Acyclic isoprenoid biomarkers and evolution of biosynthetic pathways in green microalgae of the genus Botryococcus |
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| Institution: | 1. NIOZ Royal Netherlands Institute for Sea Research, NL-1790 AB Den Burg, The Netherlands;2. CEFREM-UMR CNRS 5110, University of Perpignan, 52 avenue Paul Alduy, F-66860 Perpignan, France;3. Institut de Radioprotection et de Sûreté Nucléaire (IRSN), PRP-Env/SESURE/LERCM, BP3, Saint Paul Lez Durance F-13115, France;4. Ifremer, Géosciences Marines, BP 70, 29280 Plouzané, France;1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China;2. CAS Center for Excellence in Quaternary Science and Global Change, Xi''an 710061, China;3. Key Laboratory of Watershed Geographic Sciences, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, China;1. State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan), Hubei Province, Wuhan 430074, China;2. Institute of Oceanology, Chinese Academy of Sciences, Shandong Province, Qingdao 266071, China;3. Heisenberg Group on Marine Kerogen, Center for Marine Environmental Sciences (MARUM), Bremen University, D-28359 Bremen, Germany;4. Bureau of Economic Geology, The University of Texas at Austin, TX 78712, USA;1. State Key Laboratory for Mineral Deposits Research (Nanjing University), College of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China;2. University of Washington, School of Oceanography, Box 355351, Seattle, WA 98195, USA;1. Institut für Geologie und Paläontologie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, 48149 Münster, Germany;2. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massassuchetts Avenue, Cambridge, MA 02139, USA;3. Max-Planck Institute for Biogeochemistry, Jena, Germany;4. MARUM, University of Bremen, Building IW-3, Am Biologischen Garten 2, 28359 Bremen, Germany;1. Key Laboratory of Western China’s Environmental Systems (Ministry of Education), Lanzhou University, 222 South Tianshui Rd., Lanzhou, 730000 Gansu Province, China;2. Department of Geological Sciences, Brown University, 324 Brook Street, Box 1846, Providence, RI 02912, USA |
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| Abstract: | Acyclic isoprenoids were among the earliest lipids on Earth and today > 30,000 isoprenoid-derived compounds are known, testifying to the remarkable diversification in isoprenoid biosynthetic pathways over time. Many of the isoprenoids have proven to be useful biomarkers in geochemical studies and evidence from sedimentary studies has the potential to provide a timeline for the evolution of different types of isoprenoid biosynthesis. A single green microalgal species termed Botryococcus braunii has been recognised as a major contributor of organic matter to sediments as old as the Precambrian. Modern studies of the taxonomy of B. braunii using molecular biology techniques have shown that the major clades identified from 18S rRNA gene sequences correspond to races A, B, L and S, which are defined on the basis of their hydrocarbon composition. The biosynthetic pathways by which C30–C37 botryococcenes are produced by the B race have now been shown to be due to a duplication of the squalene synthase gene, followed by subsequent changes to the genes, such that one pathway leads to the production of the C30 botryococcene and the other to squalene. Both products are then methylated to produce botryococcenes and methylated squalenes having higher carbon numbers. The mode of biosynthesis of lycopadiene in race L is unclear, but may involve coupling of two C20 phytyl diphosphates. From an examination of the geological record of botryococcenes and lycopadiene it seems likely that these pathways probably evolved early in the Eocene (ca. 55 Ma) and thus are more recently evolved than the genes for highly branched isoprenoid (HBI) alkenes first produced by diatoms about 92 Ma ago. Botryococcane produced from botryococcenes and a monoaromatic hydrocarbon produced from lycopadiene-related lipids, presumably under anoxic conditions, show promise as age diagnostic biomarkers. In view of these results, it seems likely that the Botryococcus species recorded in sediments predating the Eocene lacked the ability to produce botryococcenes or lycopadiene, but nonetheless still contained polymeric non-isoprenoid alkyl chains in the form of an algaenan which on preservation gave rise to a multitude of organic rich rocks. |
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| Keywords: | Biomarkers Botryococcenes Lycopadiene Eocene Biosynthetic pathways Age-diagnostic Acyclic isoprenoids |
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