by Wisconsin’s Lake Mendota is transformed by the changing seasons – covered by ice in winter and algae in summer – and a new study shows how these cyclical shifts throw the lake’s bacteria into evolutionary loops.
Led by researchers from the University of Texas at Austin, the team behind the study analyzed 471 lake microbe samples collected over 20 years, looking at genetic variations within and between species over time.
The data showed that thousands of bacterial species evolved over generations and then evolved again to a nearly identical state as the seasons changed. Since microbes live for only a few days, we are talking about genetic evolution that crosses thousands of generations within a year.
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These cycles also repeated year after year, like a video playing and then playing backwards, back to the original starting point. Of the 2,855 bacterial genomes studied, cyclical seasonal changes were found in 80 percent of them.
“This study is a total game changer in our understanding of how microbial communities change over time,” said marine scientist Brett Baker of the University of Texas at Austin.
“This is just the beginning of what these data will tell us about microbial ecology and evolution in nature.”
As environmental conditions changed, the analysis found, different bacterial strains could flourish and become dominant — only to lose ground to other strains as the seasons receded. About 20 percent of the species studied showed a more lasting pattern of genetic change, spanning decades.
This was especially evident in 2012, when the lake experienced a summer that was hotter and drier than normal. The team found a substantial shift in the genes that interact with the bacteria nitrogen metabolism that year – most likely due to lower levels of nitrogen-producing algae, a result of less water flowing into the lake.
Using a supercomputer to greatly speed up the process, the researchers assembled so-called metagenomes from each water sample, a technique designed to closely examine genetic sequences from DNA fragments over time.
“Imagine that the genome of every species is a book, and that each tiny fragment of DNA is a sentence,” says microbial ecologist Robin Rohwer of the University of Texas at Austin. “Each example contains hundreds of books, all broken down into these sentences.
“To put each book back together, you have to figure out which book each sentence came from and put them back together in the correct order.”
The findings suggest that ecology and evolution appear to work together rather than working separately – and that’s an important perspective for researchers, especially when studying a planet that continues to warm.
Knowing which bacterial strains will dominate helps scientists figure out how much carbon lakes can absorb, for example — and how aquatic food chains might be affected by adaptations to warmer seasons.
“Climate change is causing seasons and average temperatures to change slowly, but it is also causing more abrupt, extreme weather events,” says Rohwer.
“We don’t know exactly how microbes will respond to climate change, but our study suggests they will evolve in response to both these gradual and abrupt changes.”
The research was published in Natural microbiology.
