Sterols in red and green algae: Quantification, phylogeny, and relevance for the interpretation of geologic steranes

Steroids, a class of triterpenoid lipids with high preservation potential, are widely distributed in sedimentary rocks. All eukaryotes have a physiological requirement for these molecules, making steroids important biomarkers for aiding our understanding of eukaryote molecular evolution and geologic history. C₂₆-C₃₀ sterols are the molecules most commonly incorporated or synthesized by eukaryotes, and correspond to C ₂₆-C₃₀ steranes ubiquitously and abundantly preserved in petroleums and sedimentary bitumens. Because these sterols occur in evolutionarily diverse taxa, it can be difficult to associate any particular compound with a single group of organisms. Nevertheless, geochemists have still been able to draw parallels between the empirical patterns in geologic sterane abundances and the age of petroleum source rocks. Paleobiologists have also used sterane data, in particular the patterns in C₂₉ and C₂₈ steranes, to support fossil evidence of an early radiation of green algae in latest Proterozoic and Paleozoic and the succession of the major modern phytoplankton groups in the Mesozoic. Although C₂₉ sterols are found in many eukaryotes, organisms that produce them in proportional abundances comparable to those preserved in Proterozoic and Paleozoic rocks are limited. Based on a large, phylogenetically based survey of sterol profiles from the kingdom Plantae, we conclude that modern ulvophyte and early diverging prasinophyte green algae produce high abundances of C₂₉ relative to C₂₇ and C₂₈ sterols most consistent with the sterane profiles observed in Paleozoic rocks. Our analysis also suggests that ancestral stem groups among the Plantae, including the glaucocystophytes and early divergent red algae are also plausible candidates.