Deep ocean earthquakes drive Southern Ocean’s massive phytoplankton blooms, study finds

Stanford University

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Deployment of an instrument used to collect water samples from different ocean depths in the northern Ross Sea to determine their iron concentration.

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Credit: Image credit: Gert van Dijken

Stanford researchers have uncovered evidence that deep underwater earthquakes can spur the growth of massive phytoplankton blooms at the ocean surface.

Phytoplankton are microscopic, plant-like organisms that float in upper ocean layers and serve as the foundation of the oceanic food chain. They also store carbon dioxide pulled from the air and supply a large amount of the planet’s oxygen.

The new findings, published Dec. 9 in Nature Geoscience, point to a previously unknown relationship between the ocean floor and life at the surface.

Building on a 2019 discovery that iron from underwater hot springs, called hydrothermal vents, fuels phytoplankton blooms in the vast Southern Ocean around Antarctica, the study authors set out to find out why a particular bloom varied so dramatically in productivity year-to-year. Productivity refers to the rate at which algae convert light, carbon dioxide, and nutrients into biomass, with higher productivity leading to denser, more extensive blooms.

“When looking back over satellite observations of this bloom, we’ve seen it swell to the size of the state of California or down to the size of Delaware,” said lead study author Casey Schine, who conducted some of the research as a PhD student in the lab of Kevin Arrigo at the Stanford Doerr School of Sustainability and is now a postdoctoral research associate at Middlebury College. “Our study ultimately showed that the main factor controlling the size of this annual phytoplankton bloom was the amount of seismic activity in the preceding few months.”

The researchers reviewed earthquake records to test a theory that increased seismicity might cause the hydrothermal vents to emit extra iron and heated fluid that can bring that iron more easily to surface waters. They found that earthquake records did indeed strongly overlap with bloom patterns.

“This is the first ever study to document a direct relationship between earthquake activity at the bottom of the ocean and phytoplankton growth at the surface,” said Arrigo, senior study author and the Donald and Donald M. Steel Professor in the Doerr School of Sustainability.

A waxing and waning bloom

The study’s origins trace to a research cruise in 2014, when Schine, Arrigo, and colleagues sampled a large phytoplankton bloom along the Southern Ocean’s Australian Antarctic Ridge. This jagged rise is a little-explored part of the mid-ocean ridge system, a volcanically active underwater mountain chain that spans the globe.

Shortly after this expedition, other scientists discovered that hydrothermal vents dotted the area. The Stanford research team then reported in a 2021 study that their previously observed bloom happened to overlay a hydrothermal vent ridge some 1,800 meters below. After looking at satellite images going back to 1997, the researchers realized that this bloom always developed in the same place at the same time but was a notably different size each year.

That observation inspired the researchers to explore what could be driving the bloom’s reliable recurrence but fluctuating productivity. Other factors that influence nutrient availability, such as changes in sea ice and ocean surface temperature, did not fully explain the bloom’s year-over-year variability. “When we ruled out more obvious, possible drivers of this variation, we started thinking about the iron-nutrient sources themselves, the hydrothermal vents,” said Schine.

Previous research has demonstrated how earthquakes can boost vent activity. The shaking of the ground can alter vents’ internal plumbing, jarring open clogged conduits and cracking new paths for heated fluids to escape. Spikes in temperature from moving subterranean magma can also increase vent emissions and alter the chemistry of the dissolved minerals in the expelled fluid.

More earthquakes could therefore pump more iron into the Southern Ocean, Schine proposed. Because iron is known to be phytoplankton’s limiting nutrient in this region – that is, the essential nutrient in shortest supply – it followed that plumes rich in the metal would help the plant-like organisms thrive.

“Casey had what I thought was a crazy idea that maybe the number of earthquakes near the hydrothermal vent was controlling the release of trace metals into surface waters that could stimulate phytoplankton growth,” recalled Arrigo. “I figured that it was a long shot but told her to go for it. And it turns out that she was right!”

Correlations confirmed

To test her earthquake hypothesis, Schine connected with study co-author Jens-Erik Lund Snee, then a Stanford geophysics PhD student researching seismicity and tectonics. The team consulted records of earthquakes captured by multiple seismic monitoring stations in the region.

Those readings showed that when earthquakes of magnitude 5 or larger occurred in the few months before the Southern Hemispheric summer, the peak phytoplankton growth period, the eventual blooms grew far denser and more productive.

The study also found that the hydrothermal iron would have to ascend nearly 6,000 feet for uptake by plankton at the surface within a few weeks and no more than a few months to influence production on the observed timescales. The prevailing view has been that it would take upwards of a decade for hydrothermal iron to reach surface waters and likely thousands of miles from the original vent source. The transport process that causes the vent fluid to come to the surface so quickly and so close to the originating vents is the subject of continuing work. A recent expedition in December 2024 to the Australian Antarctic Ridge may lead to new insights.

