Aerobic biodegradation of hopanes and norhopanes in Venezuelan crude oils F.D. Bost a , R. Frontera-Suau a,1 , T.J. McDonald b , K.E. Peters c,2 , P.J. Morris a,d, * a Department of Microbiology and Immunology, Medical University of South Carolina, 221 Fort Johnson Rd., Charleston, SC 29412, USA b B & B Laboratories, 1902 Pinon Dr., College Station, TX 77845, USA c Mobil Technology Company, PO Box 650232, Dallas, TX 75265, USA d Marine Biomedicine and Environmental Sciences, Medical University of South Carolina, 221 Fort Johnson Rd., Charleston, SC 29412, USA Received 12 January 2000; accepted 25 September 2000 (returned to author for revision 25 March 2000) Abstract The microbial degradation of two Venezuelan crude oils enriched in 25-norhopanes was examined after a 5-week aerobic incubation using a microbial enrichment culture. Analysis of the oils using gas chromatography±mass spec- trometry revealed degradation of the C 28 tricyclic terpane, the C 29 ±C 34 17a(H),21b(H)-hopanes, and the C 29 17a(H),21b(H)-25-norhopane. The C 35 17a(H),21b(H)-hopane and 18a(H)-oleanane were conserved. Further, the C 28 ± C 34 17a(H),21b(H)-25-norhopanes were degraded and no formation of 25-norhopanes was observed. Degradation caused preferential removal of the 22R versus the 22S isomer in both the extended hopanes and 25-norhopanes, implying that bacteria remove these compounds in aerobic environments. These data demonstrate 25-norhopane degradation on a time scale similar to that for other biomarkers. # 2001 Elsevier Science Ltd. All rights reserved. Keywords: Biodegradation; 25-Norhopane; Hopane; Crude oil; Venezuela 1. Introduction Biomarkers are structurally complex components of petroleum derived from biological molecular precursors, such as chlorophyll, sterols, and hopanoids (Peters and Moldowan, 1993). Many biomarkers in crude oil are resistant to biodegradation and are used by petroleum geochemists to assess genetic relationships, thermal maturity and biodegradation. The biomarker pro®le of a crude oil is distinctive and diagnostic, often allowing correlation of an oil to its source rock. Hopanes are a class of pentacyclic triterpane bio- markers that originate from hopanoids in bacterial membranes (Ourisson et al., 1984; Prince, 1987). Numerous studies show that C 30 17a,21b(H)-hopane and its extended homologs (homohopanes) are biode- graded in the environment and laboratory (Goodwin et al., 1983; Peters and Moldowan, 1991; Chosson et al., 1992; Parker and Acey, 1993; Peters and Moldowan, 1993; Moldowan et al., 1995; Morris et al., 1995). Some reservoired crude oils contain 25-norhopanes, pre- sumably formed by demethylation of the A/B ring at the C-10 position (RullkoÈtter and Wendisch, 1982; Volk- man et al., 1983). The 25-norhopanes were ®rst observed as ``degraded hopanes'' in an oil-impregnated sandstone from the Uinta Basin in Utah (Reed, 1977). Since that report, other investigators have observed 25- norhopanes in heavily degraded reservoir oils (e.g. Sei- fert and Moldowan, 1979; Volkman et al., 1983; Seifert et al., 1984; Requejo and Halpern, 1989; Moldowan et al., 1995). In most cases, a relative decrease in the 0146-6380/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0146-6380(00)00147-9 Organic Geochemistry 32 (2001) 105±114 www.elsevier.nl/locate/orggeochem * Corresponding author. Tel.: +1-843-762-5533; fax: +1- 843-762-5535. 1 Present address: Dept. of Environmental Sciences and Engineering, 104 Rosenall Hall, University of North Carolina, Chapel Hill, NC 27599-7400, USA. 2 Present address: Exxon Mobil Upstream Research Co., PO Box 2189, Houston, TX 77252-2189, USA. E-mail address: morrisp@musc.edu (P.J. Morris).