Introduction

C₂₅ and C₃₀ highly branched isoprenoid (HBI) alkenes are unusual secondary metabolites that are derived from diatoms and commonly used as biological markers in sediments and other geochemical environments. Volkman and co-workers were the first to determine biological sources of these isoprenoids, namely the marine diatoms Haslea ostrearia (C₂₅) and Rhizosolenia setigera (C₃₀). Since this initial report, we have reported on a further species of diatom capable of biosynthesising the C₂₅ HBIs (viz. Pleurosigma intermedium) and elucidated the structures of numerous C₂₅ and C₃₀ HBIs (including the most widespread and abundant isomers) following isolation from large scale diatom cultures and analysis by NMR spectroscopy. However, although an account of the C₂₅ HBIs produced by Haslea species and P. intermedium would appear to be well defined, the situation with R. setigera is less clear.

Here, we describe an investigation into the distribution of C₂₅ and C₃₀ HBI alkenes biosynthesised by R. setigera as a function of the position of the cells through their life cycle. Our observations reveal a relationship between cell size and HBI content including a dramatic change in the distribution of alkenes during the regeneration of their original size through a sexual cycle (auxosporulation).

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