New Research Reveals Vulnerability of Dominant Ocean Microbes

Scientists previously believed that SAR11 bacteria, a major group of ocean microbes, were well-adapted to environments with limited nutrients. New research indicates this assumption may be incomplete, suggesting these microbes are more susceptible to environmental shifts than initially thought.

SAR11 bacteria constitute the most abundant life forms in global surface seawater, comprising up to 40% of marine bacterial cells in some regions. Their prevalence is attributed to genome streamlining, an evolutionary process where genes are shed to conserve energy in nutrient-poor conditions. However, a study published in Nature Microbiology suggests this efficiency may also introduce significant limitations.

A Critical Genomic Anomaly

Researchers examined hundreds of SAR11 genomes and found that many strains lack genes typically responsible for regulating the cell cycle, which controls DNA replication and cell division. This absence appears to create issues when environmental conditions change.

Uncoupled Life Processes Under Stress

Under stress, many SAR11 cells continued to replicate their DNA but failed to divide, resulting in uncoupled DNA replication and cell division. This led to cells with abnormal numbers of chromosomes. Such cells often grew larger and eventually died, which reduced overall population growth, even when nutrients became available.

This finding challenges the idea that microbes will always thrive with abundant food.

Explaining Observed Declines

This research offers an explanation for the observed decline in SAR11 populations during the later stages of phytoplankton blooms, a period characterized by increased organic matter in the water. The new dissolved organic matter may disturb these organisms, making them less competitive.

Implications for Climate Change

These findings have implications for understanding how marine ecosystems may respond to climate change. SAR11 bacteria are crucial for ocean carbon cycling. Their sensitivity to warming and sudden nutrient inputs could alter microbial community balance as ocean conditions become less stable.

The study highlights that environmental change can affect marine ecosystems not solely by limiting resources but by disrupting the internal physiology of dominant microorganisms. Further research aims to identify the molecular processes underlying these disruptions.