Local and global effects

The new study paints a more complex ecological picture of the Southern Ocean: Earthquake activity could have a profound influence on the food web based on phytoplankton, which feeds the crustaceans and krill that support larger animals, including penguins, seals, and whales.

“We already know that marginal phytoplankton blooms beyond the sea ice around the Antarctic continent are an important feeding ground for whales; we’ve even documented humpback whales visiting the bloom in our new study,” Schine said. “So, there’s potentially more to the story now that we suspect seismic activity plays a role in bloom productivity.”

Because phytoplankton blooms pull carbon dioxide from the atmosphere, understanding the factors that drive their growth can help scientists improve models predicting how much carbon oceans may absorb in the future.

It’s not yet known, however, to what extent hydrothermal vents are impacted by earthquakes and may be powering blooms worldwide. “There are many other places across the world where hydrothermal vents spew trace metals into the ocean and that could support enhanced phytoplankton growth and carbon uptake. Unfortunately, these locations are difficult to sample and little is known about their global significance,” said Arrigo. “The more we learn about these systems, the better we will understand the capacity of the ocean to remove atmospheric carbon dioxide.”

Arrigo is also a senior fellow at the Stanford Woods Institute for the Environment. This research was supported by the National Science Foundation and NASA.

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Journal

Nature Geoscience

DOI

10.1038/s41561-025-01862-6 

Article Title

Southern Ocean net primary production influenced by seismically modulated hydrothermal iron

Article Publication Date

9-Dec-2025

Study reveals how ocean's most abundant bacteria diversify

University of Hawaii at Manoa

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Researchers sample surface seawater as a part of the Kāneʻohe Time-series. 

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Credit: Kelle Freel, HIMB/ SOEST/ UH Manoa

A groundbreaking study led by the University of Hawaiʻi at Mānoa's Hawaiʻi Institute of Marine Biology (HIMB) has revealed critical new details about one of the ocean's most abundant life forms, SAR11 marine bacteria. Understanding these microbes is vital because they are one of the main drivers of the global ocean's life-support system—they move and recycle the carbon and nutrients that sustain all other marine life. By better understanding them, scientists can more accurately predict how the entire ocean ecosystem—and the global climate—will react to threats like pollution and ocean warming.

The research, published in Nature Communications, found that the SAR11 bacteria are not a single, uniform population as often thought. Instead, they are organized into stable, ecologically distinct groups, essentially specialized "teams" adapted to specific environments, such as the coast versus the open ocean. This means that one of the ocean's most important engines is far more complex than previously known.

Using Kāneʻohe Bay as a natural laboratory, the team linked newly cultivated strains to ocean samples worldwide, showing that these distinct ecological groups differ significantly in habitat preference, gene content, and evolutionary history.

“Kāneʻohe Bay gave us a rare window into how microbial populations can adapt across very small spatial scales,” said Kelle Freel, lead author at HIMB. “By pairing cultivation with a long-term time series, we could directly connect genomes to real ecological differences in the ocean.”

SAR11 bacteria are tiny, streamlined cells that collectively represent one of the most abundant life forms in the ocean and play a central role in marine carbon and nutrient cycling. Despite their global importance, scientists have struggled to understand how SAR11 populations differ from one another, in part because these microbes are extremely diverse and very difficult to grow in the laboratory.

Kāneʻohe Bay provided a uniquely powerful model system to overcome 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 where these organisms live and how they survive.

“This work shows that SAR11 diversity is not random,” said Michael Rappé, principal investigator at HIMB. “By using Kāneʻohe Bay as a model system, we could integrate genomics with ecology in a way that reveals clear evolutionary structure - structure that holds across the global ocean and provides a common framework for studying one of the planet’s most important microbial groups.”

Ecologically distinct units

By culturing and sequencing the whole genomes of 81 new SAR11 isolates originating from coastal and offshore waters, the researchers tripled the number of complete genomes available for strains of this bacterial group. When these genomes were analyzed together with more than 1,300 marine metagenomes from oceans around the world, clear and repeatable ecological patterns emerged.

Rather than blending together as one large population, SAR11 bacteria consistently grouped into ecologically distinct units, whose members shared similar habitats and biological traits across space and time. 

A related study published in The ISME Journal revealed that whether SAR11 bacteria thrive near the coast or in the open ocean around Kāneʻohe Bay can depend on just a small number of genes under strong environmental selection. 

These findings show how small genetic differences can lead to significant ecological differences, helping explain how SAR11 maintains diversity despite large population sizes and global dispersal.

