PII S0016-7037(00)00464-6 Highly branched isoprenoids (HBIs): Identification of the most common and abundant sedimentary isomers SIMON T. BELT, 1, * W. GUY ALLARD, 1 GUILLAUME MASS´ E, 1,2 JEAN-MICHEL ROBERT, 2 and STEVEN J. ROWLAND 1, * 1 Petroleum and Environmental Geochemistry Group, Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, Devon, UK 2 ISOMer, Faculte ´ des Sciences et des Techniques, Universite ´ de Nantes, 2 Rue de la Houssinie `re, 44027 Nantes Cedex 3, France (Received March 6, 2000; accepted in revised form May 12, 2000) Abstract—Tri- and tetraunsaturated highly branched isoprenoid (HBI) alkenes are widespread sedimentary geochemicals but few have been isolated from sediments in sufficient quantities for rigorous identification. However, two C 25 trienes, four C 25 tetraenes and two C 25 pentaenes have now been isolated from the diatom Pleurosigma intermedium following bulk scale culture, and these have been purified by column chromatog- raphy and fully characterised by NMR spectroscopy and mass spectrometry. The compounds have been used to identify the previously unknown, but common and abundant HBIs found previously in many studies of sediments, particles and biota from around the world. These HBIs are structurally different to those reported from other diatoms. For example, unlike HBIs from the diatoms Haslea ostrearia and Rhizosolenia setigera, the alkenes in P. intermedium are unsaturated at the major branch point of the carbon skeleton and E/Z isomerism is observed for one of the trisubstituted double bonds. There is no evidence for the presence of configurational diastereoisomerism. The distributions of HBIs in P. intermedium (including the E/Z ratios) also show a dependence on the growth conditions within the five cultures studied. The positions of the double bonds in the HBIs of P. intermedium, and by inference, of the sediments, are consistent with the positions of sulphur incorporation in some of the HBI thiolanes and thiophenes which have been reported previously in some sediments and oils. Copyright © 2000 Elsevier Science Ltd 1. INTRODUCTION Since the first reports of C 25 highly branched isoprenoid (HBI) hydrocarbons in sediments (e.g., Gearing et al., 1976; Farrington et al., 1977) and the determination of the parent carbon skeleton by synthesis (I, see Appendix 1; Robson and Rowland, 1986) many studies have described the occurrence of a large number of different HBI isomers with 0 – 6 degrees of unsaturation, in a wide range of geochemical settings (reviewed by Rowland and Robson, 1990). When Volkman et al. (1994) reported the presence of several C 25 HBI alkenes in a culture of the benthic diatom Haslea ostrearia, it seemed likely that a source of the sedimentary compounds had been found, though the authors noted that the distributions in the alga were differ- ent from those in many sediments and seawater. Determinations of the structures, including double bond po- sitions and stereochemistry, and in some cases, either the relative or absolute stereochemistries of the asymmetric centres (Johns et al., 2000) of a number of dienes through hexaenes (III-X) from large scale cultures of H. ostrearia and recently from the diatom Rhizosolenia setigera, have succeeded the initial findings (Belt et al., 1996; Volkman et al., 1998; Wraige et al., 1997; Wraige et al., 1999; Johns et al., 1999; Sinninghe Damste ´ et al., 1999a;b). Pseudo-homologous C 30 HBIs have also been found in cultures of R. setigera (Volkman et al., 1994), though to date, the structures of these compounds have not been elucidated. From detailed studies on the structures of C 25 HBIs, it became apparent that there were indeed often discrepancies between the chromatographic (RI) and mass spectral properties of many of the sedimentary isomers and those found in H. ostrearia and R. setigera (c.f., Volkman et al., 1994). This led us to consider whether the biological HBI distributions were affected by algal growth conditions such as salinity or temperature (Wraige et al., 1997; Wraige et al., 1999) and if HBIs produced by diatoms such as H. ostrearia might undergo rapid diagenetic changes in sediments to yield isomeric forms of the biogenic compounds (Belt et al., 2000a). Although both phenotypic and diagenetic variables did alter HBI isomer distributions, there was often still a poor corre- spondence of the resulting distributions with sedimentary HBIs. For example, whereas HBI dienes and trienes isolated from H. ostrearia underwent facile isomerisation and cyclisation reac- tions under mild acid conditions including Montmorillonite clay, most of the products were not those widely reported in sediments to date (Belt et al., 2000a). This led us to consider the possibility that the common and abundant sedimentary HBI isomers may originate from diatom sources which had not previously been studied. In the present study, we report the isolation and characteri- sation by NMR spectroscopy, of six new HBI alkenes from several large scale cultures of the common benthic diatom, Pleurosigma intermedium. Possibly P. intermedium is a further source of these widespread and often abundant biomarkers, although many diatom species remain to be studied, including both benthic and planktonic members of the genus. Impor- tantly, in contrast to the structures of HBIs characterised pre- viously from diatoms, the chromatographic and mass spectral properties of these compounds show an excellent correlation with those HBIs most commonly reported in sediments (Row- land and Robson, 1990). Thus the common sedimentary iso- mers have at last been identified. *Author to whom correspondence should be addressed (s.belt@ plymouth.ac.uk). Pergamon Geochimica et Cosmochimica Acta, Vol. 64, No. 22, pp. 3839 –3851, 2000 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/00 $20.00 + .00 3839