For the early microbial colonists of Earth, land was rather uninviting. In the ocean, chemistry and temperatures were relatively stable, and seawater provided a shield against ultraviolet radiation. In comparison, the low-oxygen atmosphere of freshly exposed land offered little protection. Microbes had to deal with fluctuating levels of light, heat, minerals, and moisture.
Scientists have found what appears to have been a suitable refuge from the harsh terrestrial landscape: an acidic lake. In a recent study, researchers identified sediments dating back billions of years to suggest a community of microbes that adapted to life in an acidic lake that filled an ancient volcano. The volcano’s hydrothermal waters could have provided energy and minerals needed to sustain life, according to the study, published in Earth and Planetary Science Letters.
The researchers identified “a new environment for early life—that is, acidic volcanic lakes—and could pinpoint specific organisms based on their metabolisms,” said lead author Andrea Agangi, a professor in the Graduate School of International Resource Sciences at Akita University in Japan.
The findings could help scientists understand more about ancient life on Earth, as well as aid in the search for extraterrestrial life in the solar system. Similar volcanic lakes that formed during the wetter, volcanically active part of Mars’s early history, for example, may too have harbored life—giving astrobiologists a promising new location to look.
Continental land emerged during the late Archean, between 3 billion and 2.5 billion years ago. There are a number of microfossils from this period in Earth’s early history—Archean microfossils are much rarer than animal fossils—mostly in the form of stromatolites, ancient reefs formed by cyanobacteria. Similar evidence of ancient life on land, however, is very rare: Aside from the potential rarity of terrestrial-based life, tectonic activity and the rock cycle have altered many of the terrestrial rocks from the Archean.
Some of the world’s oldest crust lies in the Kaapvaal Craton in South Africa, which dates to around 3 billion years ago and is home to the Witwatersrand Basin, a vast gold deposit that has drawn miners since the late 19th century. Here lies the Dominion Group, a sequence of mostly volcanic rocks, interspersed with layers of sediment rich in pyrophyllite, known locally as wonderstone, a fine-grained mixture of light sandstone and soft, black, carbon-rich shale.
One of the first things Agangi’s group did was identify the Dominion Group sediments as terrestrial. They analyzed samples of wonderstone taken from three sites.
Distinguishing between ancient marine and nonmarine sediments is tricky, but the positioning of volcanic rocks within the Dominion Group offered clues. When lava cools quickly in seawater, it hardens in tubular formations known as pillow structures. The wonderstone is sandwiched between layers of volcanic rock that do not have pillow structures, which suggests that the lava erupted out of the ocean, in the open air. The sandwiched sediment was deposited out of the ocean, too, the authors argue, washed down by a turbidity current to settle in the bottom of a lake.
In addition to being terrestrial in origin, the South African wonderstone has properties commonly found in hydrothermal pools in modern volcanic environments such as the Yellowstone caldera complex. These properties include high levels of aluminum-rich vanadium, arsenic, and nickel.
“These are minerals you would associate with rocks that have been heavily altered by acids,” said Eva Stüeken, a lecturer in the School of Earth and Environmental Sciences at the University of St Andrews who was not involved in the research. If the South African rocks were deposited in a marine setting, seawater would have likely neutralized the acidity.
Taken together, evidence offered by nearby volcanic rock and mineral composition suggests that the sediments studied were deposited in an acidic lake.
Looking for Life
The hot, low-pH waters of an acidic lake may have leached minerals from the rocks, Agangi and his colleagues suggested. This process would dissolve into water nutrients necessary for biotic life, such as phosphorus and boron, and trace metals such as copper, selenium, and zinc.
“You have water, nutrients, energy—these are the basic components people usually look for when looking for life,” said Agangi.
To search for signs of life, the researchers used carbon isotopes. They found high ratios of lighter isotopes in the wonderstone shale. This isotopic signature suggests that the carbon is organic, the authors concluded, as it matches the signature of the modern production of methane by single-celled organisms known as Archaea. Today these hardy microbes are found in extreme environments such as hydrothermal vents, Antarctic lakes, and even the human digestive system.
“I would think that low-pH acidic environments would be hostile to life, but these researchers found good trends of carbon isotopes,” said Ilya Bindeman, a professor of stable isotope geochemistry at the University of Oregon who was not involved in the research.
“From all of this, we can say that it is very possible—though not conclusively proven—that methane-cycling microbes were living in volcanically influenced lakes on Earth 3.1 billion years ago,” said Alexander Brasier, a senior lecturer in geology at the University of Aberdeen not involved in the research.
Like Earth, Mars went through a wetter volcanic period during its early life, and similar acidic lakes may have formed on its surface. Not only does this correlation present a new ecological niche where researchers could search for past life on Mars, but also it establishes the Dominion Group as a good place to study the history of both planets.
“What makes [the Dominion Group] a good analogue is that it was deposited under an anoxic atmosphere,” Stüeken said. “The entire environmental setting was probably more similar to Mars than the modern Earth. That’s very compelling.”
—Richard Kemeny (@rakemeny), Science Writer