Editing Diatom (section)

== Life cycle ==
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 | header            = Sexual reproduction
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 | image1    = Centric diatom life-cycle.jpg
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 | caption1  = Centric diatom ([[oogamy]])
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 | image2    = Diatom pennate life cycle.jpg
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 | caption2  = Pennate diatom (morphological [[isogamy]], physiological [[anisogamy]])
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===Reproduction and cell size===
Reproduction among these organisms is asexual by [[binary fission]], during which the diatom divides into two parts, producing two "new" diatoms with identical genes. Each new organism receives one of the two [[frustule]]s – one larger, the other smaller – possessed by the parent, which is now called the [[Frustule#Thecae|epitheca]]; and is used to construct a second, smaller frustule, the [[Frustule#Thecae|hypotheca]]. The diatom that received the larger frustule becomes the same size as its parent, but the diatom that received the smaller frustule remains smaller than its parent. This causes the average cell size of this diatom population to decrease.<ref name="HasleSyvertsen1996" /> It has been observed, however, that certain taxa have the ability to divide without causing a reduction in cell size.<ref name="Werner1977">{{cite book|author=G. Drebes|editor=Dietrich Werner|title=The Biology of Diatoms|chapter-url=https://books.google.com/books?id=A4GBA7rlAroC|access-date=14 November 2013|volume=13|series=Botanical Monographs|date=1 January 1977|publisher=University of California Press|isbn=978-0-520-03400-6|pages=250–283|chapter=Chapter 9: Sexuality|archive-date=1 August 2020|archive-url=https://web.archive.org/web/20200801020328/https://books.google.com/books?id=A4GBA7rlAroC|url-status=live}}</ref> Nonetheless, in order to restore the cell size of a diatom population for those that do endure size reduction, sexual reproduction and [[auxospore]] formation must occur.<ref name="HasleSyvertsen1996" />

===Cell division===
Vegetative cells of diatoms are [[diploid]] (2N) and so [[meiosis]] can take place, producing male and female gametes which then fuse to form the [[zygote]]. The zygote sheds its silica theca and grows into a large sphere covered by an organic membrane, the auxospore. A new diatom cell of maximum size, the initial cell, forms within the auxospore thus beginning a new generation. Resting spores may also be formed as a response to unfavourable environmental conditions with germination occurring when conditions improve.<ref name="Horner2002" />

A defining characteristic of all diatoms is their restrictive and bipartite silica cell wall that causes them to progressively shrink during asexual cell division. At a critically small cell size and under certain conditions, [[auxosporulation]] restitutes cell size and prevents [[:Wiktionary:clonal|clonal]] death.<ref>Lewis, W. M., Jr. (1984). "The diatom sex clock and its evolutionary significance". ''The American Naturalist'', '''123'''(1): 73–80</ref><ref>Chepurnov VA, Mann DG, Sabbe K, Vyverman W. (2004). "Experimental studies on sexual reproduction in diatoms". In Jeon KW, ed. ''A Survey of Cell Biology''. International Review of Cytology '''237''': 91–154. London.</ref><ref>Drebes (1977) "Sexuality". In: Werner D, ed. ''The Biology of Diatoms'', Botanical Monographs '''13''' 250–283. Oxford: Blackwell Scientific Publications.</ref><ref>{{cite journal | title=Size reduction, reproductive strategy and the life cycle of a centric diatom | journal=Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences | publisher=The Royal Society | volume=336 | issue=1277 | date=1992-05-29 | issn=0962-8436 | doi=10.1098/rstb.1992.0056 | pages=191–213 | s2cid=86332060 | ref={{sfnref | The Royal Society}}}}</ref><ref>{{cite journal | last1=Koester | first1=Julie A. | last2=Brawley | first2=Susan H. | last3=Karp-Boss | first3=Lee | last4=Mann | first4=David G. | title=Sexual reproduction in the marine centric diatom Ditylum brightwellii (Bacillariophyta) | journal=European Journal of Phycology | publisher=Informa UK Limited | volume=42 | issue=4 | year=2007 | issn=0967-0262 | doi=10.1080/09670260701562100 | pages=351–366| s2cid=80737380 | doi-access=free | bibcode=2007EJPhy..42..351K }}</ref> The entire lifecycles of only a few diatoms have been described and rarely have sexual events been captured in the environment.<ref name=Moore2017 />

