Bacteria in Lake Mendota keep evolving but never change

Have you ever felt stuck in a loop, repeating the same sequence of events over and over again? Like Bill Murray in the movie Groundhog Day, bacteria in Wisconsin’s Lake Mendota appear stuck in a perpetual cycle – but in this case, it’s more aptly described as a “Groundhog Year.”

According to a study published in the journal Nature Microbiology, scientists observed that most bacterial species in the lake undergo rapid genetic changes over a year, only to return to near-identical genetic states the next year.

This cyclical evolution offers new insights into microbial ecology and how organisms adapt to their environments.

Evolutionary cycles of lake bacteria

Lake Mendota’s bacterial species evolve rapidly in response to the lake’s dramatic seasonal changes. Winters cover the lake in ice, while summers foster algal blooms.

These environmental shifts allow specific bacterial strains to dominate during one season – only to give way to others that are better suited for different conditions at another time of year. This seasonal loop plays out year after year, and creates a pattern akin to rewinding and replaying a movie.

“I was surprised that such a large portion of the bacterial community was undergoing this type of change,” said Robin Rohwer, the study’s lead researcher and a postdoctoral scientist at The University of Texas Austin.

“I was hoping to observe just a couple of cool examples, but there were literally hundreds.”

Rohwer began this work during her doctoral studies at the University of Wisconsin-Madison under Trina McMahon, and continued the research at UT Austin. The study marks a breakthrough in understanding the adaptations of Lake Mendota’s bacteria under natural, dynamic conditions.

Unique archive of bacterial DNA

The researchers used an unprecedented dataset: 471 water samples collected over 20 years by UW-Madison scientists as part of National Science Foundation-funded monitoring projects.

These samples provided the longest metagenome time series ever collected from a natural system. By reconstructing bacterial genomes from fragmented DNA, the team mapped genetic changes across generations.

“This study is a total game changer in our understanding of how microbial communities change over time,” said Brett Baker, a co-author and researcher at UT Austin. “This is just the beginning of what these data will tell us about microbial ecology and evolution in nature.”

The work revealed not just seasonal cycles but also longer-lasting genetic shifts. For example, during 2012’s unusually hot and dry summer, when algal levels were low, many bacterial species underwent significant changes in genes related to nitrogen metabolism.

This suggests that environmental extremes can drive sustained evolutionary adaptations in lake bacteria.

Implications for climate change

Climate models predict more frequent extreme weather events for the Midwestern U.S., similar to the conditions seen at Lake Mendota in 2012 .

Rohwer believes this study offers a window into how microbial communities might respond to both gradual and abrupt climate changes.

“Climate change is slowly shifting the seasons and average temperatures, but also causing more abrupt, extreme weather events,” said Rohwer. “Our study suggests they will evolve in response to both these gradual and abrupt changes.”

Technological innovations in genomic research

The team relied on the Texas Advanced Computing Center’s supercomputing resources to reconstruct over 30,000 genomes from roughly 2,800 species. Without such technology, the analysis of the Lake’s bacteria would have taken 34 years on a standard laptop.

“Imagine each species’ genome is a book, and each little DNA fragment is a sentence,” Rohwer explained.

“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.”

Lake bacteria reveal nature’s complexity

The study on the evolution of Lake Mendota bacteria highlights the intricate and resilient nature of microbial communities.

While their short lifespans allow for rapid evolution, these bacteria also demonstrate remarkable stability, repeatedly returning to near-original states despite environmental pressures.

This discovery highlights the complexity of microbial ecology and its role in broader environmental systems.

As climate change continues to alter ecosystems, studies like this provide vital insights into how life adapts. They also emphasize the importance of long-term monitoring and advanced computational tools in uncovering the secrets of the natural world.

The study is published in the journal Nature Microbiology.

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