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==Biological role== [[File:20110123 185042 Diatom.jpg|upright|thumb|A diatom, enclosed in a silica cell wall]] Although silicon is readily available in the form of [[silicate]]s, very few organisms use it directly. [[Diatom]]s, [[radiolaria]], and [[siliceous sponge]]s use [[biogenic silica]] as a structural material for their skeletons. Some plants accumulate silica in their tissues and require silicon for their growth, for example [[rice]]. Silicon may be taken up by plants as [[orthosilicic acid]] (also known as monosilicic acid) and transported through the [[xylem]], where it forms amorphous complexes with components of the cell wall. This has been shown to improve cell wall strength and structural integrity in some plants, thereby reducing insect herbivory and pathogenic infections. In certain plants, silicon may also upregulate the production of volatile organic compounds and phytohormones which play a significant role in plant defense mechanisms.<ref name="Kim2002">{{cite journal |doi=10.1094/PHYTO.2002.92.10.1095 |pmid=18944220 |title=Silicon-Induced Cell Wall Fortification of Rice Leaves: A Possible Cellular Mechanism of Enhanced Host Resistance to Blast |journal=Phytopathology |volume=92| issue=10 |pages=1095β103 |year=2002 |last1=Kim |first1=Sang Gyu |last2=Kim |first2=Ki Woo |last3=Park |first3=Eun Woo |last4=Choi |first4=Doil|doi-access= |bibcode=2002PhPat..92.1095K }}</ref><ref name="plants" /><ref name="Leroy2019">{{cite journal |last1=Leroy |first1=Nicolas |last2=de Tombeur |first2=Felix |last3=Walgraffe |first3=Yseult |last4=Cornelis |first4=Jean-Thomas |last5=Verheggen |first5=Francois |title=Silicon and plant natural defenses against insect pests: impact on plant volatile organic compounds and cascade effects on multitrophic interactions |journal=Plants |date=23 October 2019 |volume=8 |issue=444 |page=444 |doi=10.3390/plants8110444|pmid=31652861 |pmc=6918431 |doi-access=free |bibcode=2019Plnts...8..444L }}</ref> In more advanced plants, the silica [[phytolith]]s (opal phytoliths) are rigid microscopic bodies occurring in the cell.<ref>{{cite book |chapter=Silicon |page=856 |isbn=978-0-444-53181-0 |title=Studies in Natural Products Chemistry |volume=35 |first=Atta-ur- |last=Rahman|year=2008 |publisher=Elsevier Science }}</ref><ref name="abd">{{cite journal |last1=Exley |first1=C. |title=Silicon in life:A bioinorganic solution to bioorganic essentiality |journal=Journal of Inorganic Biochemistry |volume=69 |pages=139β144 |date=1998 |doi=10.1016/S0162-0134(97)10010-1 |issue=3}}</ref><ref name="plants">{{cite journal |last1=Epstein |first1=Emanuel |title=SILICON |journal=Annual Review of Plant Physiology and Plant Molecular Biology |volume=50 |date=1999 |pmid=15012222 |doi=10.1146/annurev.arplant.50.1.641 |pages=641β664}}</ref> Several [[horticultural crop]]s are known to protect themselves against [[fungal plant pathogen]]s with silica, to such a degree that [[fungicide]] application may fail unless accompanied by sufficient silicon nutrition. Silicaceous plant defense molecules activate some [[phytoalexin]]s, meaning some of them are signalling substances producing [[acquired immunity]]. When deprived, some plants will substitute with increased production of other defensive substances.<ref name="plants" /> Life on Earth is largely composed of [[carbon]], but [[astrobiology]] considers that [[extraterrestrial life]] may have other [[hypothetical types of biochemistry]]. Silicon is considered an alternative to carbon, as it can create complex and stable molecules with four covalent bonds, required for a [[DNA]]-analog, and it is available in large quantities.<ref>{{cite book |last= Aguilera MochΓ³n|first= Juan Antonio|date= 2016|title= La vida no terrestre|trans-title= The non-terrestrial life|url= |language= Spanish |location= |pages=43β45|publisher= RBA|isbn=978-84-473-8665-9}}</ref> ===Marine microbial influences === Diatoms use silicon in the [[biogenic silica]] (bSi) form,<ref>{{Cite journal|last1=Bidle|first1=Kay D.