===Sexual reproduction===

Most [[eukaryote]]s are capable of [[sexual reproduction]] involving [[meiosis]].  Sexual reproduction appears to be an obligatory phase in the life cycle of diatoms, particularly as cell size decreases with successive vegetative divisions.<ref name="Mouget2009">Mouget JL, Gastineau R, Davidovich O, Gaudin P, Davidovich NA. Light is a key factor in triggering sexual reproduction in the pennate diatom Haslea ostrearia. FEMS Microbiol Ecol. 2009 Aug;69(2):194-201. doi: 10.1111/j.1574-6941.2009.00700.x. Epub 2009 May 6. PMID 19486155</ref>  Sexual reproduction involves production of [[gamete]]s and the fusion of gametes to form a [[zygote]] in which maximal cell size is restored.<ref name = Mouget2009/>  The signaling that triggers the sexual phase is favored when cells accumulate together, so that the distance between them is reduced and the contacts and/or the perception of chemical cues is facilitated.<ref>Scalco E, Stec K, Iudicone D, Ferrante MI, Montresor M. The dynamics of sexual phase in the marine diatom Pseudo-nitzschia multistriata (Bacillariophyceae). J Phycol. 2014 Oct;50(5):817-28. doi: 10.1111/jpy.12225. Epub 2014 Sep 14. PMID 26988637</ref>

An exploration of the [[genome]]s of five diatoms and one diatom [[transcriptome]] led to the identification of 42 [[gene]]s potentially involved in meiosis.<ref name="Patil2019">Patil S, Moeys S, von Dassow P, Huysman MJ, Mapleson D, De Veylder L, Sanges R, Vyverman W, Montresor M, Ferrante MI. Identification of the meiotic toolkit in diatoms and exploration of meiosis-specific SPO11 and RAD51 homologs in the sexual species Pseudo-nitzschia multistriata and Seminavis robusta. BMC Genomics. 2015 Nov 14;16:930. doi: 10.1186/s12864-015-1983-5. Erratum in: BMC Genomics. 2019 Jul 5;20(1):544. PMID 26572248; PMCID: PMC4647503</ref>  Thus a meiotic toolkit appears to be conserved in these six diatom species,<ref name = Patil2019/> indicating a central role of meiosis in diatoms as in other eukaryotes.

===Sperm motility===
Diatoms are mostly [[Motility|non-motile]]; however, sperm found in some species can be [[Flagellum|flagellated]], though motility is usually limited to a gliding motion.<ref name="Horner2002" /> In centric diatoms, the small male [[gametes]] have one [[flagellum]] while the female gametes are large and non-motile ([[Oogamy|oogamous]]). Conversely, in pennate diatoms both gametes lack flagella ([[isogamy|isogamous]]).<ref name="HasleSyvertsen1996" /> Certain araphid species, that is pennate diatoms without a [[raphe]] (seam), have been documented as anisogamous and are, therefore, considered to represent a transitional stage between centric and raphid pennate diatoms, diatoms with a raphe.<ref name="Werner1977" />

=== Degradation by microbes ===
Certain species of bacteria in oceans and lakes can accelerate the rate of dissolution of silica in dead and living diatoms by using [[hydrolytic]] enzymes to break down the organic algal material.<ref>{{cite journal |doi=10.1038/17351 |id={{INIST|1755031}} |year=1999 |last1=Azam |first1=Farooq |title=Accelerated dissolution of diatom silica by marine bacterial assemblages |journal=Nature |volume=397 |issue=6719 |pages=508–12 |last2=Bidle |first2=Kay D. |bibcode=1999Natur.397..508B |s2cid=4397909 }}</ref><ref>{{cite journal |doi=10.1371/journal.pone.0059977 |pmid=23560063 |pmc=3613400 |title=The Structure of Microbial Community and Degradation of Diatoms in the Deep Near-Bottom Layer of Lake Baikal |journal=PLOS ONE |volume=8 |issue=4 |pages=e59977 |year=2013 |last1=Zakharova |first1=Yulia R. |last2=Galachyants |first2=Yuri P. |last3=Kurilkina |first3=Maria I. |last4=Likhoshvay |first4=Alexander V. |last5=Petrova |first5=Darya P. |last6=Shishlyannikov |first6=Sergey M. |last7=Ravin |first7=Nikolai V. |last8=Mardanov |first8=Andrey V. |last9=Beletsky |first9=Alexey V. |last10=Likhoshway |first10=Yelena V. |bibcode=2013PLoSO...859977Z |doi-access=free }}</ref>