Microbes Take their Sulfur Light


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On the ocean floor, where oxygen is in short supply, live microbes that “breathe” sulfur instead of oxygen. Far from the world’s forests, these microbes play a vital role in the planet’s carbon cycle, digesting around half of the organic matter that sinks to the seabed. New research that was recently published as a “feature article” in the Proceedings of the National Academy of Sciences (PNAS) combines biochemical principles and stable isotope theory to create a new approach to understanding what the metabolic activity of these common microorganisms tells us about their environment. This approach may prove to be a valuable tool for helping decipher crucial processes in the planet’s ecology – in the past, as well as the present and future.

Drs. Itay Halevy of the Weizmann Institute’s Earth and Planetary Sciences Department and Boswell Wing of McGill University, Montreal, sought to explain phenomena that had been observed in experiments for almost 60 years: So-called sulfur-reducing microbes discriminate between the four stable isotopes of sulfur in ways that depend on the microbes’ growth conditions. Through respiration the microbes take in a sulfur compound, sulfate, from their environment and use it to fuel their metabolic reactions. But the microbes prefer using sulfate with the lighter isotopes of sulfur, so they “fractionate” these isotopes, generating metabolic products that are enriched in 32S and depleted in 34S. The experiments reveal that fractionation drops off as respiration rates speed up, and increases as sulfate concentrations increase. These patterns have been used to interpret the geologic record of sulfur isotopes and thus past environmental conditions, but a comprehensive model that can explain and predict the patterns has been lacking.  

To create their model, the researchers had to incorporate information about biochemical reactions, the various environmental factors that regulate metabolic processes in sulfate-reducing microorganisms and the chemistry of sulfur isotopes, which are taken up in one chemical form and expelled in another (much like inhaling O2 and exhaling it in the form of CO2). Among other things, says Halevy, he and Wing made use of principles developed in part in the Plant and Environmental Sciences Department by Prof. Ron Milo’s group, which tie biological reaction rates to the energetics of the reactions and the dynamics of the enzymes involved in those reactions.

The ability to model the processes that control microbial isotope fractionation has many possible implications. If the testing that is now underway supports the model, it will be an invaluable tool for understanding microbial activity in present-day, as well as ancient, environments. The researchers suggest that it would enable geoscientists to decode ancient history by understanding how sulfur-reducing microbes left their mark long ago in sulfur-bearing rock formations. Moreover, Halevy points out, the approach may open doors to other areas of research: “The method can be applied to the microbial metabolism of many additional elements,” he says. “For example, it could be used to model the nitrogen isotope fractionation of the denitrifying bacteria that drive an important part of the planet’s nitrogen cycle, or that of the microorganisms which produce the greenhouse gas methane.”

Dr. Itay Halevy’s research is supported by the Sir Charles Clore Research Prize; the Carolito Stiftung; the Wolfson Foundation; the Wolfson Family Charitable Trust; the estate of Olga Klein Astrachan; and the European Research Council.

Dr. Boswell Wing worked in the lab of Dr. Itay Halevy through the Feinberg Foundation Visiting Faculty Program.