Silica diatom frustules

As is the case with other forms of non-crystalline silica, the conversion to quartz of phytoliths, diatom frustules, and sponge spicules under sedimentary conditions on land can be expected to be accelerated by elevated temperature, elevated pressure, and the presence of an aqueous phase of high pH (>9) or containing electrolytes. 

Marine waters also receive some biogenic silica from the land. This material is transported to the sea as windblown dust and as part of the suspended load of rivers. Rivers also deliver about 0.43 Pg of dissolved silica annually to the oceans, and some fraction of this is undoubtedly derived from biological sources as well. Locally, terrigenous biogenic silica (in particulate form) may accumulate to significant concentrations on the sea floor. For example, Kolbe (1957) reported frequent occurrences of phytoliths and freshwater diatom frustules in deep-sea cores from the equatorial Atlantic, and one locahty contained diatom tests derived exclusively from freshwater species. 

Silicon is extremely important as a building block of unicellular algae -diatoms. The main building material of the frustule (the finely sculptured protective surface layer of diatoms) is a float-stone, an opal-like water-containing polymer of silica Diatoms are the only group of organisms whose development is totally dependent on the presence of soluble forms of silica in the environment. When silicon sources run out, DNA replication stops. 

Labyerie, L. Jr. 1974. New approach to surface seawater paleotem-peratures using 0/ 0 in silica of diatom frustules. Nature, 248 40-42. 

Kyle PR, Palais JM, Delmas R (1982) The volcanic record of Antarctic ice cores Preliminary results and potential for future investigations. Annal Glaciol 3 172-177 Kyle PR, Palais JM, Thomas E (1984) The Vostok tephra an important englacial stratigraphic marker Antarctic J US 19 64-65 Kyle PR, Moore JA, Thirlwall ME (1992) Petrological evolution of anorthoclase phonolite lavas at Mount Erebus, Ross Island, Antarctica. J Petrol 33(4) 849-875 Labeyrie L (1974) New approach to surface seawater paleotem-peratures using ratios in silica of diatom frustules. 

Figure 19.2 Biogenic oxides, (a) Silica in diatom frustule (scale bar = 1 (im). (Reproduced with permission from Ref. [33]. Copyright 2003, Wiley-VCH Verlag GmbH, Weinheim.) (b) Iron oxide in magnetotactic bacteria (scale bar = 100 nm). (Reproduced with permission from...

Biogenic silica structures also exhibit interesting structural properties. In particular, the large porosity of diatom frustules makes them excellent sorbents for a wide variety of processes, including dynamite preparation by immobilization of nitroglycerin or for water remediation purposes. Noticeably, these applications use diatomite earth (i.e., geological deposits of dead diatom frustules) rather than fresh silica [67]. This is also true considering another application of natural porous silica for filtration, as used in the brewery processes [68]. 

If the association of proteins and biogenic oxides finds its main inspirational source from diatoms, the coexistence of polysaccharides and silica is most often observed in plants. In feet, fi om rice husk to sunflower leafe, a myriad of plants display variable contents of silica [37,197,198]. The occurrence of silica in plants is limited to ill-shaped ovoids, rods, or serrated leaf edges [197], some far less spectacular features than those found in diatom frustules or sea sponges. This might explain the late start in the development of materials based on the combination of polysaccharides and biogenic oxides compared with the protein-based counterparts. 

Gehlen, M., Beck, L., Galas, G., Flank, A.-M., Van Bennekom, A.J., and Beusekom, J.E.E. (2002) Umavelling the atomic structure ofbiogenic silica evidence of the structural assodation of A and Si in diatom frustules. Geochim. Cosmochim. Acta, 66,1601-1609. 

Diatomite. also known as diatomaceous earth, or kieselgubr, consists mainly of accumulated shells or frustules of intricately structured amorphous hydrous silica secreted by diatoms, which are microscopic, onc-celled golden brown algae of the class Bacillariophyceae. Diatoms exist in many different environments and arc abundant in regions of oceanic upwelling. 

Diatoms are unicellular, photosynthetic microalgae that are abundant in the world s oceans and fresh waters. It is estimated that several tens of thousands of different species exist sizes typically range from ca 5 to 400 pm, and most contain an outer wall of amorphous hydrated silica. These outer walls (named frustules ) are intricately shaped and fenestrated in species-specific (genetically inherited) patterns5,6. The intricacy of these structures in many cases exceeds our present capability for nanoscale structural control. In this respect, the diatoms resemble another group of armored unicellular microalgae, the coccolithophorids, that produce intricately structured shells of calcium carbonate. The silica wall of each diatom is formed in sections by polycondensation of silicic acid or as-yet unidentified derivatives (see below) within a membrane-enclosed silica deposition vesicle 1,7,8. In this vesicle, the silica is coated with specific proteins that act like a coat of varnish to protect the silica from dissolution (see below). The silica is then extruded through the cell membrane and cell wall (lipid- and polysaccharide-based boundary layers, respectively) to the periphery of the cell. 

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