not sure if this is the article your referring to........?
by
Randy Holmes-Farley | Jan 15, 2003
Marine Organisms that Use Silica: Sponges
Sponges are the second largest consumer of dissolved silica in the ocean, after diatoms.14 Many sponges use silica to form internal structures, called spicules, which help them retain their shape. In the case of the sponge
Tethya aurantia, these spicules are needles of amorphous silica that comprise 75% of its dry weight. These spicules are 2 mm long and 30 mm wide.15
Most frequently, the spicules in sponges are hydrated amorphous silica, and also contain collagen, an organic material (although sometimes they are calcium carbonate). In order to direct the formation of these silica spicules, sponges produce enzymes that help control the silica deposition process. In the case of the sponges
Suberites domuncula and
Tethya auranita, an enzyme called silicatein is produced. When the silicate concentration is increased in the surrounding water from 1 mM (0.06 ppm SiO2) to 60 mM (3.6 ppm SiO2), the gene responsible for silicatein is strongly up- regulated, along with that for collagen.16 These experiments suggest that the sponge may be able to take advantage of silica concentrations as high as 60 mM, or alternatively, may be restricted in it’s growth, at levels below 60 mM.
It has been shown that in the sponge
Halichondria panicea there is a correlation between the dissolved SiO2 in the seawater, and SiO2 content of the sponge.17 Moreover, it has been shown that silica uptake appears to require energy expenditure by the sponge, and that after conditions of starvation, uptake rates were greatly reduced. The ability of these sponges to remove SiO2 from the water column is sufficient so that in the summer they may actually compete with diatoms for available silicate.
In this same sponge, the rate of silica uptake is a function of the dissolved silica concentration in the seawater, with higher silica concentrations resulting in higher rates of uptake. The concentration at which the sponges take up silica at half of their maximal rate is 46 mM (2.8 ppm SiO2).14 Further, in certain waters it is believed that the growth of these sponges is limited by soluble silica, rather than by the availability of food. These sponges are capable of taking up 19 mmol/h per gram of tissue (maximally), so at 46 mM dissolved SiO2, they theoretically could take up 9.5 mmol/h per gram of tissue (though there is no evidence that the do take up silica that fast under normal growth conditions). A 50-gram sponge would then be able to take up 475 mmol/h, or 11.4 mmol (0.7 g) of silica in a day. A 100-gallon (378 L) aquarium with a high level of silica (30 mM or 1.9 ppm SiO2) only contains that much to begin with. Consequently, the potential
silica depletion in reef tanks with actively growing sponges could be substantial. Of course, if your sponges are not growing rapidly, then they likely are not using much silica. Likewise, if there is not much silica, then they may not be able to grow rapidly, even if other conditions are good.
This reason is, in fact, why I initiated dosing of silica (0.33 mM/day or 0.02 ppm SiO2/day) to my reef tank several years ago. I had a large sponge that I wanted to survive, and I had hoped that silica additions might alter the usual course of slow death for such sponges in most reef tanks. When I initiated that study, Julian Sprung told me that he had tried the same thing. In both of our cases the sponges eventually died, in my case lasting about 18 months. Perhaps we did not dose enough silica (0.33 mM/day is a small amount relative to the uptake numbers described above, and the concentration in the water column of my tank never rose above 0.8 mM (0.05 ppm SiO2) where it could be detected with a Hach kit). Alternatively, perhaps the food sources were not right or something else was wrong.
