holvoet and Van de Vyver 1985) and an iron chelator, 2,2ƍ-bipyridine, when added to
the sponge medium can inhibit skeletogenesis (as above). Model experiments on
Suberites domuncula have shown that iron concentrations of 10 micromolar and above
stimulate the formation of spicules and stimulate DNA and protein, including ferritin,