5/4/00

Mark Shwartz, News Service (650) 723-9296; e-mail: mshwartz@stanford.edu

Fool's gold may provide clues to the evolution of life on Earth

Complex life-forms -- from protozoa to people -- need oxygen to survive.

But the early atmosphere of Earth contained very little oxygen, so the first creatures to appear on our planet were probably simple bacteria that thrived on other chemicals abundant in the harsh, volcanic environment.

The question of when free oxygen began to accumulate in the atmosphere remains highly controversial among scientists.

Part of the answer may lie in ancient deposits of fool's gold that formed in the ocean, according to Adina Paytan, an assistant professor of geological and environmental sciences.

"Without molecular oxygen, life could not have evolved to its present-day level of complexity," writes Paytan in the April 28 issue of the journal Science.

"Some say that oxygen was absent in the atmosphere until about two billion years ago," she notes. "Others say it was available 3.8 billion years ago when bacterial life began on Earth."

Although scientists differ on when oxygen began to accumulate in the air, there is little debate about how it got there. Ancient microorganisms, like the modern species known as cyanobacteria, began pumping oxygen into the environment as a by-product of photosynthesis -- similar to the way modern plants take in carbon dioxide (CO₂), extract the carbon for growth, then release excess O₂ molecules into the atmosphere.

Eventually free oxygen reached its current level, comprising about 21 percent of the air we breathe.

To determine when this atmospheric build-up began, Paytan says that researchers have focused their attention on sulfates -- sulfur-based compounds that are a common ingredient in seawater today.

Because sulfates are stable only in the presence of oxygen, there had to be some free oxygen available for sulfates to accumulate in the world's oceans.

Therefore, notes Paytan, if scientists could determine when sulfates first appeared in seawater, they would also know when oxygen started to become plentiful in the atmosphere.

To find out, researchers are focusing their attention on fool's gold ­ the shiny mineral that gleams like gold but actually is made of ordinary iron and sulfur.

Most fool's gold found on the ocean floor is produced by living creatures ­ species of bacteria that thrive on sulfate in seawater.

These microbes act like tiny chemical factories, consuming sulfate and converting it into another form of sulfur that eventually turns into fool's gold after chemically bonding with iron dissolved in seawater.

According to Paytan, sulfate-consuming bacteria are likely to have evolved at about the same time that sulfates started accumulating in the ocean.

"The question of when Earth's atmosphere began to accumulate free oxygen could then be answered by determining when the oceans became sulfate-rich and sulfate-consuming bacteria became active," she concludes.

So the challenge for scientists is to find ancient deposits of fool's gold that were produced by sulfate-consuming bacteria, because those deposits could only have formed at a time when there were sulfates in the water and thus free oxygen in the atmosphere.

"Picky eaters"

How do researchers know that fool's gold is the by-product of bacterial "digestion"?

It turns out that sulfate-consuming bacteria in the sea are very picky, preferring to devour lightweight sulfur atoms, known as Sulfur 32, while passing up heavier isotopes, called Sulfur 34.

But when scientists analyze samples of fool's gold that are billions of years old, they often find that the ratio of Sulfur 32 to Sulfur 34 is too low to have been produced by marine bacteria.

Nevertheless, some researchers still maintain that these ancient samples were, in fact, biologically produced. They contend that there was plenty of oxygen in the atmosphere for sulfate-consuming bacteria to thrive, even as far back as 3.8 billion years ago when life first began. They argue that, under the extreme temperatures that prevailed back then, bacteria must have consumed sulfates at a furious pace, showing little preference for lightweight sulfur isotopes.

But Paytan points to a recent study by D.E. Canfield and a team of researchers from Odense University in Denmark demonstrating that, at high temperatures, certain populations of sulfate-consuming bacteria show a distinct preference for lighter isotopes.

The Canfield team examined bacteria that live in hydrothermal vents at the bottom of the Gulf of California, where the thermometer reaches 190^o F (88^o C).

Their results, published in a companion piece in the April 28 issue of Science, demonstrate that modern bacteria do indeed prefer lightweight sulfur, even at temperatures approaching the boiling point.

Therefore, if sulfate-consuming bacteria were alive when the Earth was hotter, 3.8 billion years ago, they would have been fully capable of producing fool's gold with high ratios of Sulfur 32 to Sulfur 34. However, ancient samples of fool's gold do not contain such high ratios.

These findings seem to contradict the theory that there might have been high concentrations of seawater sulfate and atmospheric oxygen 3.8 billion years ago.

According to the Canfield team, the accumulation of oxygen and sulfate started more recently, about 2.5 billion years ago, reaching near present-day levels around 540 million years ago. This matches the fossil record, which shows that the first complex, oxygen-breathing life-forms did not appear on Earth until roughly a half-billion years ago.

According to Paytan, the Canfield study will provide significant clues for understanding the timing of atmospheric oxygen accumulation if the bacteria Canfield found in the Gulf of California are similar to those that were alive some 2.5 billion years ago during the Archean age.

"That's not necessarily clear," she notes, adding that "firm conclusions concerning the environmental evolution of the Archean Earth cannot yet be reached. To do so, we need a more reliable record of the chemistry of the atmosphere and/or oceans. As yet, data are limited, and the biological origin of many sulfur-bearing minerals of Archean age remains questionabl Paytan concludes that, as scientists try to determine how and when life began, studying sulfur isotopes may prove useful ­ not just on Earth but also on Mars and other planets where microorganisms may once have flourished.

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By Mark Shwartz