by Leslie Mullen
for NASA Astrobiology Institute
Some of the oldest rocks on Earth can be found amid the spiky grass and orange-red dust of Northwestern Australia. While most rocks have been altered over time through geological processes, the Australian rocks have remained relatively unchanged since their inception 3.47 billion years ago.
Earlier this year, Yanan Shen of Harvard University, Donald Canfield of Odense University in Denmark, and Roger Buick of the University of Washington announced they found evidence for life in the ancient Australian rocks.
The scientists found indications of a type of bacteria that consume sulfate and produce sulfide as a waste product. Sulfate-reducing bacteria had been known to exist at least 2.72 billion years ago, but this finding pushes the date of their existence back an additional 750 million years. This would mean that sulfate-reducing bacteria are one of the oldest known life forms on the planet.
The scientists don't actually have samples of the ancient bacteria, but they believe they have proof that bacteria had been in action. After measuring the ratio of sulfur isotopes in the rocks, the scientists concluded that the sulfides were produced biologically.
Isotopes are different forms of an element that have the same number of protons in the nucleus but different numbers of neutrons. The different isotopes of an element have slightly different chemical and physical properties. For instance, the most common form of sulfur is sulfur-32, which contains 16 protons and 16 neutrons in its nucleus. Sulfur-32 is lighter than sulfur-34 - a heavier version of sulfur that has 2 extra neutrons.
When sulfate is plentiful, the bacteria prefer to eat the lighter sulfur isotope. When the bacteria eat the lighter sulfate, the sulfide they eliminate as a waste product is also lighter.
In the Australian rocks, the sulfide contains 12 parts per thousand (or "permil") less of the heavier sulfur isotopes than the sulfates. In other words, the waste product had more of the lighter isotopes than what was generally found in the available food supply. This seems to indicate that the lighter isotope sulfur was selectively eaten by bacterial organisms.
The scientists say that some natural chemical and geological processes can separate lighter and heavier sulfur isotopes, but it requires temperatures above 300 degrees Celsius (572 degrees Fahrenheit). According to Buick, the rocks have never been heated to that extent.
"We can tell their peak temperature from the associated assemblage of metamorphic minerals," says Buick. "In this instance, inorganic isotopic fractionation processes can be easily excluded."
Determining the rock's exposure to temperature is important - high temperatures would indicate sulfate reduction by either inorganic processes or by a class of organisms known as the Archaea. Sulfate-reducing archaea live in many places, including hot environments like volcanic vents under the sea.
But if the Australian rocks were only exposed to lower temperatures, this would indicate bacterial sulfate reduction.
"Low-temperature sulfate-reducers are, as far as we know, restricted to the bacteria," says Buick. "Hence, if our North Pole sulfate-reducers lived at low temperatures, then they were most likely bacteria."
The rocks were found in a hot, arid region of Australia ironically named "North Pole." But the rocks originally formed in shallow pools of water. This watery birth can be seen in the rock's sedimentary materials, in rippled features formed by waves, and in the minerals precipitated by evaporation of seawater.
Buick says that these ancient pools of water were cool rather than hot. The rock contains barite, and Buick believes this barite was originally gypsum. Gypsum chemically separates, or "precipitates," from seawater at cooler temperatures.
If, however, the barite in the rock has always been barite, this would imply high temperatures. Barite precipitates from hydrothermal fluids at high temperatures.
Bruce Runnegar of UCLA believes that the barite in the rocks has always been barite. He says the barite resulted from the rock's exposure to the high temperatures of hydrothermal vents.
Runnegar does not believe that bacteria reduced the sulfate in the rocks. Instead, he says the sulfate was reduced through exposure to hydrothermal fluids emitted from underwater volcanic vents. Runnegar says this photochemically-induced sulfate reduction can occur at temperatures ranging between 175 to 250 degrees Celsius (347 to 482 degrees Fahrenheit).
"Oxygen isotope data show that the North Pole hydrothermal fluids were heated to at least 150 degrees Celsius," says Runnegar.
