An investigation of the sources and seasonal variations of highly branched isoprenoid hydrocarbons in intertidal sediments of the Tamar Estuary, UK

The concentrations and distributions of highly branched isoprenoid (HBI) hydrocarbons in the surficial sediments of an intertidal mudflat in the Tamar Estuary, UK, were examined at monthly intervals throughout 1990. C20 and C25 HBIs were present all the year round and their concentrations covaried, maximising at several times in the year, notably in April–June. Since C25 HBIs are known to be produced by a species of diatom (Haslea ostrearia) in culture, it seemed likely that the sedimentary maxima were also due to production from sediment diatoms. It was interesting therefore that the April–June HBI maximum did not coincide with the (August) maximum concentration of the common diatom hydrocarbon n-heneicosa-hexaene (HE), even though HE is also produced by the diatom species which produces C25 HBIs in culture. Diatoms isolated from the sediments in August produced abundant HE and only a small proportion of C25 HBIs. These results suggest that HBI and HE production maximise at different diatom growth stages, or that the major sources of HBIs in the sediments are as yet unidentified diatom species. The diatoms isolated from the sediment did not produce C20 HBIs and to date no organism has produced these in culture. Nonetheless, the δ13C values of the sedimentary C20 HBIs were consistent with an algal source.