Revolutionary research published in the prestigious journal Science provides us with a deeper insight into how ocean currents, known as the global overturning circulation, play a crucial role in shaping the diversity and function of microbial life throughout the South Pacific Ocean. This study, led by scientists from renowned institutions such as the J. Craig Venter Institute (JCVI), Scripps Institution of Oceanography at UC San Diego, and the University of California Berkeley, represents the most detailed genetic map to date, revealing how microbial communities are structured by the physical movement of ocean water.

Ocean Depths and the Influence of Currents

Winds and storms affect the ocean only down to a depth of approximately 500 meters (1,640 feet), which is merely one-eighth of the total ocean depth of 4,000 meters (13,125 feet), explains lead author Bethany Kolody, who graduated in oceanography from the Scripps Institution and is currently a postdoctoral researcher at Cal. Below 500 meters from the surface, currents are driven by differences in water temperature and salinity, which affect its density, creating the global overturning circulation. This circulation acts like a conveyor belt, transporting water – and the microbes within it – across vast distances and depths.

"Until now, it was unclear whether these water masses were also distinct microbial ecosystems," Kolody stated. "Now we can answer that question with a confident 'yes'."

The research team collected over 300 water samples along a transect from Easter Island in the South Pacific to Antarctica, covering the entire ocean depth. Using advanced metagenomic and metabarcoding techniques, they reconstructed genomes for over 300 microbes and identified tens of thousands of additional microbial species using a molecular "fingerprinting" technique that examines highly conserved genes – the 16S rRNA gene for prokaryotes (which includes bacteria and archaea) and the 18S rRNA gene for eukaryotes.

Their discoveries revealed a striking pattern: microbial diversity sharply increases at around 300 meters (1,000 feet) below the ocean surface in a zone they call the "prokaryotic phylocline." This layer, similar to a pycnocline (a zone of rapid density change), marks the transition from low-diversity surface waters to rich deep-sea microbial communities.

---

Six Microbial Cohorts and Functional Zones

The study, published on July 10, 2025, identified six distinct microbial "cohorts," three of which correspond to depths, and the other three are aligned with major water masses: Antarctic Bottom Water, Upper Circumpolar Deep Water, and Ancient Pacific Deep Water. Each cohort contains unique microbial species and functional genes, shaped by temperature, pressure, nutrient levels, and water age.

For example, the Antarctic Bottom Water cohort includes microbes adapted to cold, high-pressure environments, with genes that help maintain membrane fluidity and resistance to oxidative stress. In contrast, the ancient water cohort – found in slow-circulating water that has not seen the surface for a thousand or more years – hosts microbes with genes that enable life in low-oxygen environments and the degradation of complex, low-energy carbon compounds.

Beyond taxonomy, the researchers also mapped the functional potential of microbial communities. They identified ten "functional zones" based on the presence of key metabolic genes. These zones correspond to oceanographic features such as upwelling areas, nutrient gradients, and oxygen minimum zones.

Surface zones were rich in genes for photosynthesis, iron assimilation, and photoprotection – traits essential for life in the upper, sunlit part of the ocean. Deeper zones contained genes for the degradation of complex organic molecules, survival in low-oxygen conditions, and tolerance to environmental stress.

---

Key Role of Microbes in Earth's Carbon Cycle

Microbes are the drivers of the oceanic carbon cycle. They convert carbon dioxide into organic compounds (carbon fixation), recycle nutrients, and help sequester carbon in the deep sea (carbon sequestration). Understanding how their communities are structured by ocean circulation is crucial for predicting how climate change might alter these processes.

"The study provides a baseline for how microbial ecosystems are organized under current ocean conditions," said Andrew Allen, senior author of the study and a microbial oceanographer at JCVI and Scripps Oceanography. "As climate change impacts the global overturning circulation, the distribution and function of these microbial communities could change, with unknown consequences for the global carbon cycle."

By pairing genomic data with physical and chemical measurements, scientists can build a global, species-resolved atlas of ocean life – which is essential for understanding and protecting the planet's largest ecosystem.

"This study is a reminder that life in ocean ecosystems is, in part, driven by fundamental patterns and processes unknown to us," Allen added. "Seeing and understanding them requires us to examine them more sensitively, carefully, and thoroughly. The breakthroughs reported in this study are the result of a truly interdisciplinary effort involving physical oceanographers, biological oceanographers, and genomic biologists working very closely together. Agencies, such as the National Science Foundation, which support fundamental interdisciplinary ecological research in the life and earth sciences, continue to be essential for our ability to understand the factors controlling the distribution, diversity, metabolism, and evolution of organisms in nature."

The authors advocate for the inclusion of molecular sampling in global ocean monitoring programs like GO-SHIP.

In addition to Andrew Allen, researchers from Scripps Oceanography who participated in the study include Zoltán Füssy, Sarah Purkey, and Eric Allen.

The full study, "Overturning circulation structures the microbial functional seascape of the South Pacific," was published in the journal Science. This research was supported by the National Science Foundation, Simons Foundation, National Institutes of Health, Emerson Collective, Gordon and Betty Moore Foundation, and Chan Zuckerberg Initiative.

Source: University of California