Chitinozoan chemical composition: New insights from organic geochemistry and molecular biology
Résumé
Chitinozoans are enigmatic organic-walled microfossils, which occurred in the Palaeozoic oceans from Early Ordovician to latest Devonian (Miller, 1996; Paris, 1996). They are recovered from a variety of marine sediments deposited in near-shore to deep oceanic environments. Their biology and systematic affinities remain uncertain, although the soft-bodied metazoan egg hypothesis is generally regarded as the most likely (Paris and Nõlvak, 1999). Chitinozoans have proven to be very useful in several branches of the Earth Sciences, including high-resolution biostratigraphy, palaeoenvironmental reconstructions and biogeography. New fields of additional application are being explored, especially those using chitinozoans as a source of carbon for documenting high-resolution δ13Corg curves (Lecuyer and Paris, 1997). Despite a dramatic increase in knowledge about chitinozoans, the unresolved question of their chemical composition, and especially the presence of chitin in their wall (Voss-Foucart and Jeuniaux, 1972) remains. In order to decipher the chemical composition of the chitinozoan wall, a variety of analytical techniques have been performed on individually isolated chitinozoan vesicles, including conventional elemental analysis, Rock Eval pyrolysis, pyrolysis GC-MS, Raman and Infrared spectroscopy and molecular biology. Selected rock samples from Baltica and northern Gondwana regions yielding abundant and well-preserved chitinozoan specimens have been investigated. Total organic carbon of the most suitable rock sample is 0.47%, in agreement with total carbon deduced from elemental analysis (0.69 %) while nitrogen is 0.127 %. Low Tmax values (420°C) of this rock sample, together with Raman spectroscopy performed on single vesicles, confirm a low thermal maturity for the analysed chitinozoan specimens and potential for the samples to retain the original chemical structure. Laser micropyrolysis GC-MS performed on single chitinozoan vesicles reveals the predominance of aromatic over aliphatic compounds. Within the compounds detected, no evidence for chitin derivatives was found, unlike those confirmed in fossil insect cuticles (Flannery et al., 2001). The combined results of these techniques do not support a chitin-like structure for chitinozoans. To further investigate chitinozoan vesicle composition, fluorescein-labelled Wheat Germ Agglutinin was used for selective binding and fluorescence microscopy detection of N-acetylglucosamine, the main chitin component. Preliminary molecular biological results are encouraging and further chemical analyses could ultimately verify the preservation of intact N-acetylglucosamine within chitinozoan walls. Flannery, M.B., Stott, A.W., Briggs, D.E.K., and Evershed, R.P., 2001. Chitin in the fossil record: identification and quantification of D-glucosamine. Organic Geochemistry, 32, 745-754. Lecuyer, C., and Paris, F., 1997. Variability in the δ13C of lower Palaeozoic palynomorphs: implications for the interpretation of ancient marine sediments. Chemical Geology, 138, 161-170. Miller, M.A.,1996. Chitinozoa. In: Jansonius, J., and McGregor, D.C. (eds.), Palynology: Principles and Applications. American Association of Stratigraphic Palynologists Foundation, 1, pp. 307-336. Monsigny, M., Sene, C. and Obrenovitch, A., 1979. Quantitative fluotimetric determination of cell-surface glycoconjugates with fluorescein-substituted lectins. European Journal of Biochemistry, 96, 295-300.