Biosynthesis of sterols and wax esters by Euglena of acid mine drainage biofilms: Implications for eukaryotic evolution and the early Earth

Lipid analysis of benthic and floating biofilms from an acid mine drainage (AMD) site in western Indiana revealed the dominance of photosynthetic organisms, mainly Euglena, as indicated by the detection of abundant phytadiene, phytol, phytanol, polyunsaturated n-alkenes, polyunsaturated fatty acids, short-chain (C25–32) wax esters (WE), ergosterol, and tocopherols. The WE were probably synthesized in mitochondria under anoxic conditions, whereas the sterols (ergosterol and ergosta-7,22-dien-3β-ol) were likely synthesized in the cytosol in the presence of molecular oxygen by the acidophilic, photosynthetic microeukaryotes Euglena. The dual aerobic and anaerobic biosynthetic pathways may be the biochemical relics of the anaerobic past of the Earth. The conserved compartmentalization of Euglena biosynthetic machinery may have allowed early eukaryotes to survive and diversify on early Earth, and despite their evolution, they preserved this physiology. Euglenids are capable of producing suites of short-chain WE that can be considered precursors for low-molecular weight (C12–16) n-alkanes, which can serve as proxies for the detection of such eukaryotic organisms in Precambrian strata. In addition, ergosterol and its derivative can also serve as biomarkers for such organisms.