PROTISTAN EVOLUTION IN THE PRECAMBRIAN: A NEW MULTIDISCIPLINARY APPROACH COMBINING MICROSCOPY AND MICROCHEMISTRY

Biomarkers indicate the biosynthesis of eukaryotic sterols by the late Archean. Molecular clocks also suggest an early diversification of protists. Fossils of indisputable eukaryotes (based on morphological and ultrastructural criteria) have been described by us from the 1.65 Ga McArthur Group and the ~1.4 Ga Roper Group (Australia) and Ruyang Group (China). Nevertheless, the oldest fossils attributed to an extant eukaryotic clade are 1-1.2 Ga Bangiophyte red algae. Thus new approaches other than comparative morphology alone are necessary to resolve the biological affinities of earlier autotroph and heterotroph protists .

Study of wall ultrastructure can differentiate between prokaryotic and eukaryotic acid-resistant microfossils, and, in some cases, even identify prasinophycean and chlorococcalean green algae. Combining microscopy (light microscopy, SEM and TEM) with microchemical analyses of individual microfossils offers further insights into the paleobiology and evolution of early eukaryotes. Energy Dispersive Spectroscopy (EDS), micro-Fourier transform infrared (FTIR) spectroscopy and laser micro-Raman spectroscopy are used to elucidate the macromolecular structure and biopolymer composition of individual Neoproterozoic (Tanana Fm, Australia) and early Mesoproterozoic (Roper Group, Australia and Ruyang Group, China) acritarchs of low thermal maturity. The Neoproterozoic Tanarium sp. acritarch contains a biopolymer consisting of long chained polymethylenic material similar to algaenan isolated from some micro-green algae. The Neoproterozoic Leiosphaeridia sp. may comprise a new class of biopolymer containing significant aliphatic, branched aliphatic and saturated/ olefinic carbon constituents. Mesoproterozoic acritarch cell walls contain a predominantly aromatic biopolymer consisting of short aliphatic chains that are highly branched, with a few oxygenated functionalities. Raman spectroscopy offers complementary data regarding the degree of aromaticity of the macromolecular network of the cell wall.

Further work is ongoing to characterize the morphology, ultrastructure and chemistry of microfossils, as well as extant microorganisms with resistant cell walls.