- Biogenic silica
Biogenic silica (BSi) is the equivalent to the terms
opal, biogenicopal, and amorphous opaline silica. BSi is essential to many plants and animals. Chemically, BSi is hydrated silica(SiO2•"n"H2O). Diatoms in both fresh and salt water extract silica from the water to use as a component of their cell walls. Likewise, some holoplanktonic protozoa( Radiolaria), some sponges, and some plants (leaf phytoliths) use silicon as a structural material. Siliconis known to be required by chicks and rats for growth and skeletal development. Silicon is in human connective tissues, bones, teeth, skin, eyes, glands and organs. It is a major constituent of collagenwhich helps keep our skin elastic, and it helps calciumin maintaining bone strength.
BSi is silica that originates from the production out of
dissolved silica. BSi can either be accumulated "directly" in marine sediments (via export) or be transferred back into dissolved silica in the water column.
Increasingly, isotope ratios of oxygen (O18:O16) and silicon (Si30:Si28) are analysed from BSi preserved in lake and marine sediments to derive records of past
climate changeand nutrient cycling (De La Rocha, 2006; Leng and Barker, 2006). This is a particularly valuable approach considering the role of diatoms in global carbon cycling. In addition, isotope analyses from BSi are useful for tracing past climate changes in regions such as in the Southern Ocean, where few biogenic carbonates are preserved.
The mean daily BSi rate strongly depends on the region:
Coastal upwelling: 46 mmol m-2 d-1
arcticPacific] : 18 mmol m-2 d-1
* Southern Ocean: 3–38 mmol m-2 d-1
gyres: 0.2–1.6 mmol m-2 d-1
Likewise, the integrated annual BSi production strongly depends on the region:
* Coastal upwelling: 3 . 1012 mol yr-1
* Subarctic Pacific: 8 . 1012 mol yr-1
* Southern Ocean: 17–37 . 1012 mol yr-1
* mid-ocean gyres: 26 . 1012 mol yr-1
BSi production is controlled by:
Dissolved silicaavailability, however, half saturation constant "Kµ" for silicon-limited growth is lower than "Ks" for silicon uptake.
* Light availability: There is no direct light requirement; silicon uptake at 2x depth of
photosynthesis; silicon uptake continues at night but cells must be actively growing.
BSi dissolution is controlled by:
Thermodynamicsof solubility: Temperature (0 to 25 °C - 50x increase).
Sinking rate: Food web structure—grazers, fecal pellets, discarded feeding structures, Aggregation - rapid transport.
Bacterial degradationof organic matrix(Bidle and Azam, 1999).
BSi preservation is measured by:
Sedimentation rates, mainly sediment traps (Honjo);
* Benthic remineralization rates ("recycling"), benthic
* BSi concentration in sediments, chemical leaching in
alkaline solution, site specific, need to differentiate lithogenic vs. biogenic Si, X-ray diffraction.
BSi preservation is controlled by:
* Sedimentation rate;
Porewaterdissolved silica concentration: saturation at 1.100 µmol/L;
* Surface coatings: dissolved Al modifies solubility of deposited biogenic silica particles, dissolved silica can also
precipitatewith Al as clayor Al-Si coatings.
* Brzezinski, M. A. (1985). “The Si:C:N ratio of marine diatoms: Interspecific variability and the effect of some environmental variables.” "Journal of Phycology" 21(3): 347-357.
* De La Rocha, C.L. (2006). "Opal based proxies of paleoenvironmental conditions." "Global Biogeochemical Cycles" 20. doi:10.1029/2005GB002664.
* Dugdale, R. C. and F. P. Wilkerson (1998). “Silicate regulation of new production in the equatorial Pacific upwelling.” "Nature" 391(6664): 270.
* Dugdale, R. C., F. P. Wilkerson, et al. (1995). “The role of the silicate pump in driving new production.” "Deep-Sea Research" I 42(5): 697-719.
* Leng, M.J. and Barker, P.A. (2006). "A review of the oxygen isotope composition of lacustrine diatom silica for palaeoclimate reconstruction." "Earth Science Reviews" 75:5-27.
* Ragueneau, O., P. Treguer, et al. (2000). “A review of the Si cycle in the modern ocean: recent progress and missing gaps in the application of biogenic opal as a paleoproductivity proxy.” "Global and Planetary Change" 26: 317-365.
* Takeda, S. (1998). “Influence of iron availability on nutrient consumption ratio of diatoms in oceanic waters.” "Nature" 393: 774-777.
* Werner, D. (1977). "The Biology of Diatoms." Berkeley and Los Angeles, University of California Press.
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