The ocean’s most prevalent bacteria, long thought to be perfectly suited for surviving in nutrient-scarce waters, may actually face greater challenges from environmental changes than previously understood. These bacteria, referred to as SAR11, are a dominant presence in surface seawater across the globe and can account for up to 40% of all marine bacterial cells. Their remarkable success is linked to a process known as genome streamlining, where organisms shed unnecessary genes to conserve energy when nutrients are limited.
However, recent research published in Nature Microbiology reveals that this high level of efficiency may come with significant drawbacks. "SAR11’s remarkable ability to adapt and thrive in stable, low-nutrient conditions might have inadvertently made them more susceptible to fluctuations in the ocean environment," explains Cameron Thrash, a professor of biological sciences and Earth sciences, who also serves as the corresponding author of the study.
The Hidden Flaw in Adaptation
In their investigation, researchers examined hundreds of SAR11 genomes and found that many of them lack critical genes necessary for regulating the cell cycle, which is essential for coordinating DNA replication and cell division. In most bacterial species, these regulatory genes are vital for sustaining healthy growth. When faced with changing environmental conditions, the absence of this regulation seems to lead to serious cellular malfunctions for SAR11.
Scientists had previously noted SAR11's sensitivity to environmental shifts, but what truly astonished the researchers was the manner in which these bacteria reacted to stress. Instead of merely slowing down their growth, many SAR11 cells continued replicating their DNA without successfully dividing. "We observed a disconnection between DNA replication and cell division. The cells kept duplicating their DNA yet failed to divide correctly, leading to the formation of cells with abnormal chromosome counts," remarks Chuankai Cheng, a PhD candidate in biological sciences and the lead author of the study. "The emergence of such a distinct and repeatable cellular signature was unexpected."
These atypical cells, which sometimes contained extra chromosomes, tended to enlarge and ultimately die. Consequently, even when nutrients were abundant, the overall growth rate of the population declined, challenging established notions about microbial proliferation.
The research also sheds light on why SAR11 populations frequently diminish during the later phases of phytoplankton blooms, a period characterized by an increase in organic matter. "For quite some time, we have realized that these organisms aren’t particularly well-equipped for the later stages of phytoplankton blooms," Thrash notes. "Now, we have a clearer explanation: Those late stages lead to a surge in new, dissolved organic matter that disrupts these bacteria, making their survival more challenging."
Looking Ahead for SAR11 Bacteria
The implications of this study extend beyond just SAR11, offering vital insights into how climate change and marine ecosystems interact. Given that SAR11 plays a crucial role in the ocean's carbon cycling, their heightened sensitivity to warming temperatures and nutrient surges could significantly alter microbial communities as oceanic conditions become increasingly erratic.
“This research emphasizes a new dimension of how environmental change can impact marine ecosystems—not merely by limiting resources but by disturbing the internal workings of key microorganisms,” Cheng states. He further adds that as environmental stability diminishes, organisms that possess greater regulatory flexibility may gain a competitive edge.
Looking ahead, researchers plan to delve deeper into the molecular mechanisms that underlie these disruptions. This ongoing work aims to enhance our understanding of SAR11’s contribution to marine carbon cycling, a pursuit that becomes increasingly urgent in light of the organism’s vast prevalence in ocean ecosystems.
