Swedish freshwater bacteria give new insights into bacterial evolution

Bacteria are among the most diverse and ancient forms of life on Earth. Yet, much of what we know about them comes from a small group of species, mostly studied for their roles in human health.

“The vast majority of bacterial species remain unexplored, and this really limits our understanding of how bacteria shape ecosystems and have evolved to thrive in different environments,” says Joel Hallgren, lead author of the study.

Most bacteria reproduce through simple, symmetrical cell division. However, members of one distinctive group, the Caulobacterales, known for their “stalked” appearance, deviate from this pattern. They have a more complex lifecycle, involving asymmetric cell division that results in two distinct cell types: one mobile and explorative, and the other sessile and reproductive. The evolutionary reasons behind this complex lifecycle have long puzzled scientists. Caulobacterales bacteria are also known to be environmentally widespread and are thought to be important degraders of plant matter in nature, but their ecology and evolution have remained poorly studied.

Researchers at Stockholm University analyzed the DNA of all known Caulobacterales species, including newly collected samples from Swedish and Finnish forest lakes. They discovered that several freshwater species lacked more than a hundred genes typically linked to the group’s complex lifecycle. These bacteria represent three new species in a previously unknown genus, which the team named Acaudatibacter, Latin for “bacterium without a tail.”

Intriguingly, another soil Caulobacterales member, isolated in Ecuador, also lacked the same set of lifecycle genes and, as the researchers observed by microscopy, reproduces through simple, symmetric cell division. This independent loss of complexity in separate lineages provides insight into the set of genes essential for the complex lifecycles of bacteria.

“It’s fascinating to see that evolution has reversed lifecycle complexity multiple times in the same way,” says Joel Hallgren. “It gives us a unique genetic signature of how complexity arises, and disappears, during the evolution of bacterial lifecycles.”

The researchers also made another unexpected discovery: the Swedish lake bacteria possessed all the genes required for photosynthesis, a capability not previously known in Caulobacterales, highlighting that photosynthesis is more widespread among bacteria than previously thought. Indeed, further analysis revealed that roughly 10% of species in this group carry genes for harvesting light energy.

“It’s exciting that novel bacterial species from my own country are giving us new perspectives on fundamental concepts in microbiology,” says Hallgren.

The study was a close interdisciplinary collaboration between two research groups at Stockholm University, both based at SciLifeLab, a national infrastructure for life sciences, when the work began.

“This project combined my lab’s expertise in bacterial cell biology and genetics with the strong microbial genomics and ecology expertise of Sarahi Garcia’s group,” says senior author Kristina Jonas. “Our affiliation with SciLifeLab and the flexible funding it provided made it possible for us to explore this new research direction.”