Phylogenetic investigation of the aliphatic, non-hydrolyzable biopolymer algaenan, with a focus on green algae

Algaenan, an aliphatic biopolymer found in various microalgae, has been implicated as the source of a sizable proportion of the aliphatic refractory organic matter in sedimentary rocks. Because of its recalcitrant nature, algaenan is thought to be preserved selectively in the formation of kerogen and microfossils. Its taxonomic distribution in organisms has not been studied in detail or in a phylogenetic context. Here, we evaluate the distribution and phylogenetic relationships of algaenan-producing organisms from a broad, eukaryote-wide perspective down to the level of genus and species. We focus on the kingdom Plantae, as most described algaenan producers belong to this superkingdom. The phylogenetic distribution of algaenan producers within the Plantae is actually quite limited and a detailed phylogenetic analysis of the two classes that include all green algal algaenan producers suggests that there is no finer-grained pattern of phylogenetic distribution to the production of this biopolymer. Our results suggest that the biopolymer is not widespread ecologically or phylogenetically, is not found abundantly in marine organisms and likely represents a functional description of molecular class, rather than a biomarker for green algae. This adds to a growing body of literature that questions the selective preservation hypothesis for insoluble organic matter and calls for a more detailed chemical and structural analysis of algaenan.