Elsevier

Phytochemistry

Volume 56, Issue 8, April 2001, Pages 795-800
Phytochemistry

Tetra-unsaturated sesterterpenoids (Haslenes) from Haslea ostrearia and related species

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Abstract

The structures and distributions of C25 highly branched isoprenoid (HBI) alkenes (Haslenes) have been determined following isolation from cultures of the diatoms Haslea ostrearia, Haslea crucigera, Haslea pseudostrearia and Haslea saltstonica. The distributions of the HBIs change between Haslea species and also between different cultures of the same species. Large scale culturing of H. ostrearia and H. pseudostrearia has enabled the structures of three new tetra-unsaturated alkenes to be determined by NMR spectroscopy. The structural relationships between different Haslenes together with the potential significance of the biosynthesis of HBIs by the Haslea genus is discussed.

Introduction

In a number of recent reports, we have described the structures and distributions of a series of C25 highly branched isoprenoid (HBI) hydrocarbons that are present in the diatom Haslea ostrearia (Gaillon) Simonsen (Belt et al., 1996, Johns et al., 2000, Johns et al., 1999, Wraige et al., 1997, Wraige et al., 1999). These compounds (parent structure I; Fig. 1) are ubiquitous in contemporary marine sediments and are often the major hydrocarbons present (for a review, see Rowland and Robson, 1990). Further, by use of axenic cultures, we have demonstrated conclusively that H. ostrearia is a primary producer of these lipids (Wraige et al., 1999). In culture, HBIs range in their unsaturation (e.g. IIIX) between two and six double bonds, though monoenes (e.g. II) and the parent alkane (I) have also been observed in sediments (Robson and Rowland, 1986). Cultures of H. ostrearia are usually dominated by trienes and tetraenes, often as mixtures of geometric and/or diastereoisomers (Belt et al., 1996). To date, the reasons for varying distributions of HBIs in cultures of H. ostrearia are not fully understood, though it is apparent that temperature and not salinity is a major controlling influence over unsaturation (Wraige et al., 1998, Rowland et al., 2000). In sediments, it is likely that HBIs undergo isomerisation and cyclisation reactions depending on the degree of unsaturation (Belt et al., 2000a). More recently, two new producers of C25 HBI alkenes have been reported (viz. Rhizosolenia setigera (Sinninghe Damsté et al., 1999) and Pleurosigma intermedium (Belt et al., 2000b, Belt et al., 2000c)). The C25 pentaene (IX) found in R. setigera is also produced by H. ostrearia (Wraige et al., 1997) but the HBIs from P. intermedium represent a slightly different structural type (e.g. XI-XII), since these compounds are unsaturated at the major branch point, C-7.

In the present study, we report the presence of HBI alkenes in three previously unexamined Haslea species (Haslea crucigera (Wm. Smith) Simonsen, Haslea pseudostrearia Massé et al. and Haslea saltstonica Massé et al.) together with the characterisation of three new HBI tetraenes.

Section snippets

Identification of HBIs in four species of the Haslea genus

The four Haslea species that have been cultured and examined for HBIs in this study are H. ostrearia, H. crucigera, H. pseudostrearia and H. saltstonica. Each of these species are pennate, benthic diatoms measuring ca. 30–100×6–20 μm. The main structural differences between them have been described elsewhere (Massé et al., 2000). After large scale culture of each Haslea species followed by centrifugation, freeze drying and extraction (hexane), GC and GC–MS analysis of the total hexane extracts

Algal cultures

H. ostrearia and H. crucigera were isolated from the Bay of Bourgneuf (France) while H. pseudostrearia and H. saltstonica were isolated from the Kingsbridge estuary (Devon, UK). H. ostrearia was grown in bulk between April 1991 and June 1998. Large scale cultures (440 l) of each of the other three diatoms were grown in an outdoor facility during September and October 1999 as described previously (Belt et al., 1996).

Isolation and purification of HBIs

Each of the large scale cultures of the four diatoms were centrifuged, freeze

Acknowledgements

We thank the University of Plymouth for research funds and the British Council for an Alliance travel grant.

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