Sterol-producing bacteria that may change the way geologists interpret the rock record

Geologists assume when they find molecules called sterols in soils or rocks they indicate the presence of plants, animals or fungi in ancient environments. But discovering how some bacteria also produce and modify sterols could change those interpretations.


Sterols linger in soils and rocks for billions of years and have filled in as an indication to geologists that the plants, creatures, and organisms that deliver them should once have lived nearby. A new discovery by Stanford’s School of Earth, Energy, and Environmental Sciences scientists, suggests that methane consuming bacteria produce and modify sterols in identifiable ways.

Sterols such as cholesterol, are essential molecules produced by almost all eukaryotes – a category that includes plants, animals, and fungi but not bacteria. Just a couple of microbes had been known to create sterols, thus geologist neglected that source in the stone record.

Paula Welander (Image credit: Steve Castillo)
Paula Welander (Image credit: Steve Castillo)

Paula Welander, an assistant professor of Earth system science in Stanford’s School of Earth, Energy, and Environmental Sciences said, “We were skeptical that so few bacteria produced sterols. Primarily, we found our hunch correct – we found more than 50 additional strains of bacteria with sterol-producing genes, with hints of even more. Now, we want to learn how these bacteria were making their sterols.”

Scientists noticed that as with eukaryotes, a few bacteria change sterols by evacuating chemicals called methyl bunches in a procedure called demethylation.

Walender said, “Everyone assumed the few bacteria that could remove these methyl groups were doing it the same way as eukaryotes.”

But she had her doubts. The sterol-producing genes necessary for demethylation in eukaryotes were absent in bacteria, suggesting that bacteria process sterols differently. Scientists thus decided to observe demethylation process further to determine the source of sterols in their sediment samples.

researchers analyzed the genome of the bacterium Methylococcus capsulatus, and through a series of experiments, determined two proteins that are responsible for demethylation. What’s more, they found that bacteria demethylate sterols at a different position than in eukaryotes.

Walender said, “That finding may seem trivial, but that this mechanism could be preserved in the rock record, meaning geologists can examine how a sterol is modified to learn what produced it. By studying this process in bacteria, we might get hints and ideas about how to better interpret what we see in the rock record.”

“For example, sterols appearing in the rock record had been used to determine when and how the ancient ocean became dominated by eukaryotic algae, like in the ocean today. But this interpretation of when algae became dominant is based on sterols that could also have been produced by bacteria so that might change our interpretation of how these different microbial groups emerged over time.”

“When it comes down to it, we’re very excited about microbes and proteins and biochemistry, and we want to use our molecular toolkit to continue to help answer these geologically relevant questions.”

The finding could help scientists distinguish sterols made by primitive bacteria – which would indicate an environment free of oxygen – rather than by oxygen loving plants, animals, and fungi.

Paula Welander is also a member of Stanford Bio-X. Co-authors include Alysha Lee, research technician; Amy Banta and Jeremy Wei, research scientists; and colleagues from the State University of New York.

The study is published in the Proceedings of the National Academy of Sciences.


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