A new study reveals that SAR11, the most abundant bacteria in the world's oceans, may be hindered by their own adaptations to nutrient-poor environments. Under stress, these microbes experience cellular failures that limit their growth, potentially affecting ocean ecosystems amid climate change. Researchers from the University of Southern California highlight this as a key weakness in these dominant lifeforms.
SAR11 bacteria dominate surface seawater globally, comprising up to 40% of marine bacterial cells in some areas. Their success stems from genome streamlining, an evolutionary tactic that involves shedding non-essential genes to save energy in low-nutrient conditions. However, a study published in Nature Microbiology in 2026 suggests this efficiency creates vulnerabilities when environments shift.
Led by PhD candidate Chuankai Cheng and corresponding author Cameron Thrash, both from the University of Southern California, the research analyzed hundreds of SAR11 genomes. It found that many strains lack genes crucial for cell cycle regulation, which oversees DNA replication and division. During environmental stress, such as nutrient surges, these bacteria continue replicating DNA without proper division.
"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," Cheng explained. These oversized cells with extra chromosomes often die, curbing population growth even when resources abound.
This mechanism explains a observed decline in SAR11 numbers during the late phases of phytoplankton blooms, when dissolved organic matter increases. "Late bloom stages are associated with increases in new, dissolved organic matter that can disturb these organisms, making them less competitive," Thrash noted.
The findings have broader implications for marine health and climate change. SAR11 plays a vital role in carbon cycling within ocean food webs. As oceans warm and fluctuate more, disruptions to these bacteria could reshape microbial communities. "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 added.
The study, supported by the Simons Foundation, underscores the need for further research into SAR11's molecular responses to instability.