One of the most abundant living organisms on Earth may also be one of the most vulnerable.
A group of ocean bacteria known as SAR11, which dominates surface seawater around the world, has long been considered perfectly adapted to life in nutrient-poor oceans.
But new research suggests that this remarkable efficiency may come with a hidden weakness.
SAR11 bacteria are everywhere. In some parts of the ocean, they make up as much as 40 percent of all marine bacterial cells.
Their success is largely due to an evolutionary strategy called genome streamlining.
Over millions of years, SAR11 lost many genes, allowing the bacteria to survive on very little energy in stable, low-nutrient environments. This stripped-down design made them incredibly efficient and widespread.
However, a new study published in Nature Microbiology shows that this extreme efficiency may have pushed SAR11 too far.
According to the researchers, the bacteria’s simplified genetic makeup appears to leave them poorly equipped to cope with environmental change.
By analyzing hundreds of SAR11 genomes, the scientists discovered that many strains are missing genes that most bacteria rely on to control their cell cycle. The cell cycle is the process that carefully coordinates DNA replication and cell division. In most organisms, this regulation is essential for healthy growth and survival, especially when conditions change.
Without these control genes, SAR11 bacteria behave in unusual ways when stressed.
Instead of slowing down growth when their environment shifts, many cells continue copying their DNA but fail to divide properly. This leads to cells with too many copies of their chromosomes.
The researchers observed that these abnormal cells often became larger than normal and eventually died. Even when nutrients were plentiful, the overall population grew more slowly because so many cells were malfunctioning. This finding challenges the common assumption that more nutrients automatically lead to faster microbial growth.
The study also helps explain a long-standing puzzle in ocean science. SAR11 populations are known to decline during the later stages of phytoplankton blooms, when organic material in the water increases.
Until now, the reason was unclear. The new research suggests that the sudden influx of dissolved organic matter during these blooms disrupts SAR11’s internal processes, making them less competitive than other microbes.
These findings have important implications for understanding how marine ecosystems may respond to climate change. SAR11 bacteria play a major role in the ocean’s carbon cycle, helping process vast amounts of carbon every day. As oceans warm and become more variable, with stronger nutrient pulses and shifting conditions, SAR11’s lack of regulatory flexibility could put it at a disadvantage.
The researchers emphasize that environmental change does not just affect organisms by limiting food or resources. It can also interfere with basic internal processes, such as how cells grow and divide. In a future ocean that is less stable, microbes with more flexible genetic control systems may thrive, while even the most abundant specialists could struggle.
Future research will focus on understanding exactly how these disruptions occur at the molecular level. Given SAR11’s sheer abundance and importance, learning how it responds to a changing ocean is critical for predicting the future of marine life and the planet’s carbon balance.
