Ocean Acidification

An ongoing research project exploring the effects of rising levels of oceanic CO₂ on squid neurology reveals that exposure to future levels of ocean acidification could shrink their brain volume by around 50%. This severe brain shrinkage appears to be most pronounced in the areas that interpret visual information, correlating with significant reductions in normal feeding behaviours and suggesting serious consequences for the future of squid and other cephalopods.

“Cephalopods are widely regarded as being one of the most intelligent groups of animals living in the ocean,” says Dr Garett Allen, an assistant professor at Acadia University, Canada. The Coleoidea subclass of cephalopods that includes squid, cuttlefish and octopuses are believed to be the most intelligent invertebrates on Earth, possessing a similar number of neurons to dogs.

Bigfin reef squid in the research tank. Credit: Su Huai

Ocean acidification caused by elevated levels of atmospheric CO₂ is known to be pose a serious threat to many marine species, but this project, presented at the Society for Experimental Biology conference in Florence, Italy, reveals a previously unknown impact of ocean acidification on cephalopod neural anatomy.

Preliminary data from this study suggests that bigfin reef squid, Sepioteuthis lessoniana, reared from hatching within elevated levels of dissolved CO₂ exhibited significant changes in brain physiology – the most surprising being an average 49% reduction in brain volume compared to a control group.

To investigate the effects of future ocean acidification on the squid neurology, collaborations between Dr Allen and project lead Dr Yung-Che Tseng whose his team is located at Academia Sinica’s Marine Research Station reared the squid in one of two parallel water tanks: one representing the modern oceans (pH 8.2) and one representing the oceans in the year 2100 under a predicted climate change scenario (pH 7.8). After 90 days, the squid were removed and their heads were preserved for visual analysis with diffusion magnetic resonance imaging (dMRI).

Once the images were visualized and the brain morphometrics could be assessed, Dr Allen discovered that the brains from the ocean acidification tank squid were substantially smaller than the control tank squid. “I immediately saw that their brains were half the size and had to check the diagnostic output of the software,” says Dr Allen. “It was a real surprise – I wasn’t expecting that at all.”

The study found no effect of CO₂ on whole body size, so the brain volume was normalised to mantle length to account for variation in body size. This volume reduction was observed across the whole of the brain, but the greatest reductions were found in regions identified as the optic lobes and optic tracts, which were 52% and 62% less voluminous than squid reared under modern ocean conditions, respectively.

These new findings follow on from an earlier study that linked rising CO₂ levels to reductions in hunting behaviours in bigfin reef squid, finding that an acute 7-day exposure to high CO₂ levels resulted in a 65% reduction in hunting behaviours, and squid exposed to a full 90 days exposure from hatching showed a 42% reduction in hunting behaviours compared to controls.

The ability to rapidly capture and interpret visual information is vitally important for bigfin reef squid, as they rely on their eyesight for tracking and catching their prey. “We think that the reduced willingness to feed may be linked to a decline in visual acuity,” says Dr Allen. “Not because of the retina itself, which looks to stay the same, but perhaps because the optic lobe is shrinking.”

The factors causing this brain volume shrinkage are still being investigated, but Dr Allen believes that they may be likely due to energetic constraints within the brain or oxidative damage, which could mean that the brain is unable to relay information correctly and lead to the abnormal feeding behaviour that the team have observed.

Dr Allen and Dr Tseng are currently carrying out further studies on the brains of squid reared in the same future conditions at 30 and 60 days to further validate these findings and examine how this neural shrinkage manifests as the squid grows and matures.

Published in the prestigious journal Environmental Pollution and co-authored by ISPRA researchers, the study “Climate Change and Ocean Acidification Outweigh Local Stressors in Mediterranean Mussels: A Multi-Method Convergence Analysis” presents the results of a ten-year biomonitoring programme (2014–2023) carried out in the Ligurian Sea.

The research focused on the Mediterranean mussel (Mytilus galloprovincialis) at Gorgona Island (used as the reference site) and near an offshore regasification terminal. By integrating biomarker data, tissue metal concentrations, and high-resolution oceanographic variables, the study quantified the relative contribution of different environmental stressors affecting mussel health using five complementary statistical approaches.

The study’s main finding is that climate change—particularly ocean acidification—emerged as the primary driver of biological stress, accounting for approximately 40% of the observed variance. This significantly exceeded the contribution of metal contamination (around 30%) and the local influence of the offshore regasification terminal (approximately 13%). Notably, about 64% of the climate-related impact was found to operate indirectly by enhancing the bioaccumulation of metals.

