Sulfur: As You Like It

Speaking of sulfur: This common element turns out to be highly useful for understanding planetary processes – both on Earth and Mars. Two new papers by Dr. Itay Halevy use sulfur chemistry to understand the history of sulfur-loving microbes at the bottom of the ocean and the compounds spewed from Martian volcanoes that may have created brief Martian “springs.”

Sources of sulfur: Olympus Mons on Mars and sulfate-reducing bacteria that live in water (inset) Sources of sulfur: Olympus Mons on Mars and sulfate-reducing bacteria that live in water on Earth (inset)

On the ocean beds, microorganisms “breathe” sulfur: They take in sulfate – an abundant compound of oxidized sulfur, and use it to digest organic matter that sinks to the seafloor. Thus in addition to sulfur, a great deal of the ocean’s carbon and oxygen cycle through these microorganisms. When we last mentioned these sulfate-reducing microorganisms, it was to highlight Halevy’s previous research, which traced the global sulfur cycle over the past 500 million years by accounting for the exit of sulfur from the ocean. This study revealed the importance of one destination of the microbes’ waste products: the iron-sulfide mineral pyrite, which gets buried in seafloor sediments.

In his recent research, Halevy developed a model for a closer understanding how the day-to-day activity of these microbes relates to their environment. Dr. Boswell Wing of McGill University, Montreal, who worked on this research with Halevy while on sabbatical in Halevy’s lab, says: “Along with the fact that sulfate-reducing microbes play a critical role in controlling atmospheric oxygen levels when their waste products are buried as the mineral pyrite, a.k.a. fools’ gold, what drew us to this problem was the huge amount of outstanding experimental data that was begging for an explanation.”

The two developed the model to explain a long-standing curiosity: The deep-sea microbes prefer light sulfur isotopes over heavier ones. They will, however, use the slightly heavier isotopes, and the resulting isotope ratios vary according to the availability of sulfur in their surroundings and their respiration rates. Halevy and Wing basically brought together models and formulas from across biochemistry and isotope chemistry to create a physical model that explains how the metabolic processes of the microorganisms interact with their environment. This model is already being used to uncover the planet’s sulfurous past, and it may help us to predict the future, as well, since environmental change will affect the habitats of crucial microbe populations and thus their metabolic activity. In addition, says Halevy, the basic model they developed could be adapted to work out the microbiological exchange of such other vital substances as nitrogen or methane.

On Mars, the driver of the sulfur cycle is not biological, but may be primarily volcanic – or at least it was in the planet’s more turbulent past. The signs of past volcanic activity on Mars – including the tallest volcano in the solar system – suggest that eruptions were hundreds of times more forceful than those on Earth, some lasting for up to a decade. These would have had a profound effect on the planet’s atmosphere. On Earth, the ash and sulfur compounds flung into the atmosphere from eruptions tend to have a cooling effect. But a climate model developed by Halevy suggests that the large amounts of sulfur-dioxide would act as a greenhouse gas, warming the surface below. The compounds that cool on Earth would have less of a chilling effect on Mars, as they would coat the dust particles already present in the atmosphere.

What does this model tell us? Volcanoes may have warmed the planet enough that water could have once flowed on Mars, at least in the lower latitudes for short periods of time – decades to centuries. Were such brief wet periods enough to sustain life on early Mars? Such questions are still being debated, but Halevy’s work gives us a much clearer picture of the conditions that may have prevailed on Mars’ ancient surface.


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