In addition to the production of phytol (from chlorophylls) and sterols, a limited number of diatom species are capable of synthesising unusual C25 and C30 highly branched isoprenoid (HBl) alkenes. At the outset of the current investigation, the structures of most C25 and C30 HBIs had been identified. Some environmental factors had been shown to control their production, although a detailed understanding of these remained unclear. In addition, the biological functions of the chemicals remained unknown, and the reasons for their production by some species and not by others, was not understood. Investigations on the distributions of C25 and C30 HBl alkenes biosynthesised by Rhizosolenia setigera demonstrated a dependence on the physiological status of the cells, as measured by the position of this diatom in its life cycle. Thus, while C30 HBIs were observed at every stage of the life cycle, C25 HBIs were not always present in the cells. Since the synthesis of C25 HBIs appears to be stimulated by the onset of auxosporulation (sexual reproduction), an explanation is provided as to why they have rarely been observed in previous studies. Two novel monocyclic C30 alkenes (previously reported in other strains of Rhizosolenia setigerd), and a novel monocyclic C25 alkene were also observed during life cycle experiments. The two C30 hydrocarbons structures were subsequently characterised and the potential geochemical relevance of these compounds was highlighted by comparison of their mass spectral and chromatographic properties with those of alkenes reported in sediments and suspended water column particles. An investigation of terpenoid (including HBl) biosynthesis in the diatoms Haslea ostrearia, Rhizosolenia setigera and Pleurosigma intermeclium has been performed. Evidence for species and organelle dependent biosynthetic pathways has been observed. Phytol is synlhesised by each species investigated according to the recently discovered methyl-erythritol phosphate (MEP) pathway. This pathway is also involved in the synthesis of C25 HBIs in the two species Haslea ostrearia and Pleio-osigma intermedium. In contrast, C25 and C30 HBIs, and (at least) one monocyclic C30 alkene, appear to be made predominantly via the mevalonate (MVA) route in the diatom R. setigera. Evidence for the contribution of the MVA pathway to the biosynthesis of sterols was found for the diatoms Rhizosolenia setigera, and Pleurosigma intermedium. In contrast, only contributions from the MEP pathway were found for the biosynthesis of sterols in Haslea ostrearia. Preliminary evidence for dynamic interchange between the two pathways has also been observed. Fractionation of Rhizosolenia setigera cells revealed that phytol was present in the chloroplasts, while sterols and HBIs were present in the cytoplasm.

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