A group of ocean bacteria long considered perfectly adapted to life in nutrient-poor waters may be more vulnerable to environmental change than scientists realized.
The bacteria, known as SAR11, dominate surface seawater worldwide and can make up as much as 40% of marine bacterial cells. Their success is tied to genome streamlining, an evolutionary process in which organisms lose genes to reduce energy costs in nutrient-limited environments.
A new study published in Nature Microbiology suggests that this extreme efficiency comes at a cost, however.
"SAR11's extraordinary evolutionary success in adapting to, and dominating, stable low-nutrient environments may have left them vulnerable to oceans that experience more change. They may have evolved themselves into a bit of a trap," says Cameron Thrash , professor of biological sciences and Earth sciences and corresponding author of the study.
Adaptation with a flaw for SAR11 marine bacteria
The researchers analyzed hundreds of SAR11 genomes and discovered that many lack the genes normally required to control the cell cycle, a process that coordinates DNA replication and cell division. In most bacteria, these genes are essential for healthy growth. Under changing environmental conditions, that missing regulation appears to cause serious cellular problems for SAR11.
Their sensitivity to environmental changes has been observed before by scientists. What surprised the researchers was how SAR11 cells responded to stress. Rather than simply slowing growth, many cells continued copying their DNA while failing to divide.
"Their DNA replication and cell division became uncoupled. The cells kept copying their DNA but failed to divide properly, producing cells with abnormal numbers of chromosomes," says Chuankai Cheng, a PhD candidate in biological sciences and lead author of the study. "The surprise was that such a clear and repeatable cellular signature emerged."
These abnormal cells, which carried extra chromosomes, often became enlarged and eventually died. As a result, overall population growth slowed even when nutrients were plentiful, a finding that challenges common assumptions about microbial growth.
The findings also help explain why SAR11 populations often decline during the later stages of phytoplankton blooms, when organic matter increases.
"We have known for a long time that these organisms are not particularly well suited to late stages of phytoplankton blooms," Thrash says. "Now we have an explanation: Late bloom stages are associated with increases in new, dissolved organic matter that can disturb these organisms, making them less competitive."
What's next for SAR11 bacteria
The study has broader implications for understanding climate change and marine ecosystems. SAR11 bacteria play a major role in ocean carbon cycling, and their sensitivity to warming and nutrient pulses could reshape microbial communities as oceans become more variable.
"This work highlights a new way environmental change can affect marine ecosystems, not simply by limiting resources, but by disrupting the internal physiology of dominant microorganisms," Cheng said. As environmental stability declines, he added, organisms with greater regulatory flexibility may gain an advantage.
Researchers say future work will focus on uncovering the molecular mechanisms behind these disruptions. Their work will help improve our understanding of SAR11's role in marine carbon cycling, an effort made critical by the organism's sheer abundance.
About the study
In addition to Cheng and Thrash, the study's authors include Brittany Bennett, Pratixa Savalia, Hasti Asrari, Carmen Biel and Kate Evans at USC Dornsife; and Rui Tang of the University of California, San Diego.
This research was supported by a Simons Foundation Early Career Investigator in Marine Microbial Ecology and Evolution Award and a Simons Foundation Investigator in Aquatic Microbial Ecology Award .
