Bold claim: Ocean’s most common bacteria aren’t one uniform crowd but a constellation of specialized, ecologically distinct groups that adapt to local shores or the open sea. And this nuance changes everything we thought we knew about the ocean’s carbon and nutrient cycles. A new study led by the University of Hawaiʻi at Mānoa’s Hawaiʻi Institute of Marine Biology (HIMB) has uncovered these crucial details about SAR11 marine bacteria, one of the ocean’s most abundant life forms. Understanding SAR11 is essential because they drive the movement and recycling of carbon and nutrients that underpin the entire marine food web and shape global climate responses to threats like pollution and warming seas. The work, published in Nature Communications, shows that SAR11 are not a single, homogenous population. Instead, they organize into stable, ecologically distinct clusters—like teams tuned to particular environments, such as coastal zones versus open-ocean waters. This reveals a level of complexity in the ocean’s primary engine that researchers hadn’t fully appreciated before.
Using Kāneʻohe Bay as a natural laboratory, the researchers connected newly cultivated SAR11 strains to ocean samples from around the world, demonstrating that these ecological groups differ markedly in where they live, what genes they carry, and their evolutionary history.
“Kāneʻohe Bay gave us a rare window into how microbial populations adapt across very small spatial scales,” said Kelle Freel, the study’s lead author from HIMB. “By combining cultivation with a long-running time series, we could directly link genomic data to real ecological differences in the ocean.”
SAR11 bacteria are tiny, highly streamlined cells that collectively rank among the ocean’s most abundant life forms, playing a central role in marine carbon and nutrient cycling. Despite their global importance, scientists have struggled to understand how different SAR11 populations differ from one another, partly because these microbes are incredibly diverse and difficult to grow in the lab.
Kāneʻohe Bay provided a uniquely powerful model system to address these challenges. Years of sustained sampling through the Kāneʻohe Bay Time-series (KByT) allowed researchers to pair environmental measurements with newly grown SAR11 strains, creating an opportunity to connect microbial DNA with the organisms’ habitats and survival strategies.
“This work shows that SAR11 diversity is not random,” noted Michael Rappé, HIMB principal investigator. “Using Kāneʻohe Bay as a model system, we could integrate genomics with ecology in a way that reveals clear evolutionary structure—structure that persists across the global ocean and provides a common framework for studying one of the planet’s most important microbial groups.”
Ecologically distinct units
By sequencing the whole genomes of 81 new SAR11 isolates from both coastal and offshore waters, the team tripled the number of complete genomes available for this bacterial group. When these genomes were analyzed alongside more than 1,300 marine metagenomes from oceans worldwide, the data revealed clear and repeatable ecological patterns.
Rather than blending into a single, large population, SAR11 bacteria consistently grouped into ecologically distinct units, with members sharing similar habitats and biological traits across space and time.
A related study in The ISME Journal showed that whether SAR11 populations thrive near the coast or in the open ocean around Kāneʻohe Bay can hinge on a small set of genes under strong environmental selection. These findings illustrate how small genetic differences can drive meaningful ecological divergence, helping explain how SAR11 maintains diversity despite enormous population sizes and wide global dispersal.
Publication details
Journal: Nature Communications
Article: New SAR11 isolate genomes and global marine metagenomes resolve ecologically relevant units within the Pelagibacterales
Date: 14-Dec-2025
COI statement: The authors declare no competing interests.
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