Resistant macromolecules of extant and fossil microalgae

The occurrence and composition of macromolecular resistant walls of microalgae and their fossil macromolecular counterparts are reviewed. To date, several algal groups have been identified to produce fossilizable biomacromolecules. Only two biosynthetic pathways seem to be responsible for this, of which the acetate/malate pathway used by Chlorophyta, Eustigmatophyta and Dinophyta is considered to lead to a series of closely related resistant biomacromolecules, called algaenans. Algaenans consist of a network of predominantly linear carbon chains. A different, as yet unidentified, pathway is used by the Dinophyta to produce the aromatic walls of their cysts. The polyketide or acetogenic pathway may have been responsible for resorcinol-based algae or bacteria-derived microfossils of the acritarch Gloeocapsamorpha prisca, either through synthesis of the biomacromolecule or through a third pathway, the post-mortem polymerization of its resorcinol lipids. The post-mortem polymerization of lipids also appears to be responsible for the formation of fatty acid-based macromolecules in Eocene dinoflagellate-shaped remains from Pakistan. Finally, there is a clear need for elucidating the chemical differences between the biomacromolecules produced by the algae and their fossil analogs in the sediments. This notably applies to the release and condensation of aliphatic and aromatic moieties both at normal and at elevated temperature and pressure conditions.