Climate change and ocean acidification outweigh local stressors in Mediterranean mussels: A multi-method convergence analysis

International institutions managing fisheries and climate change operate in silos, leaving a critical gap in protection for communities dependent on blue foods.

London, 18^th June 2026 – A new study, launched during Our Oceans Conference being held in Mombasa, finds that international governance frameworks are poorly equipped to address the combined threat of ocean acidification (OA) and blue food insecurity — despite both issues becoming increasingly urgent for global sustainability. The research, published in Environmental Research: Food Systems, maps the international governance landscape for both issues and finds that, while the two policy domains overlap significantly, they are rarely addressed together.

Annika Frosch, lead author and Research Fellow at the UCL Energy Institute Shipping and Oceans Research Group, said: “Without coordinated action across climate and food systems, the growing impacts of ocean acidification will continue to undermine fisheries and aquaculture that underpin global nutrition and livelihoods.”

The thematic nodes represent issue areas related to blue food security (blue) and ocean acidification (green). The nodes in the centre are those that apply to both blue food security and ocean acidification, and issues that were identified as critical governance areas at the OA-blue food nexus. The fisheries, aquaculture, and climate change nodes represented in red are discussed further in the paper.

Blue foods — fish, shellfish, and algae sourced from marine and freshwater environments — provide essential nutrients to millions of people worldwide, and global consumption has grown more than fivefold over the past 60 years. At the same time, ocean acidification, driven primarily by rising CO₂ emissions, is already harming shellfish aquaculture and fisheries, degrading coral reef ecosystems, and altering the nutritional quality and safety of seafood. Communities in the Global South, which rely most heavily on blue foods for daily protein, face the greatest risks — yet also receive the least research attention and policy support.

Using a structured review of international governance literature and a thematic mapping exercise, the researchers identified thirteen policy domains relevant to blue food security and ten relevant to ocean acidification, seven of which overlap — including fisheries and aquaculture, biodiversity, climate change, and land-based pollution.

Despite this overlap, the study finds that most international actors and instruments treat the two issues in isolation. A detailed examination of two central institutions — the Food and Agriculture Organization (FAO) and the UN Framework Convention on Climate Change (UNFCCC) — illustrates the problem clearly:

• The FAO lacks an explicit mandate to govern ocean acidification. When OA is mentioned in FAO instruments, it typically appears as one item in a longer list of climate stressors, rather than being treated as a distinct and direct threat to food security.
• The UNFCCC addresses ocean acidification only indirectly, through its broader CO₂ mitigation goals. Neither the UNFCCC nor the Paris Agreement explicitly names OA, and it does not feature as a core governance priority. Food security is referenced in UNFCCC objectives but is not linked to OA even in documents that mention both.

The study also highlights a troubling asymmetry: blue food security tends to receive legal and political recognition, while ocean acidification remains largely confined to scientific discussion — making coordinated policy action harder to achieve.

Keiko Nomura, co-author and Postdoctoral Researcher at the University of Colorado Boulder, said “Our findings suggest that ocean acidification remains more visible in scientific discussions than in international policy frameworks. Bridging this science-policy divide could help improve governance of climate risks to blue food systems.”

The paper makes recommendations for policy makers to bridge these governance gaps, including:

• Strengthening FAO instruments to treat OA as a distinct climate-related stressor to fisheries and aquaculture, including through binding policies, vulnerability thresholds for key species, and the promotion of socioeconomic strategies.
• Developing a dedicated OA workstream or technical body under the UNFCCC
• Incentivising countries to include OA monitoring and adaptation measures in their Nationally Determined Contributions (NDCs)
• Creating formal institutional links and coordination between climate bodies and marine science and fisheries organisations modelled on the Joint Capacity-building Programme of the three ‘Rio Conventions’, adapted for the OA–blue food security nexus

Inken Dressler, co-author and European Programme Lead at the International Alliance to Combat Ocean Acidification (OA Alliance): “The siloed recognition and approach to ocean acidification and blue food security hinders effective governance of both issues. To bridge this gap, closer cooperation between existing frameworks such as the UNFCCC and FAO must be achieved.”

The researchers note that some Pacific Island nations — including Fiji and the Solomon Islands — are already leading the way by integrating ocean acidification into their national climate strategies and fisheries management plans, demonstrating what more coherent governance could look like.

