Real-time in situ pH monitoring in the hadal zone is essential for resolving deep-sea carbon dynamics but is severely challenged by extreme hydrostatic pressures and complex biochemical environments. Current sensors often lack the necessary robustness and calibration protocols for full-ocean-depth applications. To address these challenges, we developed a solid-state electrochemical pH sensor system comprising a fouling-resistant sulfonated poly(ether ether ketone)/ionic liquid composite IrOx (SP/IL-IrOx) working electrode and a pressure-tolerant silica-stabilized ionic liquid (Si-StabIL) reference electrode. Using Tris-artificial seawater (Tris-AS) buffers, we established a standardized high-pressure calibration protocol and systematically evaluated sensor performance over the full-ocean-depth pressure range (0.1−120 MPa) under simulated hadal pressure conditions. The sensor exhibited near-Nernstian sensitivity with high reversibility and repeatability, with potential deviations of no more than 1.6 mV, corresponding to less than 0.03 pH units across the investigated pressure range. Long-term reliability was demonstrated by a minimal drift of only 0.01 pH units during continuous operation in the Tris-AS buffer at 120 MPa for 65 h. Crucially, the sensor captured the nonlinear, pressure-driven acidification of simulated hadal-zone seawater during a 7 day pressurization experiment while maintaining stable response in calibration buffers. These results demonstrate the robustness of the sensor system and provide an experimental basis for calibration and pH assessment under simulated full-ocean-depth pressure conditions.
Marine ecosystems are undergoing rapid transformation under climate change, yet the responses of many marine invertebrates remain vastly understudied. In particular, for many benthic gastropods there is a striking imbalance between their traditional appreciation by shell collectors—and, consequently, their consistent representation in Natural History Collections—and the limited attention they receive in ecological and conservation studies. Focusing on the northeastern Atlantic and the Mediterranean, the cowries Luria lurida, Naria spurca, Zonaria pyrum and the frog-shell Talisman scrobilator are emblematic examples of this knowledge gap, despite being frequently mentioned as species of conservation concern. Using long-term occurrence records spanning more than a century, we modelled past and present distributions of these species and explored their potential responses to future climate scenarios through a multi-temporal Species Distribution Modelling framework. Our results show that intermediate climatic conditions—both in time (2050–2060 vs. 2090–2100) and scenario intensity (moderate SSP2-4.5 versus high-emission SSP5-8.5)—may represent a critical transition phase, leading to habitat contractions without compensatory gains in newly emerging suitable areas. The Mediterranean Sea is expected to increasingly function as a cul-de-sac, with the dominant circulation patterns strongly limiting outward movements towards cooler regions for species relying on planktic larvae for dispersal. Furthermore, incorporating larval sensitivity to reduced pH suggests that large areas of the Atlantic Ocean may actually result unsuitable for larval persistence, substantially reducing the habitat effectively available for completion of the full life cycle; this highlights the need to account for connectivity, life-history constraints and juvenile-stage sensitivity when assessing climate-driven range shifts in shelled organisms with planktic larvae.
