C 25 highly branched isoprenoid alkenes from the marine benthic diatom Pleurosigma strigosum Vincent Grossi a, * , Be ´atriz Beker b , Jan A.J. Geenevasen c , Stefan Schouten d , Danielle Raphel a , Marie-France Fontaine e , Jaap S. Sinninghe Damste ´ d a Laboratoire de Microbiologie, Ge ´ochimie et Ecologie Marines (UMR 6117 CNRS), Centre dÕOce ´anologie de Marseille (OSU), Campus de Luminy, case 901, F-13288 Marseille, France b Centre dÕOce ´anologie de Marseille (OSU), Station Marine dÕEndoume, F-13007 Marseille, France c Institute of Molecular Chemistry (IMC), University of Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Netherlands d Department of Marine Biogeochemistry and Toxicology, Royal Netherlands Institute for Sea Research (NIOZ), PO Box 59, 1790 AB Den Burg, The Netherlands e Laboratoire de Diversite ´ Biologique et Fonctionnement des Ecosyste `mes Marins Co ˆ tiers (UMR 6540 CNRS), Centre dÕOce ´anologie de Marseille (OSU), Station Marine dÕEndoume, F-13007 Marseille, France Received 25 June 2004; received in revised form 30 August 2004 Abstract The hydrocarbon composition of the marine diatom Pleurosigma strigosum isolated from coastal Mediterranean sediments is described. A suite of five C 25 highly branched isoprenoid (HBI) alkenes with 2–5 double bonds were detected together with n- C 21:4 and n-C 21:5 alkenes and squalene. The analysis by 1 H and 13 C NMR spectroscopy of two isolated HBI alkenes allowed the structural identification of a novel C 25 HBI triene (2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadeca-5E,13-diene) and the first identification in diatom cells of 2,6,10,14-tetramethyl-7-(3-methylpent-4-enyl)-pentadec-5E-ene, an HBI previously detected in marine sediments and particulate matter. The other minor C 25 HBIs detected were a tetraene and a pentaene that have been pre- viously identified in other diatoms from the genera Haslea and Rhizosolenia, and one other C 25 tetraene that could not be structur- ally identified. The structures of the HBI alkenes of P. strigosum were compared with those of C 25 homologues previously identified in three other Pleurosigma sp. (Pleurosigma intermedium, Pleurosigma planktonicum and Pleurosigma sp.). Unlike most structures previously reported, none of the HBI alkenes produced by P. strigosum showed an unsaturation at C7–C20, or E/Z isomerism of the trisubstituted double bond at C9–C10 (whenever present). Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Pleurosigma strigosum; Diatoms; Bacillariophyceae; Highly branched isoprenoid alkenes; Structural identification; NMR spectroscopy; Mass spectrometry 1. Introduction Highly branched isoprenoid (HBI) hydrocarbons are widespread components in modern marine sediments (Rowland and Robson, 1990; Belt et al., 2000a). During the last decades, the identification of C 25 HBI alkenes in natural populations of diatoms (Nichols et al., 1988; Summons et al., 1993; Johns et al., 1999) and of C 25 and C 30 HBI alkenes in laboratory cultures of isolated diatoms (e.g. Volkman et al., 1994, 1998; Wraige et al., 1997) clearly established that diatoms are the major source of HBI alkenes in sediments. 0031-9422/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.phytochem.2004.09.002 * Corresponding author. Tel.: +33 491 829 651; fax: +33 491 829 641. E-mail address: grossi@com.univ-mrs.fr (V. Grossi). www.elsevier.com/locate/phytochem Phytochemistry 65 (2004) 3049–3055 PHYTOCHEMISTRY