The study accompanies Dr Frosch’s new book ‘Navigating the Souring Seas: The Global Experimentalist Governance of Ocean Acidification’ which explores how OA is being addressed at the global stage. Bridging science, law, and international policy, this interdisciplinary book provides a clear overview of the scientific background of OA and maps the international governance landscape, identifying it as a regime complex. Through detailed interview-based case studies of the Ocean Acidification Alliance and the International Maritime Organization, the book evaluates real-world efforts to govern OA and highlights how flexibility, learning, and multilevel collaboration, can enhance their effectiveness.

CO2 hydrates releasing droplets of liquid CO2, filmed in 2021 at a depth of 1,367 meters by the Victor 6000 ROV in the Fer à Cheval area during the Geoflamme campaign aboard the Pourquoi Pas. (Image credit: Ifremer)

More than 120 CO2 hydrate deposits were discovered at the Fer à Cheval site, located 10 km east of Petite-Terre (Mayotte), during the Geoflamme expedition co-led by Ifremer and the Paris Institute of Earth Physics (IPGP) in 2021. No comparable site had ever been documented before. Published in Nature Geoscience, the study shows that this site is unique worldwide for investigating the mechanisms of transient CO2 sequestration in the ocean and the impacts of ocean acidification on biodiversity.

The data collected on these CO2 hydrates discovered in the Indian Ocean were analyzed by an international team from Ifremer, IPGP, the French Alternative Energies and Atomic Energy Commission (CEA), the French National Center for Scientific Research (CNRS), the National Oceanic and Atmospheric Administration (NOAA), and the University of Milan.

Solid CO2 Deposits at the Bottom of the Ocean

Hydrates are solid compounds similar to ice, consisting of water and gas molecules. In nature, hydrates are usually composed of methane, and it is extremely rare to find carbon dioxide hydrates on the ocean floor.

Cécile Cathalot, marine geochemistry researcher at Ifremer and the study’s lead author, said: “This is the first time we have observed clusters of CO2 hydrates that remain stable for several years on the ocean floor, of this size and in such quantities. Composed of agglomerated CO2 droplets, these domes range in height from a few centimeters to 2 meters. This discovery raises new questions about the natural mechanisms of temporary CO2 storage in the ocean. It could also fuel discussions on certain geoengineering approaches aimed at limiting climate change.”

These hydrates were observed within the active Fer à Cheval volcanic structure, located 10 km east of the island of Mayotte. Surrounded by cliffs reaching 250 meters in height, this 6 km² underwater feature is one of many structures in the underwater volcanic chain that extends east of Mayotte to the Fani Maore underwater volcano. It forms a semi-enclosed space within which CO2 released onto the seafloor accumulates periodically with the tides.

Furthermore, this site offers the conditions necessary for the formation of hydrates: the combination of cold water—here at 4 degrees Celsius—and sufficient pressure exerted by the water column at a depth of 1,400 meters.

Olivia Fandino, a specialized physical chemistry of gas hydrates researcher at Ifremer, said: “At the Fer à Cheval site, CO2 hydrates form when droplets of liquid CO2 come into contact with cold water under high pressure. A solid film then develops on their surface, the growth of which depends closely on temperature, salinity, and emission rate. What is remarkable here is that, despite the ocean currents, these hydrates were able to grow and form large, relatively stable structures.”

Structures Associated with the Fani Maore Volcano

It is likely that the emergence of these magmatic sources of liquid CO2 in the Fer à Cheval area is linked to the seismic-volcanic crisis affecting the island of Mayotte, which was notably marked by the formation of the new Fani Maoré volcano discovered in 2019. This activity likely destabilized the volcanic structure of the Fer à Cheval, which formed long before the eruption of Fani Maoré.

Unlike Fani Maoré, which has shown no activity since 2021, the Fer à Cheval site remains highly active in terms of seismicity and fluid emissions, particularly CO2.

A joint campaign conducted by Ifremer and OceanX made it possible to revisit this site of interest four years later.

Carla Scalabrin, a specialized water-column acoustics researcher at Ifremer, said: “Using the ROV Argus, deployed from the OceanXplorer vessel, we observed that the field of hydrate mounds appeared to have remained stable since 2021. The formation of these hydrates depends on the balance between incoming and outgoing carbon dioxide fluxes over time. This provides a first indication of the ability of hydrate mounds to store carbon dioxide over periods of several years.”

Studying the Adaptation of Biodiversity to Environmental Acidification

Marjolaine Matabos, benthic ecology researcher at Ifremer, said: “The dynamics of these domes, which sequester liquid CO2 and then release it as they dissolve, will be monitored over the long term to better understand the mechanisms involved and assess their viability in the medium to long term. This monitoring, conducted during the MAYOBS missions (IPGP, IPGS, BRGM, IFREMER) and as part of the Mayotte Volcanological and Seismological Monitoring Network (REVOSIMA, IPGP), could also help determine the consequences of ocean acidification for biodiversity.”

