Résumé
Known for more than 150 years, Mid-Cretaceous French ambers already provided more than 1500 arthropods (mostly Dipterans and Hymenopterans, but also Coleopterans, Neuropterans, Arachnids...). A first inventory of the microfossils preserved in these ambers, carried out in 2003, revealed a great amount of microorganisms. But, it mostly showed that an intense contamination of the amber pieces occurs, mostly by actinomycetes, fungi and diatoms. A decontamination protocol was elaborated to destroy all the contaminants without attacking the amber itself. The investigation of decontaminated pieces allowed to realize an inventory of the microfossils from Mid-Cretaceous French ambers. If the Actinomycetes were found in all the studied ambers, the cyanobacterium Palaeocolteronema cenomanensis is the most frequent microorganism. The eukaryotes showed a great diversity, with numerous specimens of fungi (including Ascomycetes, Basidiomycetes, a carnivorous fungus...), unicellular algae (four taxa of green algae) and protozoans (various amoebae and few ciliates). These discoveries allowed to highlight different microassemblages of fossils due to their place and modality of trapping. Some were embedded in the resin directly on the tree bark, other when the resin fell down on the forest floor or when resin flows were produced by aerial, underground or subaquatic roots. However, the most spectacular is the discovery of a rich assemblage of marine microfossils (mostly diatoms and sponge spines), transported into the resin during high sea levels or by sprays. The analysis of all the different inclusions found in the French ambers, mostly from Charentes, showed that complex trophic links existed in the soil of the Mid-cretaceous forest. Prokaryotes, fungi and few arthropods were the saprotrophic organisms. Amoebae and ciliates were the most common bacterivores whilst few Acari and Coleopterans were the main fungivores. Diverse Dipterans, Hymenopterans, Neuropterans, Heteropterans... were the main predators, and the food web was dominated by the spiders.