First example of an algaenan yielding an aromatic-rich pyrolysate. Possible geochemical implications on marine kerogen formation

Non-hydrolysable macromolecules were isolated from the marine green microalga Chlorella marina where they represent ca. 5% by weight of the dry biomass. Such resistant biomacromolecules, termed algaenans, were previously identified in numerous microalgae (mostly comprising freshwater species) and they were shown to play a major role in the formation of a number of source rocks and oil shales via the selective preservation pathway. Scanning and Transmission Electron Microscopy revealed that the algaenan of C. marina is located in the cell wall but is completely amorphous once isolated. All the previously studied algaenans were also cell wall constituents but they always retained some morphological features of the initial walls. Another conspicuous difference was also observed regarding chemical structure. In agreement with spectroscopic observations, the pyrolysate of C. marina algaenan is sharply dominated by compounds comprising various aromatic units whereas all the so far studied algaenans appeared highly aliphatic and based on long polymethylenic chains. As a result, the selective preservation of algaenans could not be restricted to the formation of aliphatic sedimentary organic matter. In fact, it can be anticipated that some aromatic fractions of kerogens should also be derived from such a pathway, as suggested in recent studies.