Campaign Visual - International Coccolithophore Day

Image: Campaign visual celebrating International Coccolithophore Day, combining an original coccolithophore illustration with a field photograph and clean graphic design for public outreach.
Credit: Illustration: Rosie Sheward; Photo: Dr Jelena Godrijan, Ruđer Bošković Institute; Design: Andrej Kajganić, Ruđer Bošković Institute

Microscopic plankton that regulate Earth’s climate and sustain ocean ecosystems take centre stage in a new awareness campaign.

Smaller than a speck of dust and shaped like tiny discs, coccolithophores are microscopic ocean organisms with a big climate job. They draw carbon out of seawater, help produce oxygen, and their calcite plates sink to form chalk and limestone that preserve Earth’s climate history. Today, scientists from the five European research organisations have launched an initiative to make 10 October International Coccolithophore Day, highlighting their crucial role in regulating the planet’s carbon balance, producing oxygen, and sustaining the ocean ecosystems that underpin all life.

The campaign is led by the Ruđer Bošković Institute (Zagreb, Croatia) Lyell Centre at Heriot-Watt University (Edinburgh, UK), NORCE (Norwegian Research Centre) (Bergen, Norway) and the Marine and Environmental Sciences Centre  (MARE) University of Lisbon (Portugal), and the International Nannoplankton Association (INA). 

A Delicate Balance Under Threat 

Most people have never heard of coccolithophores, yet without them, Earth’s climate and oceans would be profoundly different. These single-celled, chlorophyll-containing organisms drift in the sunlit surface waters, adorned with calcium carbonate plates called coccoliths.

Despite their microscopic size, coccolithophores are among the planet’s most powerful carbon processors. Each year, they produce more than 1.5 billion tonnes of calcium carbonate, removing carbon dioxide from the atmosphere and helping to store carbon in deep-sea sediments. They also produce oxygen, support marine food webs, and influence global climate by helping to regulate our planet's greenhouse effect.

Coccolithophores thrive and often dominate vast areas of the ocean. But climate change is altering water temperature, pH chemistry, and nutrient flows, threatening their survival and the ecosystems they support. 

Why Coccolithophore? 

Coccolithophores are unique among plankton due to both their role in regulating the global carbon cycle and the ability to track their long-term impact. “Unlike other groups, they build intricate calcium carbonate plates that not only help draw down carbon dioxide from the atmosphere, but also transport it into deep ocean sediments, where it can be locked away for millennia. This biomineralisation leaves behind an exceptional geological record, allowing us to study how they’ve responded to past climate shifts and better predict their future role. In short, their dual role as carbon pumps and climate archives makes them irreplaceable in understanding and tackling climate change,” says Professor Alex Poulton of the Lyell Centre.

“They are the ocean’s invisible architects, crafting the tiny plates that become vast archives of Earth’s climate,” says Dr Jelena Godrijan, a leading coccolithophore researcher at the Ruđer Bošković Institute. “By studying their past and current responses to changes in the ocean, we can better understand how marine ecosystems function and explore how natural processes might help us tackle climate change.''

Cutting-Edge Science: From Plankton to Planetary Processes 

The launch of International Coccolithophore Day spotlights the tiny ocean plankton that quietly help regulate atmospheric carbon dioxide.

At the Lyell Centre in Scotland, the OceanCANDY team, led by Prof. Alex Poulton, studies how these plankton pull CO₂ from the air and store it in the sea, and tests how warmer, more acidic oceans could alter this process. Computer forecasts compare which species do this job best, today and tomorrow.

In Norway, scientists at NORCE Research, led by Dr Kyle Mayers and his team, track coccolithophore life stories, how they grow, who eats them, and the viruses that infect and ultimately kill them, to show how carbon moves through the ocean. Ancient DNA in seafloor mud adds a long view of past climate shifts. “Coccolithophore interactions with viruses and grazers matter,” says Dr Kyle Mayers of NORCE. “These links shape food webs and how the ocean stores carbon.”

In Croatia, the Cocco team at the Ruđer Bošković Institute study how they shape the ocean’s carbon cycle, from the decay of organic matter to bacterial interactions that influence seawater chemistry and CO₂ uptake. “In understanding coccolithophores, we’re really uncovering the living engine of the ocean’s carbon balance,” says Dr Jelena Godrijan “Their interactions with bacteria determine how carbon moves and transforms—processes that connect the microscopic scale of plankton to the stability of our planet’s climate.” 

At MARE, University of Lisbon, Dr Catarina V. Guerreiro leads studies to trace how aerosol-driven fertilisation shapes the distribution of coccolithophores across the Atlantic into the Southern Ocean, and what that means for the ocean’s carbon pumps today and in recent times. Her approach consists of combining aerosol and seawater samples with sediment records, satellite data and lab microcosms to pin down cause and effect. “We’re connecting tiny chalky organisms to planetary carbon flows,” says Dr Guerreiro.

At INA, scientists connect living coccolithophores to their fossil record, using their microscopic plates to date rocks and trace Earth’s climate history. By refining global biostratigraphic frameworks and calibrating species’ evolutionary timelines, INA researchers transform fossils of coccolithophores into precise tools for reconstructing ancient oceans, linking modern plankton ecology with the geological record of climate change.

Why Coccolithophore Day Matters?

Designating a day for Coccolithophores may seem like a small gesture, but its advocates argue it could have a big impact. “This could contribute to changing the way we see the ocean. “We most often talk about whales, coral reefs, and ice caps, but coccolithophores are a vital part of the planet’s climate system. They remind us that the smallest organisms can have the biggest impact, and that microscopic life plays a crucial role in shaping our planet’s future,’’ says Dr Sarah Cryer from the CHALKY project and OceanCANDY team.

