Contamination of Amber Samples by Recent Microorganisms and Remediation Evidenced by Mid-Cretaceous Amber of France

International audienceOne of the best ways to obtain data about the ecology of past forests is to study amber fossils. Meso- and macroinclusions in amber have been intensively studied for a long time, however microinclusions have been largely neglected despite their high potential of fossilization in amber. Because of their abundance in recent forest ecosystems, many microorganisms are prone to be trapped in tree resin and thus fossilized in amber. According to their tiny size, from a statistical point of view we should find more microfossils in amber than arthropods. From this observation, investigations of microorganisms in the mid-Cretaceous amber of SW France have been performed. Studies of this material since 1999 already allowed the discovery of a rich arthropod fauna. From first investigations rich microfauna and microflora were also found: they were mainly composed ofcyanobacteria and actinomycetes but also included green algae, protists, nematodes. The most spectacular was the discovery of a rich diatom flora composed of both freshwater and marine taxa. More detailed studies of these inclusions revealed that a non-quantifiable part of these microorganisms was only contaminants and biased our estimation of the microcoenosis of mid-Cretaceous amber forests of SW France. This paper details the different types of amber contamination by modern microorganisms (particularly modern diatoms) highlighted during the study. A simple and accessible decontamination protocol, inexpensive enough to be used routinely in the laboratory, has been developed accordingly. This protocol comprises three main steps: (1) ultrasonic cleaning, (2) oxidation byH2O2 to eliminate organic contaminants, and (3)HFacid attack to dissolve inorganic contaminants such as siliceous skeletons. SEM observations controlled for the efficiency of our protocol. The contamination of samples is one of the most important sources of error in science, particularly in microbiology and paleomicrobiology. Mid-Cretaceous ambers found in Charentes (SW France), due to their outcropping conditions, are excellent tools for the investigation of such phenomenon of sample contamination and showed how this problem could bias paleoecological interpretation from erroneous amber preserved microcoenosis. We suggest that this protocol should be applied as a preliminary step when studying microorganisms embedded in amber to avoid erroneous scientific results