The northern Gulf of Mexico (nGOM) is a river‑dominated marginal sea with strong physical‑biogeochemical variability. We reconstruct sea surface partial pressure of CO2 (pCO2) at 4‑km, 8-day resolution from 2003 to 2024 using a satellite‑based, season‑specific random forest model (independent validation R² = 0.82, RMSE = 27.6 μatm). The climatological pCO2 distribution exhibits a sharp coastal‑to‑offshore gradient: river‑influenced coastal waters (SSS < 33) have persistently low pCO2 with high spatial variability, while offshore waters (SSS > 33) have higher pCO2 with weaker heterogeneity and lower seasonal amplitude. The nGOM acts as a net CO2 sink for atmospheric, largely concentrated in the river‑influenced plume region due to riverine nutrient‑stimulated biological uptake. Seasonal pCO2 variation is dominantly controlled by temperature but counteracted by spring‑summer biological drawdown (reducing pCO2) and autumn‑winter vertical mixing with CO2‑rich deeper water (raising pCO2). Interannual pCO2 variability is dominantly affected by year-to-year changes in river discharge and nutrient loading, with higher discharge leading to lower pCO2 via enhanced biological uptake. On a decadal timescale, sea surface pCO2 increased at a rate of 0.50 ± 0.20 μatm yr-1, much slower than atmospheric pCO2 (2.13 ± 0.04 μatm yr-1), leading to a strengtheningoceanic CO2 sink with the sea-to-air flux becoming more negative at −0.41 ± 0.06 mmol C m-2 d-1 yr-1. Furthermore, a decreasing frequency of easterly winds has reduced the westward transport of the Mississippi River plume, causing a higher pCO2 increasing rate on the western Texas‑Louisiana shelf.
Marine radionuclides with well-constrained input histories have proven to be sensitive tracers.
Radionuclides in oceanic compartments enable to study transport and biogeochemical processes.
A decline in vertical mixing of upper waters in the North Pacific over recent decades was identified.
Radionuclides were used in climate change studies in marginal seas of the NW Pacific.
Abstract
Observed global warming has profoundly affected the world’s oceans, which are experiencing increasingly frequent marine heatwaves and a slowdown of the Global Meridional Overturning Circulation. These changes disrupt ocean circulation patterns, alter biogeochemical cycles, enhance surface ocean acidification, and drive poleward migration of marine organisms. Marine radionuclides (e.g., 3H, 14C, 90Sr, 129I, 134Cs, 137Cs, and Pu isotopes), released from nuclear activities since the 1940s, provide time-resolved tracers of oceanic processes owing to their well-documented input functions and distinct chemical behaviors. Their distributions in seawater, bottom sediments, and marine biota have recorded climate-driven modifications in ocean circulation and stratification. The Pacific Ocean, the largest ocean basin on Earth, has undergone changes in recent decades under ongoing climate forcing. Long-term radionuclide observations indicate a decline in vertical mixing in the upper North Pacific Ocean, likely associated with enhanced stratification. Variability linked to Asian monsoon systems and El Niño–Southern Oscillation (ENSO) events is also clearly reflected in radionuclide records from the marginal seas of the Northwest Pacific. Radionuclide datasets provide essential reference benchmarks for calibrating and validating Ocean General Circulation Models and Earth System Models under future climate scenarios. To strengthen predictive capability, coordinated international, high-resolution sampling programs covering the entire world ocean are required, together with measurement campaigns employing newly developed ultra-sensitive analytical techniques. Particular attention should be given to the Southern, Arctic, and Subarctic Oceans because of their critical role in the global climate system and the current scarcity of comprehensive radionuclide data.
Ocean acidification, driven by increasing atmospheric CO2 concentrations, poses a growing threat to marine ecosystems and biogeochemical processes. The Mediterranean Sea, characterized by complex circulation patterns and distinct hydrographic sub-basins, represents a sensitive region for assessing basin-scale pH variability. However, long-term in situ pH observations remain spatially sparse and unevenly distributed, limiting the assessment of coherent spatiotemporal trends across the basin. Here, we present a basin-wide spatiotemporal assessment of pH trends in the Mediterranean using an 18-year biogeochemical reanalysis dataset from the Copernicus Marine Environment Monitoring Service. Our analysis reveals a consistent vertical structuring of pH trends, with negative trends in surface waters and contrasting, often neutral to weakly positive tendencies at depth. The magnitude and vertical extent of these trends vary regionally and are closely linked to local circulation regimes, water-mass formation processes, and remineralization dynamics. In deep-water formation regions such as the Adriatic, Ionian, and Aegean Seas, negative pH trends extend throughout much of the water column, whereas in the Levantine Basin, mesoscale circulation structures confine pH changes primarily to a relatively thin surface layer. These results demonstrate that basin-scale analyses based on high-quality, publicly accessible biogeochemical reanalysis products, such as CMEMS, can provide a spatially integrated perspective on long-term pH variability, complementing existing observational records by bridging spatial and temporal gaps. The framework presented here offers a reproducible approach for systematically assessing depth and region resolved pH trends.
