Crude oil is the blood of the national industry. Biogeology is an important means to predict the presence of underground oil and gas reservoirs. The 4-methyl steranes serve as molecular fossils and are used for studying both eukaryotic evolution and geological history. The 4α-methyl sterenes are assumed to be formed diagenetically from 4α-methyl sterols produced by ancient organisms, such as dinoflagellates. However, this route has not yet been experimentally validated. In contrast, the origin of 4β-sterenes is still under debate given that their precursors, the 4β-methyl sterols, have rarely been detected in any organism. 4β-methyl steranes are attributed entirely to diagenetic generation from 4α-methyl steroids. Thermal breakdown occurs with increasing temperature and pressure during deeper burial of sediments where the abundance of less stable compounds (e.g., 4β-methyl steranes) is converted into their stable isomerization products (4α-methyl steranes). The 4α/4β-methyl sterane ratio can thus serve as an indicator of thermal maturity, which indicates the extent of the conversion of sedimentary organic material into gas, petroleum, and other products. Therefore, the 4α/4β-methyl sterane ratio has been used to interpret geological history and help distinguish actual oil and gas source rocks from merely potential source rocks.
Till now, the biosynthetic pathway for 4-methyl sterols has not been identified in any organisms that could be connected with fossils. Now, a research team led by Prof. LU Yandu from Marine Single-Cell BioEngineering Group (SCBE) of MRU collaborated with scientists from Massachusetts Institute of Technology and CSIRO Oceans and Atmosphere report a previously unknown C4-methyl sterol biosynthetic pathway involving a sterol methyltransferase. They show that both C4α- and C4β-methyl sterols are end products of the sterol biosynthetic pathway in an endosymbiont of reef corals, Breviolum minutum, while this mechanism exists not only in dinoflagellates but also in eukaryotes from alveolates, haptophytes, and aschelminthes.
“These organisms comprise a major proportion of the phytoplankton community that are globally distributed and exert large-scale impacts on ocean biogeochemistry.” team leader ZHOU Wenxu explained.
“The discovery provides a previously untapped route for the generation of C4-methyl steranes and overturns the paradigm that all 4β-methyl steranes are diagenetically generated from the 4α isomers. This may facilitate the interpretation of geological history and the discovery of crude oil and gas” said LU Yandu, group leader of SCBE and the senior author of the study.
Their findings were published on May 12 in Journal of the American Chemical Society https://pubs.acs.org/doi/10.1021/jacs.2c01401.
This work was supported in part by grants from the National Key R&D Program of China, the National Natural Science Foundation of China, the Key R&D Program of Hainan Province, and the Project of the State Key Laboratory of Marine Resource Utilization in South China Sea.
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