This discovery will allow researchers to study the ability of the surrounding biodiversity to thrive and adapt to changes in the acidity of their environment.

Scientists have shown conclusively for the first time that tiny marine organisms in polar oceans survived the mass extinction event that wiped out prehistoric dinosaurs because they needed less energy and were more tolerant to darkness.

The study, led by the University of Bristol and published in the journal Nature, sought to solve a longstanding evolutionary enigma: what factors determined whether marine species would survive a mass extinction event known as the Cretaceous-Paleogene (K-Pg) boundary some 66 million years ago? Findings revealed that being small and accustomed to darkness proved to be the vital attributes.

Image shows microscopic marine plankton, which are larger in size and went extinct. Credit: Brian Huber Smithsonian

Study lead author Dr Rui Ying said: “It’s an exciting breakthrough. For so many years scientists have been unable to test what actually decided whether a species prevailed or perished because the extinction event involves multiple environmental changes like ocean acidification and darkness.”

“It is difficult to understand the causality because of the lack of fossil data and environmental proxy data, especially at century timescale. Using a numerical model, I looked at the base of the food chain – plankton – which helped us to identify the most likely cause and the best survival strategies for plankton.”

The Cretaceous–Paleogene (K–Pg) boundary is an ancient and much-studied geological signature marking the mass extinction that wiped out non-avian dinosaurs, separating the Mesozoic Era (the age of reptiles) from the Cenozoic Era (the age of mammals). It is thought that the impact of an asteroid, called Chicxulub, caused the extinction of around 75% of species in the fossil record by triggering catastrophic environmental changes.

Despite decades of research, the mechanisms linking the environmental changes to the selective extinction patterns observed in the fossil record have until now been unresolved. But by creating and deploying a unique model which maps ecosystem traits globally, the scientists have been able to establish what attributes resulted in the marine plankton community’s survival.

Dr Ying, who is now a Senior Research Associate at the University of East Anglia, said: “The model is based on trades and the trade-off of how often they are eaten by predators and what they can eat against specific attributes, such as temperature, light level and body size.”

Study co-author Dr Fanny Monteiro, Associate Professor in Ocean Sciences at the University of Bristol, explained: “The body size and abundance of small plankton mean the organisms rely on less energy, increasing their likelihood of survival. An ability to deal with lower light and darkness and turbulent waters in higher latitudes also makes them more adaptable to polar regions. In contrast, species adapted to higher light and warmer waters were more vulnerable to this type of mass extinction.”

The model allowed the traits of millions of organisms to be analysed and quantified with unprecedented accuracy, providing important insights into the physical and chemical changes linked to diversity. Besides shining a light on the distant past of marine life, the research can also help inform forecasts of how ecosystems might respond in future.

Study co-author Professor Daniela Schmidt, Professor of Earth Sciences at the University of Bristol, said: “This study not only demonstrates how trait-based models can help us better understand biodiversity crises in ancient history, but it also has potential to indicate how less light and hotter environments, as a result of global warming, might impact current and future ecosystems.”

The research was funded by China Scholarship Council (CSC)-Bristol PhD Scholarship and NERC grants.

For years, scientists have known that the ocean does a huge amount of the planet’s climate work for us.

The ocean absorbs a large share of the carbon dioxide, taking in roughly 20 to 30 percent of human-caused CO2 emissions since the industrial era. At the same time, tropical cyclones are among the most violent things that happen on Earth’s surface. They churn the upper ocean, stir up deep water, cool the sea surface, and leave behind lingering physical changes that can last for weeks. What has been much less clear is how those storms affect the ocean’s role in the carbon cycle. Do tropical cyclones help the ocean absorb carbon, or do they cause it to release more back into the atmosphere?

A new study suggests the answer is not simple, and it may also be changing.

A role reversal for tropical cyclones

The study was led by experts at the National University of Defense Technology (NUDT), Chinese Academy of Sciences, the NSF National Center for Atmospheric Research, and the GEOMAR Helmholtz Centre for Ocean Research Kiel.

The team pulled together a large set of observations to build a globally available daily dataset of air-sea CO2 flux. Using that, they were able to track how tropical cyclones have influenced the exchange of carbon between the ocean and the atmosphere over time. Their conclusion is that tropical cyclones have tended to push carbon out of the ocean and into the air. But that effect has been weakening in recent decades, and if warming continues under high emissions, the role of these storms may eventually flip.

