Long-lasting ergot lipids as new biomarkers for assessing the presence of cereals and cereal products in archaeological vessels

International audienceCereals were very important in ancient diets, however evidence from archaeological sites of the vessels used for processing or storing cereals is comparatively rare. Microorganisms , as well as chemical-physical effects can easily degrade cereals during the burial period. This can lead to a complete cereal decay and to serious difficulties in estimating the intensity of use of the cereals by ancient populations. Here, we present a novel biomarker approach entailing the detection of secondary lipid metabolites produced by ergot fungi (genus Claviceps), which are common cereal pests. The aim was to identify the original presence of Gramineae and to indirectly establish if vessels were used for cereal storage/ processing. The fatty acid and TAG-estolide profiles of the remains from more than 30 archaeological vessels were investigated by gas chromatography/mass spectrometry (GC/MS) and high performance liquid chromatography/high resolution mass spectrometry (HPLC/ESI-Q-ToF). The detection of lipids derived from ergot in archaeological and historic contexts rests on its complex chemistry, providing a unique and relatively recalcitrant chemical signature for cereals. This research demonstrated that the combination of our innovative biomarker approach along with environmental and archaeological evidence can provide unprecedented insights into the incidence of cereals and related processing activities in ancient societies. Although cereals played a key role in ancient times, e.g. in the Neolithic diet, their botanical and chemical remains are relatively rare at archaeological sites. When cereals are morphologically preserved or even found as charred remains, the analysis of the DNA, phytoliths as well as mass spectrometric analysis of the organic remains can lead to interesting conclusions regarding the artefact functions and cereals diffusion 1-4. Cereals are lignocellu-losic substrates and thus prone to attack by insects, bacteria or other microorganisms , as well as degradation by chemical-physical processes occurring during the burial period 5. Charred cereals are comparatively less prone to microbial degradation, and are thus more easily found at archaeological sites. Some attempts to determine the presence of cereals from the analysis of charred remains have been reported in the literature 6-9. However, understanding the importance of cereals in ancient times remains a difficult and unexplored task. This paper presents an innovative biomarker approach entailing the recognition of the exclusive lipids bio-synthesized by ergot fungi of the genus Claviceps. Claviceps fungi (ergot) are common pests of Gramineae 10 , and they metabolically produce lipids characterized by a complex mixture of more than 70 compounds, consisting of a series of diglycerides, triglycerides and high molecular weight estolides, with ricinoleic acid (12-hydroxy-9-octadecenoic acid) being the most abundant acyl substituent 11. Estolides (Fig. 1) result from secondary acylation reactions, where additional fatty acids are esterified to the hydroxyl moieties of ricinoleic acid