Highly branched isoprenoids: a novel tracer of diatom-based energy pathways in freshwater food webs

In complex food webs, it is often difficult to classify all trophic interactions, especially when the number of potential energy sources and interacting species can be high. Biochemical markers (biomarkers) can help trace energy-flow pathways from basal sources up to top predators, but can suffer from poor resolution when multiple sources all produce the same biomarker (e.g. many algae produce long-chain unsaturated fatty acids). Highly branched isoprenoids (HBIs) are unique lipids produced by diatoms, which have been successfully applied as biomarkers of diatom-derived energy pathways through marine food webs. However, currently, the existence and trophic transfer of HBIs has not been explored in freshwater food webs. Here, we confirm, for the first time, the presence of two HBI isomers (IIb and IIc) across two temperate-lake food webs, from lower basal sources up to higher trophic-position consumers (predatory fishes). Lake ecosystems are facing multiple interacting threats that could influence food-web structure and function in complex ways. HBIs could provide a novel method for tracing the outcome of altered temperature, nutrient loading and water clarity on high-quality, diatom-derived energy pathways through freshwater food webs