The campaign to establish October 10 as International Coccolithophore Day is a call to action. By highlighting the profound, yet often overlooked, role of coccolithophores, scientists want to inspire a new wave of ocean literacy, policy focus, and public engagement.

ADDITIONAL INFORMATION: 

The push for a global day of recognition is also an opportunity to showcase some of the research projects and associations that are deepening our understanding of these tiny but mighty organisms and their role in Earth’s climate future.

Celebrating coccolithophores means celebrating the bridge between the modern ocean and deep time—tiny organisms that both drive today’s carbon dynamics and write the archive geologists use to forecast our future.

At the Lyell Centre: Tracking the Carbon Highway

The OceanCANDY research group at the Lyell Centre are involved in several major projects that investigate how coccolithophores shape the ocean’s carbon cycle.

  • CHALKY explores how the alkalinity associated with coccolithophore growth and death in their large-scale blooms affects the exchange of carbon dioxide between the ocean and the atmosphere. Led by OceanCANDY and funded under the UK’s BIO-Carbon strategic research programme, CHALKY unites scientists from across the UK, Europe (including NORCE), the US, and Canada to better understand how biology controls the ocean’s ability to absorb and store CO₂.
  • OceanICU takes this work further, examining the ocean’s role in the carbon cycle on regional and global scales. The OceanCANDY team co-leads efforts to assess how climate “multistressors” — such as warming, acidification, and nutrient shifts — influence the biological carbon pump. Their work also includes mapping the scale and distribution of the carbonate pump, driven by organisms like coccolithophores, foraminifera, and pteropods.
  • CoccoTrait zooms in even further, studying how species-level differences in coccolithophores affect their climate impact. The project has built global databases on species distribution and carbon content, and is now using machine learning to model species-specific calcite production — critical data for future climate models.

At MARE: Aerosol Fertilisation and the Ocean’s Carbon Balance

At MARE in Lisbon, scientists are tracing the effects of dust and currents that shape coccolithophore lives, and their carbon afterlives.

  • CHASE is a multidisciplinary project exploring how atmospheric, oceanic, and ecological processes drive the north–south distribution of coccolithophores across the Atlantic and Southern Oceans. It focuses on aerosol-driven ocean fertilisation and its impacts on coccolithophore dynamics, biogeochemistry, and the marine carbon cycle. Funded by the Portuguese Science Foundation (FCT-CEECIND CHASE), the work combines atmospheric aerosol and water-column samples from AMT, OPERANTAR, DUST, and Ocean ICU expeditions with sediment-trap records, satellite observations, microcosm experiments, and Lagrangian modelling outputs. Lead Investigator: Dr Catarina V. Guerreiro. Collaborators: IDL, PML, UEA, NIOZ, IOCAG, FURG.
  • In parallel, OAEPIIP, supported by the Carbon-to-Sea Initiative, is a global intercomparison assessing how Ocean Alkalinity Enhancement (OAE) may affect pelagic ecosystems. Bringing together 19 groups worldwide, it harmonises methods and datasets to evaluate OAE’s potential as a carbon dioxide removal pathway. Dr Guerreiro leads Portugal’s contribution, using plankton microcosms from Cascais Bay to test impacts on diversity, productivity, and carbon-cycling feedbacks, feeding results into modelling frameworks.

At NORCE: Life, Death, and the Ocean’s Memory

Scientists at NORCE are uncovering the ecological forces that shape their lives, and their afterlives.

  • The AEGIS project investigates the mortality of coccolithophores such as Gephyrocapsa huxleyi, exploring the complex predator-prey and virus-host dynamics that determine bloom cycles and the fate of carbon in the ocean.
  • AGENSI/ARCHIE looks deep into the past, using ancient DNA preserved in marine sediments to reconstruct historic plankton communities. Here, G. huxleyi serves as a marker of ice-free conditions in the Arctic and Antarctic, offering a powerful tool for understanding how marine ecosystems have responded to climate shifts over millennia.
  • i2B – Into The Blue is a €12.5M ERC Synergy project integrating geological records, ancient DNA, and climate models to reconstruct Arctic greenhouse states, refine drivers and feedbacks, and assess what an ice-free Arctic means for policy and society ( https://in2blue.eu/).

In Croatia: Testing Ocean Solutions in Real Time

Meanwhile, hundreds of kilometres to the south, scientists from the Ruđer Bošković Institute are working on several projects that have coccolithophores in their focus:

  • The Future Ocean project investigates the fate of organic matter produced by coccolithophores. In large-scale microcosm experiments, scientists are cultivating coccolithophore biomass under conditions that simulate future ocean scenarios to unravel how their organic matter decomposes and transforms over time.
  • The Cocco-Channel project is uncovering the hidden relationships between coccolithophores and the bacteria that live around them. Scientists are exploring how closely these bacteria stay connected to their hosts, how they interact with each other, and how these microscopic partnerships can affect the chemistry of seawater—including how much carbon dioxide the ocean can absorb.

OAEPIIP - the Ocean Alkalinity Enhancement Pelagic Impact Intercomparison Project is testing how a process called Ocean Alkalinity Enhancement—which increases the ocean’s natural ability to absorb carbon dioxide—affects marine life. Working with large 55-litre seawater tanks at the Martinska field site on Croatia’s coast, the team is tracking how local marine communities respond to these changes.