Caption

Researchers conducted field work in Kāneʻohe Bay to study SAR11 bacterial diversity.

Credit

Kelle Freel, HIMB/ SOEST/ UH Mānoa

Journal

Nature Communications

DOI

10.1038/s41467-025-67043-6 

Method of Research

Observational study

Article Title

New SAR11 isolate genomes and global marine metagenomes resolve ecologically relevant units within the Pelagibacterales

Article Publication Date

14-Dec-2025

 

Without campus leftovers to pick through, the beaks of this bird changed shape during the pandemic

Urban junco bills at UCLA became more like mountain junco bills

University of California - Los Angeles

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Key takeaways

• Dark-eyed juncos, a bird that typically live in mountain forests, have established thriving populations in Southern California cities, where they eat food people leave behind.

• A UCLA biologist studying the biological adaptations that help them survive an urban environment has found that the bill shape of this species became more like that of their non-urban counterparts in the absence of people during the pandemic closures at UCLA.

• After campus re-opened, the bills gradually returned to their previous shape, suggesting that the presence of people and their trash is driving the evolution of bill shape.

Juncos on the UCLA campus evolved a different bill shape during the pandemic closure, before returning to their previous shape once university life returned to normal, UCLA biologists who have been studying this population of birds for a decade reported in a new study. The finding, published in Proceedings of the National Academy of Sciences, shows how quickly traits can evolve when an important selective pressure is altered — in this case, humans making food easier to find.

“We have this idea of evolution as slow, because in general, over evolutionary time, it is slow,” said corresponding author and UCLA professor of ecology and evolutionary biology Pamela Yeh. “But it’s amazing to be able to see evolution happening before your eyes, and to see a clear human effect changing a living population.”

Dark-eyed juncos are a small member of the sparrow family that usually live in mountain forests. But as climate change shrinks their preferred habitat, Southern California’s juncos have surprised biologists by establishing themselves in cities and suburban neighborhoods. Non-urban juncos forage for seeds, but urban juncos have learned to exploit various crumbs, scraps and food waste that people leave lying around. Yeh’s lab has been banding, observing, measuring and taking blood samples from UCLA’s junco population to understand the adaptations that are making it possible for the birds to flourish in their city digs.

The new study found that during the pandemic closure, UCLA’s junco bills evolved to more closely resemble the bills of their forest relatives. Previously, Yeh and her students had seen relatively short, stubby bills. Over nearly two years of minimal human activity on campus, junco parents raised babies with gradually longer, more slender bills, which are typical of non-urban juncos.

“We were quite shocked, to be honest, when we saw just how strong that change was,” said first author Eleanor Diamant, who did the research as a doctoral student at UCLA and is now a visiting assistant professor at Bard College. “We think that that likely happened because when humans are not around, they’re not leaving their trash around and they’re not leaving their food around.”

At UCLA, juncos love to forage on plazas where students gather, patios around dining venues and walkways. When the food they used to find in these places disappeared in the absence of people, the birds were forced to forage in the more challenging landscape of bushes, grass and leaves like their non-urban counterparts. Leaving campus for easier pickings around supermarkets and the handful of other businesses that remained open during the pandemic closure was not a good option for UCLA’s juncos because these birds do not usually venture far from a territory they defend.

“We think that juncos with different bill shapes were more successful when campus was closed. Those with bills that improved their success at foraging for seeds probably got more food and raised more babies,” said Diamant.

But the change didn’t last. Once campus life returned to normal, the bills returned to their previous shape, which drove home the idea that the presence of people is having a strong effect on the evolution of bill shape in urban juncos.

“Wild animals have to work hard to find and get their food. When humans make it that much easier, the parts of their bodies, such as their mouths, that animals use for foraging adapt,” said Yeh. 

Although no one expected to see this change in juncos, the evolution of shorter snouts in other urban wildlife, such as rats and raccoons, has been documented. Given that an estimated 3 billion birds have vanished over the past 50 years — around 168 million of them juncos — their resilience offers hope for their future survival.

Yeh said that other urban birds like house sparrows and pigeons are in some ways pre-adapted to live with people because of their generalist diet, tendency to live in large flocks  and ability to nest off the ground in human structures. But juncos are territorial, ground-nesting birds that don’t usually thrive in cities.

“The fact that juncos are doing very well, popping up in cities all over Southern California, and that we are unintentionally changing them in a way that helps them survive around us is encouraging,” said Yeh. “I don’t feel like we have a lot of success stories when we think about how human behavior affects wildlife. I wouldn’t fully call it a success story yet, but it’s not a disaster story, and that’s no small thing.”

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Journal

Proceedings of the National Academy of Sciences