Microbes take their sulfur light

Dr. Itay Halevy of the Weizmann Institute's Department of Earth and Planetary Sciences and Dr. Boswell Wing of McGill University 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 Dr. Halevy, he and Dr. Wing made use of principles -- developed in part Prof. Ron Milo's group in the Department of Plant and Environmental Sciences -- that 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, Dr. 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."