Biogenic silica

Biogenic silica (BSi) is the equivalent to the terms opal, biogenic opal, and amorphous opaline silica. BSi is essential to many plants and animals. Chemically, BSi is hydrated silica (SiO2•nH2O).

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. Silicon is 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 collagen which helps keep our skin elastic, and it helps calcium in 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 change and 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.

BSi production
The mean daily BSi rate strongly depends on the region:
 * Coastal upwelling: 46 mmol m-2 d-1
 * Sub-arctic Pacific]]: 18 mmol m-2 d-1
 * Southern Ocean: 3–38 mmol m-2 d-1
 * mid-ocean 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 silica availability, 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.
 * Micronutrient availability.

BSi dissolution
BSi dissolution is controlled by:


 * Thermodynamics of solubility: Temperature (0 to 25 °C - 50x increase).
 * Sinking rate: Food web structure&mdash;grazers, fecal pellets, discarded feeding structures, Aggregation - rapid transport.
 * Bacterial degradation of organic matrix (Bidle and Azam, 1999).

BSi preservation
BSi preservation is measured by:
 * Sedimentation rates, mainly sediment traps (Honjo);
 * Benthic remineralization rates ("recycling"), benthic flux chamber (Berelson);
 * 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;
 * Porewater dissolved silica concentration: saturation at 1.100 µmol/L;
 * Surface coatings: dissolved Al modifies solubility of deposited biogenic silica particles, dissolved silica can also precipitate with Al as clay or Al-Si coatings.