A bacterium that lives in the depths of the ground, that lives from the chemical reactions triggered by radioactive decay, makes it unchanged for millions of years, new research has discovered.
A genetic analysis of the species’ microbes Candidatus Desulforudis audaxviator (CDA) collected from three different continents revealed that the bacterium only evolved since they were last together on the same land mass, Pangea.
This means that it is in what scientists call “evolutionary stasis” of at least 175 million years, making the CDA the only known living underground microbial fossil. This could have important implications for understanding microbial evolution.
“This finding shows that we need to be careful when making assumptions about the speed of evolution and how we interpret the tree of life,” said microbiologist Eric Becraft of the University of North Alabama.
“Some organisms may enter a fully evolving sprint, while others slow down to a creep, causing reliable timelines to be established.”
CDA is a strange little organism. It was first discovered in 2008, living 2.8 kilometers (1.7 miles) below the Earth’s surface, in the groundwater of a gold mine in South Africa. Moreover, it contained 99.9% of the microorganisms in the place where it was found – effectively constituting an ecosystem with a single species.
This, as you can imagine, is quite rare. Small microbes live in cavities filled with water on the rock, relying on chemosynthesis for food; Unlike photosynthesis, which relies on sunlight for conversion to energy, chemosynthetic organisms derive their energy from chemical reactions.
In the case of CDA, it is the decomposition of water molecules due to ionizing radiation generated by the radioactive degradation of uranium, potassium and tor.
Therefore, unlike most life on Earth, the bacterium does not rely on sunlight or other organisms for its survival – it can only be kept there, in the damp darkness.
The team wanted to know more about CDA and how it evolved and adapted, so they searched for deep groundwater samples from other continents and found the bacteria in Siberia and California and other locations in South Africa.
They collected 126 microbes from all three continents and – being extremely careful, the investigators in each laboratory who did not approach the others – sequenced their genome. They believed that by comparing microbes from separate continents in different physico-chemical environments, they would see how they evolved and diversified as each adapted to their particular circumstances.
“We wanted to use this information to understand how it evolved and what kind of environmental conditions lead to what kind of genetic adaptations,” said microbiologist Ramunas Stepanauskas of the Bigelow Laboratory for Ocean Sciences in Maine.
“We thought of microbes as if they were inhabitants of isolated islands, like the quintessences Darwin studied in the Galapagos.”
They had no reason not to believe this – how could an isolated microbe 3 kilometers underground in South Africa come into contact with an isolated microbe 3 kilometers underground in Siberia? However, when the team compared the genomes, they found that the microbes on the three continents were almost identical.
A more detailed investigation showed no evidence that CDA can survive on the surface or in the air without losing long distances and twice checked for cross-contamination of the samples. Once all of this was ruled out, the researchers had to find a different answer.
The most plausible explanation? Microbes have barely evolved.
“The best explanation we have at the moment is that these microbes have not changed much since their physical locations separated during the collapse of the Pangea supercontinent, about 175 million years ago,” he said. Stepanauskas.
“They seem to be living fossils from those days. That sounds pretty crazy and goes against the contemporary understanding of microbial evolution.”
We know that bacteria can evolve extremely quickly; in fact, this has been a huge problem in the development of antibiotic drugs, as some microbes have managed to evolve resistance to these drugs. However, we do not hear much about the opposite scenario. Some scientists have suggested that some species of cyanobacteria may be in a state of evolutionary stasis, although this has been disputed.
CDA it could still be the best case of evolutionary stasis in a microbe. The team believes it may be due to microbes having specialized mechanisms that help them resist mutation. Researchers have identified genes for DNA repair mechanisms that could reduce mutation rates, along with polymerase – the enzymes that bind long chains of genetic material together – that have better accuracy than is seen in other organisms.
This has potential applications in biotechnology, from diagnostic tests to gene therapy, scientists said. Beyond how we can use it for our own benefit, however, the discovery shows us how little we know about our strange, wonderful, and diverse planet.
“These findings are a powerful reminder that the different microbial branches we see on the tree of life may differ greatly over time from their last common ancestor,” Becraft said.
“Understanding this is essential to understanding the history of life on Earth.”
The research was published in ISME Journal.