Did its great-great-great-great-great-grandson grow back to look like it? The creatures in this lake have been trapped in the same year

Every February 2nd, North America welcomes the traditional festival - Groundhog Day. People predict when spring will come by observing whether the groundhog sees its own shadow. In the movie "Groundhog Day", the protagonist Phil Connors is trapped in a time loop. No matter how hard he tries, he will return to this day every day when he wakes up.

This "ghost wall" experience sounds like an absurd setting in a movie, but there are actually similar phenomena in nature, except that the protagonists are not humans, but microorganisms that are difficult to see with the naked eye. In January this year, a study was published in Nature Microbiology, telling such a "cyclic story" that took place in Lake Mendota, Wisconsin, USA.

Bacteria's Groundhog Year

Lake Mendota is a freshwater lake with four distinct seasons. In summer, the lake is covered with algae, and in winter, it freezes. Over the past 20 years, a group of scientists have collected 471 water samples here. This time, they analyzed the microbial genomes in these samples and eventually constructed the longest-spanning metagenome (all genetic material sequences in the environment) dataset from the natural environment.

After analyzing the data, the researchers discovered an astonishing phenomenon: the bacteria in the lake actually showed a seasonal cycle pattern. From spring to winter, their genomes kept changing, but in the spring of the following year, everything returned to the starting point. Year after year, it seemed as if they were trapped in a time loop.

You might think, isn't this just the cycle of seasons in nature? Humans wear different clothes throughout the year, and start all over again at the beginning of the new year. Isn't it the same for bacteria? But the key here is the time scale: bacteria reproduce very quickly, and thousands of generations have already passed in a year.

So the cycle is not a cycle experienced by an individual, but rather a whole population repeating the same pattern over and over again as it evolves through the generations. In other words, it's like you look exactly like your great-great-great-great-grandfather, and your great-great-great-great-grandchild looks like you again, and so on and so forth.

Bacteria "trapped in the same year" (Image source: original paper)

What's even more surprising is that this phenomenon is not limited to individual bacteria, but widespread. The researchers analyzed a total of 2,855 species of bacteria and found that 80% of them showed similar seasonal genomic changes. "I was surprised that such a large proportion of bacterial communities would experience such changes," said Robin Rohwer, the first author of the study and a postdoctoral researcher at the University of Texas at Austin. "I thought I would only observe a few unusual examples, but I found hundreds or even thousands."

So why does the bacterial community show such a seasonal cycle? The researchers speculate that this may be related to the distinct four seasons in Lake Mendota. Even for the same bacteria, different individuals may dominate in different seasons: some strains may be more adaptable in the summer, while others may perform better in the winter. As the seasons change, the dominant members of the bacterial community continue to alternate, forming a dynamic pattern.

Crossing the stairs

In addition to this seasonal cycle, the researchers also found long-term genomic changes in 20% of the bacterial species in the lake , but these trends often overlapped with seasonal patterns. Moreover, the long-term change patterns of different bacterial species were different, some gradually evolved over decades, some changed suddenly like "stepping across", and others quickly returned to their original state after a short-term change.

Among these change patterns, "step-by-step" mutations were the most common and much more common than the other two patterns. For example, in 2012, many bacteria in Lake Mendota underwent a dramatic change in their genomes, especially genes related to organic nitrogen metabolism.

As for why the genomes of these bacteria suddenly changed, scientists have proposed some possible explanations. One reason may be extreme weather . In 2012, Lake Mendota experienced unusually warm and dry weather, which caused a sharp drop in the number of algae in the lake, which is an important source of organic nitrogen for bacteria. The lack of this key nutrient source may have prompted the bacterial community to undergo adaptive changes.

In addition, species invasion may also be one of the reasons.

In 2009, Lake Mendota welcomed an "uninvited guest" - Bythotrephes cederstroemi, an invasive zooplankton, whose proliferation may have caused water hypoxia. Due to the lag effect of species invasion, they may change the composition of bacterial communities in the water three years later. Although the researchers only deeply analyzed the mutation process of one of the bacteria, they speculate that other microorganisms may have undergone similar adaptive changes.

Different bacterial genomes have different patterns of change (Image source: original paper)

In the past, scientists rarely tracked changes in microbial communities over such a long time scale. They usually focus on samples at a specific moment, so the long-term evolutionary trends of microbial communities are often obscured by short-term seasonal fluctuations. This study reveals these overlooked deep evolutionary patterns by studying 20 consecutive years of data.

More importantly, this study changes our understanding of how microorganisms evolve over time. In the past, people tended to think of evolution as a force that constantly pushes forward. But this study shows that evolution does not always move forward linearly. Sometimes it also goes back and forth along a specific trajectory , as if time has drawn a "reincarnation" in the microbial world.

This study also raises an intriguing question: Is the dynamic change of microbial communities an ecological phenomenon or an evolutionary process? Traditionally, ecological research focuses on the interactions between different species, while evolutionary research focuses on genetic changes within species. However, in the microbial world, the boundaries between species are inherently blurred, and the dividing line between ecology and evolution is therefore difficult to clarify.

For example, scientists used to consider population competition and niche differentiation as typical ecological phenomena, but if the research is based on changes inferred from genomic data, should such research be classified as evolution? For another example, positive selection of organic nitrogen metabolism genes is generally considered an evolutionary process, but if this change affects the ecological transition between strains with different phenotypes, does it belong to ecology or evolution? Furthermore, if ecology and evolution occur on the same time scale, driven by similar environmental pressures, and the boundaries between microbial species are so blurred, do we need to rethink the traditional boundaries between evolution and ecology?

Ecology and evolution may be different chapters of the same story. And future research by scientists will help us understand the history of life more deeply.

References

[1]https://www.nature.com/articles/s41564-024-01888-3

[2]https://phys.org/news/2025-01-lake-bacteria-evolve-clockwork-seasons.html

Planning and production

Source: Global Science (ID: huanqiuekexue)

Author: Huang Yujia

Editor: Wang Mengru

Proofread by Xu Lailinlin

Note: The cover image is a copyrighted image. Reprinting it may lead to copyright disputes.