Quote:
Originally posted by Geoff
i was wondering, when the bloom dies down where do all of the siliates go? would now when the diatoms die they release the silicates again, would not the cycle keep going? ...does IO or any other salt mix contain silicates?
|
Silicates are a necessary part of the elements used by some creatures in the maintenance of their structure. In particular, sponges use quite a bit of silicate to form the structure of their colonies. Silica in the form of silicates and organosilicates are in their highest concentration in waters near river mouths and coastlines in general where there is terrestrial runoff, and lowest in surface waters away from the influences of estuary areas. It is also found in high concentrations in deep pelagic waters (i.e., trenches and end-oceanic pools). The primary users are still diatoms and silicoflagellates, as they use the silicates to form their tests. In locations where blooms of these organisms have occurred, the silicates in the water column often fall to near zero.
Silicon in seawater exists as both particulate silicon and as dissolved silicate. Silicon is actually hydrated in the presence of water to form a weak acid:
SiO2 + 2H2O --> Si(OH)4 (aqueous)
which in aqueous solutions then dissociates to form two protons and the hydrated silicates:
Si(OH)4 --> H+ + Si(OH)3O-
and... Si(OH)3O- --> H+ + Si(OH)2O2- -
based on the pKa of each reaction (I will post it for those of you that want this info_) in NSW at 25C and a pH of 8.1, the ionization of each reaction accounts for 95.9% for the first reaction and 4.1% of the second reaction (i.e., expect to see about 96% of the first product in seawater, and about 4% of the second product). Polymerized forms of the above are of little consequence, as the concentration of Silica in seawater is so low to start with, although should these ions form ionic pairs with either Mg++ of Ca++, their concentrations may climb if the raw materials are available (this may be a small factor in closed systems, but not significant in the ocean)
In addition to the solubilized silicates, seawater contains a wide variety of different sized particulates of silicate origin. These particulates are formed during the weathering process of terrestrial rocks, primarily quartz, feldspar, and some silicate-based clay materials. They are transported to the sea as suspended fines in river water and by wind erosion. A good portion of these materials are deposited as silts in the river deltas at their mouth, the rest may stay in suspension for years (70 to 90% of these particulates are in the 10 micrometer
size range...) A percentage (? %) is taken up by diatoms and a surface planktonic organism known as radiolarians (they form opal for their skeletons, a hydrated noncrystalline form of SiO2), often to the point where the concentration of silicates drops to near zero. When these organisms die, their tests and skeletons sink to the sediment to form diatom oozes (where do you think diatomaceous earth comes from???). Alternatively, in deep oceanic pools these same tests/skeletons breakdown to release inorganic silicon, where silicon concentrations then climb to their highest oceanic levels. As an aside, the concentration in deep Pacific waters is much higher than in Atlantic waters (much the same as CO2), as the waters there have had more time to accumulate SiO2.
Not a lot is known about the physiology of SiO2 absorption and its conversion to hydrated silicates in diatoms. Specialized proteins are involved, and are found in particular centers on the cytoplasmic membrane of the diatom. Deposition of Silica hydrates spread from these centers quickly to form the diatom's test. Depending on the specie, diatoms may consist of as much as 50% of the dried weight of the organism as SiO2. The SiO2 in living diatoms is insoluble, but after their death, becomes soluble and may account for the levels of Silicates seen in fresh water supplies. Treatment with EDTA accelerates this solubility (which means that when divalent ions like Fe++ or Mg++ or Ca++ are removed, the tests become much more soluble... HHHHmmmmmmm....) If the organisms are cultured in SiO2-deficient media (some closed aquaria systems), the cells themselves will become SiO2 deficient, but may live as such for several weeks, and are capable of absorbing needed SiO2 from the water column even in the dark. If these same SiO2-deficient organisms are exposed to strong illumination, they will photosynthesize for a short period of time, but will soon die.
In our aquaria, we will need to see some silicates in the water column, but we do not want to reintroduce waters with high levels of silicate into our systems via topoff, specimen introduction, etc. Sponges will remove some of the silicate over time, but if we continually reintroduce silicates, we must expect recurrent
diatom blooms. This makes the case for making our ASW with silicon-deficient RO/DI water. Skimming will remove most of the particulates and the tests of dead diatoms, sponges will utilize the dissolved portion to build their colonies, but the rest (not very much) will sink to the substrate where they may slowly release their silicates in the most usable form for diatoms. Another good reason to skim 24/7 when establishing your system initially (heh, weaning the system back after the first 6 to 12 months IS a viable alternative with a mature system).
I couldn't think of much else, I guess I could do some reading, but that would take all the fun out of your research (hint hint...
)
HTH