|last2=Manganelli|first2=Maura|last3=Azam|first3=Farooq|date=2002-12-06|title=Regulation of Oceanic Silicon and Carbon Preservation by Temperature Control on Bacteria|url=https://www.science.org/doi/10.1126/science.1076076|journal=Science|language=en|volume=298|issue=5600|pages=1980β1984|doi=10.1126/science.1076076|issn=0036-8075|pmid=12471255|bibcode=2002Sci...298.1980B|s2cid=216994|url-access=subscription}}</ref> which is taken up by the silicon transport protein (SIT) to be predominantly used in the cell wall structure as frustules.<ref>{{Cite journal|last1=Durkin|first1=Colleen A.|last2=Koester|first2=Julie A.|last3=Bender|first3=Sara J.|last4=Armbrust|first4=E. Virginia|date=2016|title=The evolution of silicon transporters in diatoms|url=|journal=Journal of Phycology|language=en|volume=52|issue=5|pages=716β731|doi=10.1111/jpy.12441|issn=1529-8817|pmc=5129515|pmid=27335204|bibcode=2016JPcgy..52..716D }}</ref> Silicon enters the ocean in a dissolved form such as silicic acid or silicate.<ref name="Dugdale-2001">{{Cite journal|last1=Dugdale|first1=R. C.|last2=Wilkerson|first2=F. P.|date=2001-12-30|title=Sources and fates of silicon in the ocean: the role of diatoms in the climate and glacial cycles|journal=Scientia Marina|volume=65|issue=S2|pages=141β152|doi=10.3989/scimar.2001.65s2141|issn=1886-8134|doi-access=free|bibcode=2001ScMar..65S.141D }}</ref> Since diatoms are one of the main users of these forms of silicon, they contribute greatly to the concentration of silicon throughout the ocean. Silicon forms a nutrient-like profile in the ocean due to the diatom productivity in shallow depths.<ref name="Dugdale-2001" /> Therefore, concentration of silicon is lower in the shallow ocean and higher in the deep ocean. Diatom productivity in the upper ocean contributes to the amount of silicon exported to the lower ocean.<ref>{{Cite journal|last1=Baines|first1=Stephen B.|last2=Twining|first2=Benjamin S.|last3=Brzezinski|first3=Mark A.|last4=Krause|first4=Jeffrey W.|last5=Vogt|first5=Stefan|last6=Assael|first6=Dylan|last7=McDaniel|first7=Hannah|date=December 2012|title=Significant silicon accumulation by marine picocyanobacteria|url=https://www.nature.com/articles/ngeo1641|journal=Nature Geoscience|language=en|volume=5|issue=12|pages=886β891|doi=10.1038/ngeo1641|bibcode=2012NatGe...5..886B|issn=1752-0908|url-access=subscription}}</ref> When diatom cells are [[Lysis|lysed]] in the upper ocean, their nutrients such as iron, zinc, and silicon, are brought to the lower ocean through a process called [[marine snow]]. Marine snow involves the downward transfer of particulate organic matter by vertical mixing of dissolved organic matter.<ref>{{Cite journal|last=Turner|first=Jefferson T.|date=January 2015|title=Zooplankton fecal pellets, marine snow, phytodetritus and the ocean's biological pump|url=http://dx.doi.org/10.1016/j.pocean.2014.08.005|journal=Progress in Oceanography|volume=130|pages=205β248|doi=10.1016/j.pocean.2014.08.005|bibcode=2015PrOce.130..205T|issn=0079-6611|url-access=subscription}}</ref> It has been suggested that silicon is considered crucial to diatom productivity and as long as there is silicic acid available for diatoms to use, the diatoms can contribute to other important nutrient concentrations in the deep ocean as well.<ref>{{Cite journal|last1=Yool|first1=Andrew|last2=Tyrrell|first2=Toby|date=2003|title=Role of diatoms in regulating the ocean's silicon cycle|url=https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2002GB002018|journal=Global Biogeochemical Cycles|language=en|volume=17|issue=4|pages=n/a|doi=10.1029/2002GB002018|bibcode=2003GBioC..17.1103Y|s2cid=16849373|issn=1944-9224|citeseerx=10.1.1.394.3912}}</ref> In coastal zones, diatoms serve as the major phytoplanktonic organisms and greatly contribute to biogenic silica production. In the open ocean, however, diatoms have a reduced role in global annual silica production. Diatoms in North Atlantic and North Pacific subtropical gyres only contribute about 5β7% of global annual marine silica production. The [[Southern Ocean]] produces about one-third of global marine biogenic silica.<ref name="The World Ocean Silica Cycle" /> The Southern Ocean is referred to as having a "biogeochemical divide"<ref>{{cite journal |last1=Marinov |first1=I. |last2=Gnanadesikan |first2=A. |last3=Toggweiler |first3=J. R. |last4=Sarmiento |first4=J. L. |title=The Southern Ocean biogeochemical divide |journal=Nature |date=June 2006 |volume=441 |issue=7096 |pages=964β967 |doi=10.1038/nature04883|pmid=16791191 |bibcode=2006Natur.441..964M |s2cid=4428683 }}</ref> since only minuscule amounts of silicon are transported out of this region. ===Human nutrition=== There is some evidence that silicon is important to human health for their nail, hair, bone, and skin tissues,<ref>{{cite book|first1=Keith R.