Abstract:
Organic-walled microfossils of uncertain origin, classified to an informal group named
acritarchs, are most commonly interpreted as the resting cysts of marine eukaryotic
phytoplankton. Some acritarchs have recently been interpreted as vegetative cells of
chlorococcalean green algae, based on internal bodies that have been interpreted as their
asexual reproductive structures (spores). To verify this interpretation, we applied confocal
Raman spectroscopy and atomic force microscopy (AFM) to study the ultrastructure and
nanostructure of exceptionally preserved acritarchs with internal bodies from the early
Silurian cherts (c. 430 Ma-old) of Frankenwald (Germany). Three-dimensional Raman
mapping showed the spatial distribution of carbonaceous material and other minerals in the
walls of the analysed internal bodies and confirmed that these structures are comparable with
spores of chlorococcalean microalgae. Our findings document therefore the oldest thus far
known vegetative cells of sporulating green algae. The combination of confocal Raman and
AFM techniques yielded detailed information about the nanostructure and fossilisation mode
of the mineralised organic walls of both the central vesicles and the enclosed spore-like
bodies.
