Yeast cells are capable of synthesizing steroid molecules—the basic building blocks of the biological membranes that are essential to eukaryotic life—using only trace amounts of oxygen, according to a study published on the website of the Proceedings of the National Academy of Sciences this Monday (August 8).
Life began its 3.8-billion-year evolutionary journey when the Earth was largely devoid of oxygen. For eons, anaerobic organisms ruled the planet until photosynthetic, ancestral bacteria proliferated in the ancient seas and filled the atmosphere with the gas about 2.4 billion years ago, opening the door to aerobic life.
But fossils of ancestral yeast cells harboring ancient steroid molecules—of which oxygen is a key component—indicated that these aerobic organisms were around prior to this profusion of atmospheric oxygen. How these early, aerobic yeast-like cells manage to eke out a living when their oceanic habitats contained so little oxygen has baffled biologist for decades.
Biochemist Jacob Waldbauer and colleagues subjected brewer's yeast (Saccharomyces cerevisiae) to conditions that mimicked the ancient seas, with orders of magnitude less oxygen than today's oceans contain, and found that the cells were still able to construct steroid molecules. He and his colleagues suggest that ancient yeast-like cells may have survived on similarly low oxygen about 3 billion years ago, taking up small amounts of O2 molecules produced by early forms of cyanobacteria to build their steroid molecules, Waldbauer told ScienceNOW.