| Revision as of 21:14, 25 February 2017 editSprstrfghtr (talk | contribs)3 editsm →Chemical argument for Precambrian sponges← Previous edit | Latest revision as of 16:27, 19 March 2017 edit undo786b6364 (talk | contribs)Extended confirmed users2,143 edits This page is a duplicate of 24-Isopropylcholestane. Redirecting there. | ||
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| '''24-isopropyl cholestane''' is an organic molecule produced by specific ] and marine ].<ref name=":4">{{Cite journal|last=Hofheinz|first=Werner|last2=Oesterhelt|first2=Gottfried|date=1979-06-08|title=24-Isopropylcholesterol and 22-Dehydro-24-isopropylcholesterol, Novel Sterols from a Sponge|url=http://onlinelibrary.wiley.com/doi/10.1002/hlca.19790620443/abstract|journal=Helvetica Chimica Acta|language=en|volume=62|issue=4|pages=1307–1309|doi=10.1002/hlca.19790620443|issn=1522-2675}}</ref> The identification of this molecule at high abundances in ] rocks has been interpreted to reflect the presence of ] prior to the rapid diversification and radiation of life during the ].<ref name=":1">{{Cite journal|last=Love|first=Gordon D.|last2=Grosjean|first2=Emmanuelle|last3=Stalvies|first3=Charlotte|last4=Fike|first4=David A.|last5=Grotzinger|first5=John P.|last6=Bradley|first6=Alexander S.|last7=Kelly|first7=Amy E.|last8=Bhatia|first8=Maya|last9=Meredith|first9=William|title=Fossil steroids record the appearance of Demospongiae during the Cryogenian period|url=http://www.nature.com/doifinder/10.1038/nature07673|journal=Nature|volume=457|issue=7230|pages=718–721|doi=10.1038/nature07673}}</ref><ref name=":2">{{Cite journal|last=Gold|first=David A.|last2=Grabenstatter|first2=Jonathan|last3=Mendoza|first3=Alex de|last4=Riesgo|first4=Ana|last5=Ruiz-Trillo|first5=Iñaki|last6=Summons|first6=Roger E.|date=2016-03-08|title=Sterol and genomic analyses validate the sponge biomarker hypothesis|url=http://www.pnas.org/content/113/10/2684|journal=Proceedings of the National Academy of Sciences|language=en|volume=113|issue=10|pages=2684–2689|doi=10.1073/pnas.1512614113|issn=0027-8424|pmc=4790988|pmid=26903629}}</ref> In this transitional period at the start of the ], single-celled organisms ] to produce many of the evolutionary ] present on Earth today.<ref>{{Cite journal|last=Maloof|first=A. C.|last2=Porter|first2=S. M.|last3=Moore|first3=J. L.|last4=Dudas|first4=F. O.|last5=Bowring|first5=S. A.|last6=Higgins|first6=J. A.|last7=Fike|first7=D. A.|last8=Eddy|first8=M. P.|title=The earliest Cambrian record of animals and ocean geochemical change|url=http://mr.crossref.org/iPage?doi=10.1130%2FB30346.1|journal=Geological Society of America Bulletin|volume=122|issue=11-12|pages=1731–1774|doi=10.1130/b30346.1}}</ref> Interpreting 24-isopropyl cholestane in ancient rocks as indicating the presence of ] before this rapid diversification event alters the traditional understanding of the evolution of ] and the coupling of biology to changes in ]. However, there are several arguments against causally linking 24-isopropyl cholestane to ] based on considerations of ] and the ].<ref name=":0">{{Cite journal|last=Antcliffe|first=Jonathan B.|date=2013-09-01|title=Questioning the evidence of organic compounds called sponge biomarkers|url=http://onlinelibrary.wiley.com/doi/10.1111/pala.12030/abstract|journal=Palaeontology|language=en|volume=56|issue=5|pages=917–925|doi=10.1111/pala.12030|issn=1475-4983}}</ref> Interpreting the presence of 24-isopropyl cholestane in the context of changing ] at the ]-] transition remains an area of active research. | |||
| ==24-isopropyl cholestane== | |||
| ===Chemical argument for Precambrian sponges=== | |||
| ] | |||
| 24-isopropyl cholestane (figure 1, left) is a C<sub>30</sub> ] with ] C<sub>30</sub>H<sub>54</sub> and ] 414.76 g/mol. The ] has a ] skeleton with an ] moiety at C<sub>24</sub> and is the geologically stable form of 24-isopropyl cholesterol.<ref name=":1" /> A related and important molecule is 24-n-propyl cholestane (figure 1, right), also with the cholestane skeleton, but with an ] moiety at C<sub>24</sub>. | |||
| 24-isopropyl cholestane is produced copiously by a particular group of ] in the class ] within the ] ].<ref name=":4" /><ref>{{Cite journal|last=Bergquist|first=Patricia R.|last2=Hofheinz|first2=W.|last3=Hofheinz|first3=W.|last4=Oesterhelt|first4=G.|date=1980-11-18|title=Sterol composition and the classification of the demospongiae|url=http://www.sciencedirect.com/science/article/pii/0305197880900459|journal=Biochemical Systematics and Ecology|volume=8|issue=4|pages=423–435|doi=10.1016/0305-1978(80)90045-9}}</ref> Like other ], the presence of 24-isopropyl cholestane in rocks may indicate whether demosponge were living in or near the rock’s ]. High abundances of 24-isopropyl cholestane are identified in the ] from the ] in ], suggesting the presence of sponges prior to the ].<ref name=":1" /> However, sponges are not the only organisms that produce 24-isopropyl cholestane, so the identification of this biomarker is not uniquely linked to the presence of demosponge. | |||
