Identification of a novel di-unsaturated C25 highly branched isoprenoid in the marine tube-dwelling diatom Berkeleya rutilans

Highly branched isoprenoids (HBIs) are known to be biosynthesised by diatoms and are a common component of many marine and freshwater environments. However, the ability to produce these unusual hydrocarbons appears to be restricted to a few diatom species that are represented by just four genera (Haslea, Pleurosigma, Rhizosolenia and Navicula). Despite this, we routinely observe some HBIs in the natural environment that are absent from cultures of known HBI-producing diatoms, indicating the possibility of further sources. Having identified one commonly observed, yet unknown HBI isomer in estuarine sediments, we isolated and cultured diatoms in the laboratory to identify the source of this novel di-unsaturated C₂₅ HBI. Here, we show that analysis of purified extracts obtained from a laboratory culture of the tube-dwelling diatom Berkeleya rutilans enabled determination of the structure of this new compound by combined NMR spectroscopic and mass spectrometric analysis. This represents the first identification of an HBI alkene within the Berkeleya genus and adds to the growing number of reports of genera that produce these unusual hydrocarbons. The newly characterised HBI diene appears to be common in marine sediments and has also been reported in a range of marine biota, thus making it a potential tracer of source organic matter, as has been found for other HBI alkenes.