|last1=Martin|chapter=Silicon: The Health Benefits of a Metalloid |editor=Astrid Sigel|editor2=Helmut Sigel|editor3=Roland K.O. Sigel|title=Interrelations between Essential Metal Ions and Human Diseases|series=Metal Ions in Life Sciences|volume=13|date=2013|publisher=Springer|pages=451β473|doi=10.1007/978-94-007-7500-8_14|pmid=24470100|isbn=978-94-007-7499-5}}</ref> for example, in studies that demonstrate that premenopausal women with higher dietary silicon intake have higher [[bone density]], and that silicon supplementation can increase bone volume and density in patients with [[osteoporosis]].<ref name="jugdaohsingh2007silicon">{{cite journal |last1=Jugdaohsingh |first1=R. |title=Silicon and bone health |journal=The Journal of Nutrition, Health and Aging |date=MarβApr 2007 |volume=11 |issue=2 |pages=99β110 |pmc=2658806 |pmid=17435952}}</ref> Silicon is needed for synthesis of [[elastin]] and [[collagen]], of which the [[aorta]] contains the greatest quantity in the human body,<ref name="LoeperLoeper1978">{{Cite book |last1=Loeper |first1=J. |last2=Fragny |first2=M. |title=Biochemistry of Silicon and Related Problems |chapter=The Physiological Role of the Silicon and its AntiAtheromatous Action |year=1978 |pages=281β296 |doi=10.1007/978-1-4613-4018-8_13 |isbn=978-1-4613-4020-1}}</ref> and has been considered an [[mineral (nutrient)|essential element]];<ref name="Niels">{{cite journal |doi =10.1146/annurev.nu.04.070184.000321 |pages=21β41 |journal=Annual Review of Nutrition |volume=4 |date=1984 |title=Ultratrace Elements in Nutrition |first=Forrest H. |last=Nielsen |pmid=6087860}}</ref> nevertheless, it is difficult to prove its essentiality, because silicon is very common, and hence, deficiency symptoms are difficult to reproduce.<ref name="lipp">{{Cite book |publisher=University Science Books |isbn=978-0-935702-72-9 |page=411 |last=Lippard |first=Stephen J. |author2=Jeremy M. Berg |title =Principles of Bioinorganic Chemistry |year=1994 |location=Mill Valley, CA}}</ref><ref>{{cite journal |author1=Muhammad Ansar Farooq |author2=Karl-Josef Dietz |title=Silicon as Versatile Player in Plant and Human Biology: Overlooked and Poorly Understood Muhammad Ansar Farooq and Karl-J |journal=Front. Plant Sci. |date=2015 |volume=6 |issue=994 |page=994 |doi=10.3389/fpls.2015.00994 |pmid=26617630|pmc=4641902 |doi-access=free }}</ref> Silicon is currently under consideration for elevation to the status of a "plant beneficial substance by the Association of American Plant Food Control Officials (AAPFCO)."<ref>{{cite web |title=AAPFCO Board of Directors 2006 Mid-Year Meeting |archive-url=https://web.archive.org/web/20120106010519/http://www.aapfco.org/MY06BODAgenda.pdf |url=http://www.aapfco.org/MY06BODAgenda.pdf |archive-date=6 January 2012 |publisher=Association of American Plant Food Control Officials |access-date=2011-07-18 |quote=A presentation was made for Excell Minerals to recognize Silicon as a recognized plant nutrient |url-status=dead }}</ref><ref name="presentation">{{cite web |last1=Miranda |first1=Stephen R. |last2=Barker |first2=Bruce |title=Silicon: Summary of Extraction Methods |url=https://docs.google.com/viewer?a=v&q=cache:SzfW40-2DDcJ:www.aapfco.org/AM09/LSC_Si_Methods_DC.ppt+aapfco+siicon&hl=en&gl=us&pid=bl&srcid=ADGEESj4Jo-RFFj54kb6Sun3ikgJW9DMHzRAuUS045YkFErzE5NaSA084KvIyRxJp0IVX5ktDhaPPqcYLRx2hVu6K5YVWj95h2kgvkvDLQLyrxcJXXD3tQ3P5YLJ7J5F8rRYzenxznHp&sig=AHIEtbSPNk7BtSIpiRnvNI1F-2jSLN5LYA |archive-url=https://web.archive.org/web/20121112232117/https://docs.google.com/viewer?a=v&q=cache%3ASzfW40-2DDcJ%3Awww.aapfco.org%2FAM09%2FLSC_Si_Methods_DC.ppt+aapfco+siicon&hl=en&gl=us&pid=bl&srcid=ADGEESj4Jo-RFFj54kb6Sun3ikgJW9DMHzRAuUS045YkFErzE5NaSA084KvIyRxJp0IVX5ktDhaPPqcYLRx2hVu6K5YVWj95h2kgvkvDLQLyrxcJXXD3tQ3P5YLJ7J5F8rRYzenxznHp&sig=AHIEtbSPNk7BtSIpiRnvNI1F-2jSLN5LYA |url-status=dead |archive-date=November 12, 2012 |publisher=Harsco Minerals |date=August 4, 2009 |access-date=2011-07-18 }}</ref>
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