| While marine ] predominantly produce ],<ref>{{Cite web|url=http://search.proquest.com/openview/d12fe18efbcfa7bc8d20c95508904585/1.pdf?pq-origsite=gscholar&cbl=1256|title=Sedimentary 24-n-Propylcholestanes, Molecular Fossils Diagnostic of Marine Algae - ProQuest|website=search.proquest.com|access-date=2016-11-24}}</ref> they also produce 24-isopropyl cholestane. The two possible sources of 24-isopropyl cholestane to rocks, the demosponge and the algae, can be decoupled by considering the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane. In many rocks, this ratio is 0.2-0.3.<ref name=":1" /> However, in rocks from ], this ratio of steranes is 0.52-16.1, with an average value of 1.51, which strongly suggests input of sponge organic matter.<ref name=":1" /> Notably, these elevated values disappear during the ], and the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane is used an age-specific proxy for the ]-] transition.<ref>{{Cite book|url=https://books.google.com/books?hl=en&lr=&id=RjcgAwAAQBAJ&oi=fnd&pg=PR11&dq=the+biomarker+guide+volume+2&ots=060tpWLUSa&sig=4WqbVMgYirh2Oq1Y-jVrZ0OLyDo#v=onepage&q=the%20biomarker%20guide%20volume%202&f=false|title=The Biomarker Guide: Volume 2, Biomarkers and Isotopes in Petroleum Systems and Earth History|last=Peters|first=K. E.|last2=Walters|first2=C. C.|last3=Moldowan|first3=J. M.|date=2007-08-16|publisher=Cambridge University Press|isbn=9781107079625|language=en}}</ref> | |||
| Recent research in molecular clocks has argued that the ability to produce 24-isopropyl cholesterol evolved independently in both the demosponge and algae.<ref name=":2" /> However, it appears that the ] evolved earlier in the sponges, during the ], and that the ability to perform the ] was not present in algae until the ]. If correct, these results would give scientists much more confidence in interpreting elevated levels of 24-isopropyl cholestane in ancient rocks as reflecting the presence of sponges. | |||
| ===Observations of Precambrian sponges=== | |||
| Additional evidence for sponge evolution before the ] is found in ]ic ] from ].<ref name=":3">{{Cite journal|last=Maloof|first=Adam C.|last2=Rose|first2=Catherine V.|last3=Beach|first3=Robert|last4=Samuels|first4=Bradley M.|last5=Calmet|first5=Claire C.|last6=Erwin|first6=Douglas H.|last7=Poirier|first7=Gerald R.|last8=Yao|first8=Nan|last9=Simons|first9=Frederik J.|title=Possible animal-body fossils in pre-Marinoan limestones from South Australia|url=http://www.nature.com/doifinder/10.1038/ngeo934|journal=Nature Geoscience|volume=3|issue=9|pages=653–659|doi=10.1038/ngeo934}}</ref> Through repeated grinding and ], researchers constructed 3D models of ] structures with ~1 mm-diameter interconnected ]s contained within this rock. The complex network of tunnels appears inconsistent with ] or ], and the researchers tentatively suggested that they are primitive sponges. This interpretation is controversial because the structures pre-date the first appearance of ] and the structures are only known to occur within a single ]. | |||
| ==Implications== | |||
| While Love ''et al.'' (2009) argues for the presence of sponges in rocks below the Marinoan cap carbonate at ~635 Ma (millions of years ago),<ref name=":1" /> Antcliffe (2013) estimates the age of the biomarker-bearing rock to be between 645 Ma and ~580 Ma.<ref name=":0" /> Most recently, Gold ''et al.'' (2016) writes that the age of rocks containing 24-isoproylcholestane have an age between ~650 Ma and 540 Ma.<ref name=":2" /> In all cases, estimates agree that the age of the rocks containing 24-isoproylcholestane pre-date the ] at ~541 Ma. | |||
