By Leslie Mullen
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."
What Next?
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!"
