Note C 25 highlybranchedisoprenoidalkenesin planktonicdiatomsofthe Pleurosigma genus SimonT.Belt a, *,GuillaumeMasse´ a,b ,W.GuyAllard a ,Jean-MichelRobert b , StevenJ.Rowland a a Petroleum and Environmental Geochemistry Group, Department of Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, PL4 8AA, Devon, UK b Faculte ´ des Sciences et des Techniques, Universite ´ de Nantes, 2 Rue de la Houssinie `re, 44027 Nantes Cedex 3, France Received30July2001;accepted20August2001 (returnedtoauthorforrevision7August2001) Abstract TheC 25 highly branched alkenes which have been widely reported in coastal and deep sea marine sediments and sedimentingparticleshavetodateonlybeendetectedinbenthicdiatoms.Logically,aplanktonicorganismwouldbea much more likely source for the HBIs found in deep sea environments. Here we report the structures of C 25 HBI alkenes isolated from two planktonic diatoms belonging to the Pleurosigma genus. The HBI structures have been determinedusingNMRandmassspectraltechniques.TheHBIsfrom Pleurosigma sp.wereidenticaltothosefoundin manydeepseamarinesampleswhereasthoseof P. planktonicum consistedoftwoisomerictetraeneswhosestructures arenovelandwhicharereportedhereforthefirsttime. # 2001ElsevierScienceLtd.Allrightsreserved. Keywords: HBIalkenes; Pleurosigma planktonicum;Diatoms;Tetraenes;Plankton 1. Introduction Intworecentreports,wedescribedthestructures(e.g. Fig.1)ofthemostcommonandabundantsedimentary isomers of C 25 highly branched isoprenoid (HBI) alkenes, which are widely distributed in recent coastal marinesediments.Thiswasachievedbycomparisonof thegaschromatographic(GC)andmassspectral(MS) features of the sedimentary compounds which those of C 25 alkenes isolated from the benthic diatom, Pleur- osigma intermedium whichwerecharacterisedbynuclear magnetic resonance (NMR) spectroscopy (Belt et al., 2000a,b). However,sinceHBIalkeneshavebeenwidelyrepor- tedfromwatercolumnparticles(e.g.Prahletal.,1980; Osterrohtetal.,1983;Volkmanetal.,1983;Albaige´set al.,1984;Batesetal.,1984;MatsuedaandHanda,1986; Wakeham, 1990) from as far afield as Dabob Bay, Washington, USA, the Peru Upwelling zone, the Ebro Delta, Spain, and the eastern North Pacific Ocean, it seems unlikely that benthic diatoms such as P. inter- medium and Haslea ostrearia (Volkmanetal.,1994)are amajorsourceofC 25 HBIs in deep sea marine envir- onments. Whilst Rhizosolenia setigera is also a plank- tonic diatom producer of HBI alkenes, this organism produceseitheranuncommonC 25 pentaene(Sinninghe Damste´ etal.,1999)orbothC 25 andC 30 HBIsdepend- ing on the particular strain (Rowland et al., in press). Thus, R. setigera isprobablynotthesolesourceofC 25 HBIsinthedeepsea.Wehavethereforeexaminedtwo planktonicdiatomspeciesofthe Pleurosigma genusfor HBI alkenes. Significantly, one of these (Pleurosigma sp.) produced C 25 tetraenes and pentaenes identical to thosepreviouslyfoundinsuspendedparticlesandsedi- ments, whilst another (Pleurosigma planktonicum) pro- ducednovelandisomericC 25 tetraenes. 0146-6380/01/$-seefrontmatter # 2001ElsevierScienceLtd.Allrightsreserved. PII:S0146-6380(01)00111-5 OrganicGeochemistry32(2001)1271–1275 www.elsevier.com/locate/orggeochem * Correspondingauthor.Tel.:+44-1752-233042;fax:+44- 1752-233035. E-mail address: sbelt@plymouth.ac.uk(S.T.Belt).