Research by Montana State professors provides insight into early evolution of life on primitive Earth
By Diana Setterberg MSU NEWS SERVICE
BOZEMAN — In one of the few places on the planet where conditions are most like those that existed on primordial Earth, Montana State University researchers have discovered two species of a microorganism that provide new insight into the processes that supported early life here and potentially on other rocky planets.
In a paper published this week in the journal Proceedings of the National Academy of Sciences, Daniel Colman and Eric Boyd describe the discovery of two ancient, previously uncharacterized types of bacteria inhabiting a subterranean environment similar to the rock-water interface where Earth’s earliest organisms could have formed.
The anaerobic bacteria, called acetogens, feed on hydrogen and carbon dioxide to produce acetate. Both novel types of bacteria were identified by the researchers over a five-year period from groundwater in a unique geological formation called the Samail Ophiolite, a chunk of Earth’s mantle that was thrust to the surface along the southeastern coast of the Arabian Peninsula.
“Here’s an environment reminiscent of what would have been omnipresent on early Earth – that’s the connection and motivation for going there and studying it,” said Boyd, corresponding author on the paper and professor in MSU’s Department of Microbiology and Cell Biology in the College of Agriculture.
The work was funded by MSU’s share of a five-year, $6.4 million NASA “rock-powered life” grant, which was awarded in 2015 to scientists from a group of institutions working together to understand how rocks and water interact to release energy capable of supporting microbial life. The group’s fieldwork was done at the Samail Ophiolite because it hosts an ecosystem most like those that existed on Earth more than 3 billion years ago – before any life forms visible to the naked eye existed, including green plants, algae and cyanobacteria that produce oxygen.
Despite the absence of oxygen in those conditions, scientists have long hypothesized that acetogens lived in geological formations similar to those found at the present day Samail Ophiolite and that they were among the earliest evolving organisms on the planet. The ophiolite is composed of dense, iron-rich rock called serpentinite, which reacts with water to create hydrogen gas through a process called serpentinization. The serpentinization reaction creates alkaline pH waters, whereas unreacted waters have neutral pH. In this way, pH can indicate the influence that serpentinization has had on the chemical and microbial composition of those waters.
Boyd’s team pumped waters from different depths beneath the ophiolite to find the acetogens. One population of the organisms, labeled Type I, was abundant in shallower, more neutral water, while the other population, labeled Type II, was abundant in very alkaline waters undergoing extensive serpentinization at depths of almost 280 feet.
Back in the lab, Colman, the paper’s lead author and assistant research professor in the Department of Microbiology and Cell Biology, used DNA sequenced from the organisms to reconstruct their genomes and began noticing interesting patterns in their distribution across environment types, Boyd recalled. To explain them, the two researchers conducted computational analyses on the DNA of both microbes to reconstruct the series of events that led to their diversification. It’s the same process scientists use to determine the evolutionary relationships among organisms – for example, between dogs and coyotes – as well as the ancestry of microorganisms and when key divergence events took place in relative time.
Colman and Boyd concluded that the metabolisms of the two lineages differed, allowing them to diversify into and inhabit their respective environments. Notably, the Type I acetogens living in the more neutral water used a process similar to that found in some bacteria to produce acetate; the Type II acetogens, from the deeper and more alkaline waters, used a different process most likely derived from an early gene transfer or shared origin with archaeal methanogens, which are ancient microorganisms that produce methane rather than acetate as a metabolic byproduct.
“What’s interesting is that these two organisms inhabit distinct subsurface chemical environments, they have distinct physiological components that align with their respective environments, and these physiological components have different evolutionary histories, with the Type II acetogens more reflective of ‘older’ life,” Colman said. “It all aligns very well with long-standing theoretical hypotheses for what we thought should live in these types of environments, and now we have real world examples of actual organisms that have previously been elusive.”
While scientists have speculated that acetogens were among the first forms of life on Earth and that they evolved in environments where serpentinization took place, the new discovery offers the clearest evidence yet of that connection, according to Boyd and Colman.
“We were specifically aiming to find life like this, but we certainly didn’t think we’d find such a tight connection between the organisms and their environments,” Boyd said. “Here’s an organism living on Earth today that can be used to study and advance those theories for the origin of life even further.”
In fact, Boyd and Colman are continuing their research on acetogens with a sequencing grant from the U.S. Department of Energy to study strain-level diversity within the acetogen populations.
“These observations provide a blueprint for us to now really think about the earliest forms of life on Earth and how the geosphere (rocks) supported early microbial life,” Boyd said.