Runnegar and his team also measured an additional sulfur isotope in the Australian rocks. While Buick and his colleagues measured the ratios of two different isotopes - sulfur-34 and sulfur-32 - Runnegar's team measured sulfur-32, sulfur-33, and sulfur-34. Measurements of the sulfur-33 isotope led Runnegar's team to a different conclusion of how the sulfate was reduced.
"The extra dimension shows effects that cannot be explained by ordinary chemistry of the kind that bacteria use," says Runnegar. "The only known explanation for the chemistry we observe involves reactions in gases - hence the need to bring atmospheric chemistry into the picture."
Runnegar says that less energy is required to separate oxygen atoms from sulfur-32 than sulfur-34. This is why the bacteria preferentially choose to consume sulfur-32, but also why sulfur-32 tends to be reduced more often than sulfur-34 by inorganic processes.
"Bacteria just make use of the rules of chemistry," says Runnegar. "Consequently, it can be very difficult to recognize the difference between bacterial sulfate reduction and non-biological thermochemical sulfate reduction from the pyrite [sulfides] preserved in rocks."
Runnegar also points out that the sulfides measured by Buick's team came from the insides of large crystals of barium sulfate. He says that this location makes it unlikely that bacteria were involved.
Buick acknowledges that hydrothermal processes affected the sulfur minerals after they were deposited. He also says that atmospheric processes could also have affected the sulfur isotopes before deposition. But he argues that these two processes cannot fully explain the features found in the rocks.
"Above and beyond these events, there are mineralogical and isotopic features that can't be easily explained by one or the other or both," says Buick. "These features are best interpreted as biological."
In addition, Buick says there are other signs of biological activity in the rocks. Immediately overlying the rocks are stromatolites - layers of sediment that are constructed by microbes in shallow pools of salt water. In modern environments, stromatolites are formed by photosynthetic bacteria.
"As there are two types of microbial photosynthesis, one yielding oxygen by the familiar plant process but the other producing sulfate, it could be that the organisms responsible for building the stromatolites were the same bugs that filled the water with sulfate," says Buick.
Buick says that the presence of sulfate-reducing bacteria almost 3.5 billion years old suggests that a wide range of microorganisms had already colonized the early Earth, forming a rudimentary food chain.
"Sulfate reducers need dead organic matter to be able to reduce the sulfate, so there must have been other organisms that were primary producers," says Buick. "They also need a source of sulfate, which may have come from anoxygenic photosynthesizers." This would constitute "a simple but complete ecosystem - photosynthetic sulfate-producers that fed other bacteria that lived by reducing the sulfate."
Because the North Pole rocks are rare in their age and state of preservation, the chance of pushing the record even further back in time is not great. In order to find evidence of sulfate-reducing bacteria in rocks older than 3.5 billion years, Buick says we may have to look beyond Earth.
"From spectral analysis, we know that there are lots of sulfate minerals on the surface of Mars," says Buick. "If that planet was warmer, wetter and inhabited more than 3.5 billion years ago, we might be able to find older signs of biological sulfate-reduction there, provided of course that NASA sends a bloody good field geologist with lots of experience of particularly ancient rocks in remote places."
Runnegar and his colleagues are still writing up data from their tests on the Australian rocks. They plan to continue their work in Australia and elsewhere.
Buick and his colleagues are likewise continuing their studies of the ancient Australian rocks. Yanan Shen is doing further analyses of some new samples that Buick collected at North Pole while on a NASA Astrobiology "Mission to Early Earth" field trip. Donald Canfield, meanwhile, is studying many living sulfate-reducers to test their contention that low-temperature sulfate-reducers are only found in one part of the Tree of Life.
"These will hopefully add even more strength to our conclusions," says Buick. "I would dearly love to go to South Africa to look at some almost-as-ancient sulfate minerals there, to see if they have similar physical and chemical features to the North Pole rocks. And to Mars, of course."
NASA Astrobiology Institute
Sulfates: Barite and Gypsum
The Sulfur Cycle
Introduction to the Archaean
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