| The presence of ]s before ~540 Ma has profound implications for the evolution of multicellular life and the coupling of the ] to ] climate. Climate change before the ] and the subsequent diversification of life are intricately intertwined with understanding the causes of ] episodes,<ref>{{Cite journal|last=Hoffman|first=Paul F.|last2=Kaufman|first2=Alan J.|last3=Halverson|first3=Galen P.|last4=Schrag|first4=Daniel P.|date=1998-08-28|title=A Neoproterozoic Snowball Earth|url=http://science.sciencemag.org/content/281/5381/1342|journal=Science|language=en|volume=281|issue=5381|pages=1342–1346|doi=10.1126/science.281.5381.1342|issn=0036-8075|pmid=9721097}}</ref> the deposition of ],<ref>{{Cite journal|last=Kappler|first=Andreas|last2=Pasquero|first2=Claudia|last3=Konhauser|first3=Kurt O.|last4=Newman|first4=Dianne K.|title=Deposition of banded iron formations by anoxygenic phototrophic Fe(II)-oxidizing bacteria|url=http://mr.crossref.org/iPage?doi=10.1130%2FG21658.1|journal=Geology|volume=33|issue=11|doi=10.1130/g21658.1}}</ref> and the second step in the ]<ref>{{Cite journal|last=Lyons|first=Timothy W.|last2=Reinhard|first2=Christopher T.|last3=Planavsky|first3=Noah J.|title=The rise of oxygen in Earth’s early ocean and atmosphere|url=http://www.nature.com/doifinder/10.1038/nature13068|journal=Nature|volume=506|issue=7488|pages=307–315|doi=10.1038/nature13068}}</ref> In particular, the presence of sponges raises questions of the minimum dissolved O<sub>2</sub> content of the oceans in the late ] and the transition from a ] ] to the modern oxygenated deep-ocean. However, sponges appear to require very little O<sub>2</sub> to survive, so their presence in the ] may not provide strong constraints on ] O<sub>2</sub> levels.<ref>{{Cite journal|last=Mills|first=Daniel B.|last2=Ward|first2=Lewis M.|last3=Jones|first3=CarriAyne|last4=Sweeten|first4=Brittany|last5=Forth|first5=Michael|last6=Treusch|first6=Alexander H.|last7=Canfield|first7=Donald E.|date=2014-03-18|title=Oxygen requirements of the earliest animals|url=http://www.pnas.org/content/111/11/4168|journal=Proceedings of the National Academy of Sciences|language=en|volume=111|issue=11|pages=4168–4172|doi=10.1073/pnas.1400547111|issn=0027-8424|pmc=3964089|pmid=24550467}}</ref> | |||
| ==Caveats== | |||
| There are several lines of logic against interpreting 24-isopropyl ] as a ] for ]:<ref name=":0" /> | |||
| # Much of the argument for ] sponges is grounded in the observation that ] produce organic matter with a low ratio of 24-isopropyl cholestane to 24-n-propyl cholestane, but that this ratio is high in ancient rocks. However, the observed change in the ] ratio could also be explained if ] changed the relative abundances in which they produce ]s over the past 600 million years. In a similar line of argument, it is possible that another ] organism from which the algae descended produced organic matter with a higher ratio of 24-isopropyl cholestane to 24-n-propyl cholestane. As argued above, recent evidence has suggested that the algae's synthesis pathway only arose during the ],<ref name=":2" /> which tempers this argument. More generally, these concerns address the issue of insufficient specificity in ], which plagues many ] studies. | |||
| # 24-isopropyl cholestane can be formed through ] of other ], so the high ratio of 24-isopropyl cholestane to 24-n-propyl cholestane could simply reflect the ] of organic matter. Some research has addressed this concern by showing a lack of alteration in other organic molecules,<ref name=":1" /> such as ]s, but subsequent analysis has questioned whether the molecules with minimal alteration could have been contamination from modern ].<ref name=":0" /> | |||
| # There may have been a group of ] living symbiotically with the sponges that also produced 24-isopropyl cholestane.<ref>{{Cite journal|last=Siegl|first=Alexander|last2=Kamke|first2=Janine|last3=Hochmuth|first3=Thomas|last4=Piel|first4=Jörn|last5=Richter|first5=Michael|last6=Liang|first6=Chunguang|last7=Dandekar|first7=Thomas|last8=Hentschel|first8=Ute|date=2011-01-01|title=Single-cell genomics reveals the lifestyle of Poribacteria, a candidate phylum symbiotically associated with marine sponges|url=http://www.nature.com/ismej/journal/v5/n1/full/ismej201095a.html|journal=The ISME Journal|language=en|volume=5|issue=1|pages=61–70|doi=10.1038/ismej.2010.95|issn=1751-7362|pmc=3105677|pmid=20613790}}</ref> If these ] produced the biomarker throughout ], its presence would not be strictly indicative of ]. However, as with the marine algae, analysis of the ratio of 24-isopropyl cholestane to 24-n-propyl cholestane may clarify the source of the compounds. | |||
| # It is strange to find sponge ]s before the ] without accompanying sponge ]s (although there is tentative evidence for sponge-like structures in the latest-]<ref name=":3" />). Moreover, once fossils of sponges do appear during the ], the ratio of 24-isopropyl cholestane to 24-n-propylcholestane returns to its background value. This is surprising because we might expect the ratio to stay elevated or even to increase as ] diversified and sponges proliferated. | |||
| == References == | |||
| {{Reflist}} | |||
| ] | |||
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