by Tanja Bosak
In the beginning, there were fats, and in the end, only fats will remain. This is, in simple words, the basis of a new field of research: the study of molecular fossils. Molecular fossils are remnants of the actual lipid molecules that formed the cell membranes of ancient organisms. Because all organisms have membranes, most old fats may not tell us anything more than that some living organisms were present in the past. Fortunately, relatives of a disreputable lipid – cholesterol – are more informative and come to the geochemist's rescue. Old fats, it turns out, also contain various cholesterol-like ringed lipids (called polycyclic terpenoids) that are amazingly stable under both acidic and basic conditions, at elevated temperatures, and when faced with other adverse conditions that destroy most organic compounds in sediments. By some estimates, the mass of five-ringed terpenoids in sedimentary rocks and oil reservoirs (~1012 tons) is comparable to the total mass of carbon in all living organisms! (1).
Just as the shape of a fossil bone can tell us whether an organism was a dinosaur or a mammal, chemical structures of various polycyclic terpenoids may be specific to certain microbes or environmental conditions.
Characteristically, four-ringed cholesterol or other sterols are essential and abundant in eukaryotic membranes, but are absent from the membranes of almost all present-day bacteria. Some eukaryotic sterols are also modified differently from all known bacterial sterols. Thus, when the fossils of these modified molecules are found in old sediments, they can be attributed to ancient eukaryotic organisms. Fossil sterols can tell us something about past environments as well, because all modern organisms require molecular oxygen to produce sterols. By extension, geologically-stable derivatives of sterols are thought to indicate that molecular oxygen was present in those ancient environments. Intriguingly, molecular fossils of sterols are 2.4-2.7 billion years old. Since other independent geochemical evidence had placed the rise of molecular oxygen at about 2.2 billion years ago (2), this implies that oxygen had been around for quite some time before it accumulated in the atmosphere.
What about bacterial lipids? Very few bacterial species are known to produce four-ringed sterols, but many produce very similar five-ringed polycyclic terpenoids called hopanoids. Although bacteria are by far the most important source of hopanoids in modern sediments, these molecules were detected in modern bacteria only about thirty years ago, after they had been detected in eukaryotes and in the rock record. (They even owe their very name to a eukaryotic organism, a tropical hardwood Hopea).
Bacterial hopanoids and other polycyclic terpenoids are thought to act as membrane strengtheners, and may well have done so for at least 2.4–2.7 billion years. But there are many unknowns that obscure the value of these compounds as molecular fossils. First, hopanoids seem not to be essential to bacteria (in contrast to the sterols of eukaryotes.) Not more than one-sixth of all bacteria have the ability to synthesize hopanoids. Second, the biosynthesis of hopanoids and similar compounds in some soil-dwelling bacteria only takes place when they sporulate (e.g., B. subtilis) (3), or when they form aerial structures on solid surfaces (S. coelicolor) (4). Third, various bacteria modify hopanoids, and these modifications can persist in rocks. Until recently, such modified hopanoids had been found only in cyanobacteria, but other bacteria are now known to produce the same molecules under appropriate culture conditions (5). Fourth, hopanoids can be made in the absence of molecular oxygen, but more hopanoids seem to be made by oxygen-loving bacteria. Lastly, sediments of all ages also contain abundant three-ringed cheilanthanes, compounds that may be ancestral to both sterols and hopanoids. These lipids are often called "orphans" because modern microbes that produce these compounds have yet to be identified.
Microbial mysteries surrounding modern and fossil fats often result in controversial stories about the parallel evolution of organisms and the environment in the deep past. The old fats are far from being stale. At the very least they will have sparked a new appreciation for modern microbial lipids.
1. Ourisson, G. & Albrecht, P. (1992) Accounts of Chemical Research 25, 398-402.
2. Brocks, J. J., Logan, G. A., Buick, R., & Summons, R. E. (1999) Science 285, 1033-1036.
3. Bosak, T., Pearson, A., and Losick, R. (2006) Geochimica et Cosmochimica Acta 70 (Issue 18, Supplement 1), A60.
4. Poralla, K., Muth, G., & Hartner, T. (2000) Fems Microbiology Letters 189, 93-95.
5. Rashby, S. E., Sessions, A. L., Summons, R. E., & Newman, D. K. (2007) Proceedings of the National Academy of Sciences of the United States of America 104, 15099-15104.
Tanja recently joined the Faculty of the Department of Earth, Atmospheric, and Planetary Sciences at MIT.