28-01-2025
This Strange Life Form Is Caught in an Evolutionary Time Loop
Evolution doesn't always move inextricably forward, instead sometimes taking the occasional detour or even getting stuck in never-ending circles.
In Madison, Wisconsin's Lake Mendota, bacteria evolves cyclically in lockstep with the lake's changing conditions.
A new study detailing this phenomenon includes data from water samples gathered over 20 years to create the largest metagenomic time series of a natural system.
Evolution is often portrayed as a forward-moving natural force. Even the famous 1965 illustration of human evolution—from Pliopithecus to Modern Man—is known as the 'March of Progress.' But of course, like most things in nature, things aren't really that simple.
Take, for instance, Lake Mendota. Located in Madison, Wisconsin, this lake goes through rapid changes throughout the year—freezing in the winter before being covered in algae in the summer. Because the lake has long been the subject of an ongoing study as part of the National Science Foundations long-term monitoring project, 471 water samples collected over 20 years creates a rich data set of the genome changes of the bacteria that call the lake home.
However, scientists at the University of Wisconsin-Madison and the University of Texas (UT) undertook a study that came to another startling conclusion—there wasn't any evolutionary change at all. Or to put it more succinctly, the evolution of the bacteria in the lake was like 'a movie run back to the beginning each time and played over again,' the researchers said in a press statement. The results of the study were published in the journal Nature Microbiology.
'I was surprised that such a large portion of the bacterial community was undergoing this type of change,' said UT's Robin Rohwer, lead author of the study. 'I was hoping to observe just a couple of cool examples, but there were literally hundreds.'
To understand the changes occurring within the lake (especially under the body of water's constantly changing conditions), the researchers constructed a 'metagenome,' which contained all the genetic sequences from DNA fragments left behind by bacteria. If that sounds complicated, rest assured, it definitely is. The researchers relied on the supercomputer powers of the Texas Advanced Computing Center to essentially reconstruct bacterial genomes from the 471 water samples spread across two decades.
After months of number-crunching (a task that would've taken 34 years for a typical computer to accomplish), the resulting metagenomic time series was the largest of its kind collected from a natural system.
'Imagine each species' genome is a book, and each little DNA fragment is a sentence,' Rohwer said in a press statement. 'Each sample has hundreds of books, all cut up into these sentences. To reassemble each book, you have to figure out which book each sentence came from and put them back together in order.'
Because these lake-bound microbes measure lifespan in days—not years—the bacteria appeared to evolve in never-ending circles to adjust to the changing environment. However, when the lake experiences any kind of disturbance, those changes can reach into the very genomes of these bacteria. For example, the authors note that in 2012, the lake experienced a hotter and drier summer than usual, and within the bacteria's genome, the team saw hints of major shifts in genes related to nitrogen metabolism, likely spurred on by the scarcity of algae in the lake. With hotter, drier conditions expected for this region of the U.S. as climate change progresses, understanding these conditions on a genetic level is incredibly important.
'This study is a total game changer in our understanding of how microbial communities change over time,' UT's Brett Baker, a co-author of the study, said in a press statement. 'This is just the beginning of what these data will tell us about microbial ecology and evolution in nature.'
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