Numerous kerogens from marine and lacustrine source rocks and oil shales, previously considered as amorphous, were recently shown to comprise very thin (10-30 nm thick) lamellar structures, termed ultralaminae. Comparative morphological and chemical studies of a lacustrine ultralaminae-rich kerogen and of the non-hydrolysable biopolymer (algaenan) isolated from the thin resistant outer walls of a freshwater microalga (Scenedesmus quadricauda) established a close relationship between these two materials. Chemical correlation was mainly based on the production upon pyrolysis of a series of n-alkylnitriles with a bimodal distribution. The non-hydrolysable biopolymer isolated from the outer walls of the ubiquitous freshwater microalga, Chlorella fusca, exhibits the same morphological features as those observed in S. quadricauda and in fossil ultralaminae. The investigation of C. fusca algaenan by solid-state C-13 NMR and analysis of pyrolysis products revealed that the correlation previously established between S. quadricauda and lacustrine ultralaminae also holds for C.fusca since the same series of n -alkylnitriles with the same distribution is formed upon pyrolysis (probably via cleavage of amide functions). This confirms that fossil ultralaminae in lacustrine kerogens derive from the algaenans building up the thin outer walls of freshwater microalgae via the selective preservation pathway. In addition, the analysis of the 2-n-alkanones generated on pyrolysis indicates that their distribution depends on the algal species considered. Accordingly, variations in ketone distributions in the pyrolysates of lacustrine ultralaminae-rich kerogens may reflect differences in populations of the source microalgae. On the contrary, changes in n-alkylnitrile distribution would not be species-dependent but would reflect the lacustrine or marine origin of the considered material.