A messy carbon signal

At first, the result sounds a little counterintuitive. The researchers found that tropical cyclones generally cause net ocean carbon outgassing. The main reason is that the intense winds of a cyclone greatly strengthen the transfer of CO2 from sea to air. But there is another process happening at the same time. After a tropical cyclone passes, it often leaves behind a cold wake, a patch of sea surface that has cooled because the storm mixed the upper ocean so strongly. That cooling can increase the ocean’s ability to take up carbon dioxide from the atmosphere, partly offsetting the carbon being released. Tropical cyclones are doing two things at once: they are helping CO2 escape because of their winds, while also setting up conditions that can later encourage carbon uptake.

The new study suggests that, historically, the first effect has usually won out. Even so, that balance has not stayed fixed.

As ocean temperatures rise and marine ecosystems change, scientists are working to understand how valuable species like the American lobster will respond. New research from William & Mary’s Batten School of Coastal & Marine Sciences & VIMS suggests one source of resilience may come from the microscopic bacterial communities living on lobster embryos.

The study, published in Scientific Reports, found that lobster eggs host surprisingly diverse microbiomes that change as the embryos develop but otherwise remain remarkably stable even under conditions simulating future environmental conditions. The findings challenge decades of assumptions that lobster eggs contained only a few key bacterial species and could help scientists better understand disease risks in one of North America’s most valuable fisheries.

“We were hoping to discover one dominant microbe early on,” said study coauthor Jeffrey Shields, a professor at the Batten School & VIMS who collaborated with several of his students on the research, including lead author Sarah Koshak. “Instead, it was a mishmash, a rich community of different bacteria whose roles we don’t yet fully understand.”

The exterior surface of a late-stage lobster embryo covered by filamentous bacteria at 400x magnification.Credit: Jeffrey Shields.

A hidden ecosystem on lobster eggs

Using advanced genetic sequencing techniques, researchers analyzed microbial communities on lobster embryos and newly hatched larvae raised under varying temperatures and pH conditions designed to mimic present-day and future ocean conditions in the Gulf of Maine.

Rivers deliver freshwater, nutrients, and carbon to Earth’s oceans, influencing the chemistry of coastal seawater worldwide. Notably, a river’s alkalinity and the levels of dissolved inorganic carbon it brings to the sea help to shape regional conditions for marine life, including shellfish and corals. These factors also affect the ability of coastal seawater to absorb carbon dioxide from Earth’s atmosphere—which can have major implications for climate change.

However, the factors influencing river chemistry are complex. Consequently, models for predicting worldwide carbon dynamics typically simplify or only partially account for key effects of river chemistry on coastal seawater. That could now change with new river chemistry insights from Da et al. By more realistically accounting for river inputs, the researchers demonstrate significant corrections to overestimation of the amount of carbon dioxide absorbed by the coastal ocean.

The researchers used real-world data on rivers around the world to analyze how factors such as forest cover, carbonate-containing rock, rainfall, permafrost, and glaciers in a watershed influence river chemistry. In particular, they examined how these factors affect a river’s levels of dissolved inorganic carbon as well as its total alkalinity—the ability of the water to resist changes in pH.

Climate change is nowhere more apparent in its disruptions than in the world’s oceans. The vast bodies of water that make our planet unique are currently undergoing fast and extensive transformations that are unlike anything scientists have seen before, according to a study published Tuesday in the journal Nature Climate Change.

Researchers from the Institute of Atmospheric Physics at the Chinese Academy of Sciences in Beijing, Mercator Ocean International in Toulouse, France, and the Laboratoire de Météorologie Dynamique at the École Normale Supérieure in Paris created a framework and tool to standardize and assess ocean variables and figure out when those variables are changed due to the warming climate.

The scientists examined how “compound changes” — the simultaneous effects of the Earth’s oceans losing oxygen, acidifying and becoming saltier or fresher — are pushing ecosystems past the point where adaptation is possible. And different layers of the pelagic world — a place where scientists have estimated over two million species reside, with approximately only 250,000 known to humanity — are affected to various degrees.

“For example, some specific species, e.g. some kind of fish, may be okay if their living environment is only saltier, but may become more vulnerable if they are exposed to the ‘compound change,’” Zhetao Tan, lead author of the study and a researcher at the Laboratoire de Météorologie Dynamique at the École Normale Supérieure, said in an email.