Shallow-water CO2-rich hydrothermal systems provide natural laboratories for studying localized ocean acidification under realistic environmental conditions. Here, we present a multidisciplinary characterization of the Calent mound CO2-rich system (Columbretes Islands, Western Mediterranean), based on oceanographic surveys conducted in 2020 and 2021. Localized pH anomalies were detected directly above active vents, reaching maximum reductions of 1.12 pH units, whereas water-column temperature anomalies were minimal and subsurface sediment temperatures exceeded ambient seawater by 5.67 °C. Gas analyses indicated high CO2 concentrations (0.094 ± 0.008 mol L− 1), with heterogeneous degassing regimes, ranging from sporadic to continuous emissions and an average flux of 189.4 ± 15.4 kg CO2 m− 2 yr− 1 at the active vent field. Vent fluids were significantly enriched in dissolved inorganic nutrients, particularly silicate, phosphate, nitrate+nitrite, and ammonium. Benthic microbial mats hosted metabolically diverse prokaryotic and eukaryotic communities, including hydrothermal-associated taxa such as Zetaproteobacteria, Campylobacterota, and Nitrosophaeria, consistent with iron, sulfur, and ammonia oxididation metabolisms. Several microbial core taxa persisted across years despite shifts in relative abundance. These findings demonstrate that Calent mound sustains an intense yet highly localized biogeochemical environment within the photic zone, where CO2 venting and nutrient inputs jointly influence carbonate chemistry and microbial community structure.
Environmental Sciences Professor Brooke Love and graduate student Rhiannon Holmes explore the link between proteins and resistance to ocean acidification
Rhiannon Holmes holds up two female Dungeness crabs. Photo by Luke Hollister.
Professor Brooke Love was already studying the effects of ocean acidification on sea life, but wanted to look into some new tools to aid her studies. After Love received the National Science Foundation’s Mid-Career Advancement Grant in 2020, she decided to learn molecular tools such as mass spectrometry to explore a microscopic angle.
Soon after, she found a study by Paul McElhany, who was researching Dungeness crabs at the National Oceanic and Atmospheric Administration (NOAA).
McElhany had found a difference in survivability between the offspring, or zoea, of multiple Dungeness crab mothers collected in different regions when living in water with a high concentration of CO2.
Close-ups of some zoea used in the experiment. Photos courtesy of Rhiannon Holmes.
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The NOAA group initially hypothesized that water conditions, such as oxygen and CO2 levels, at the sites where the crab mothers were collected could influence the zoeae’s survivability and resistance to ocean acidification (OA), but ultimately they found that location had less of an impact than matrilineal lineage.
As Love continued the research, she knew she would need more resources. Support from the Washington Ocean Acidification Center allowed her to move ahead and bring WWU undergraduate Rhiannon Holmes onto the project as an intern. Together, they took the first NOAA experiment’s preserved zoea gathered from the Puget Sound and compared all the proteins present in each brood. Of the three Puget Sound females, one mother’s brood had far higher survivability under high CO2 conditions than the other two. The team discovered that these survivors had elevated amounts of a few key proteins. The function of these proteins could indicate the reason why those zoea survived the best.
Now, the team is working towards the next phase of their experiment, which will consist of testing the resistance to OA of additional broods to see if the first experiment’s data is repeatable. Love’s team is working with local tribal fishery experts to collect new egg-bearing females and will use the Shannon Point OA system to test how their offspring survive, and to see if their targeted proteins are once again associated with more resilient broods in the next trial.
“We’ve been using mass spectrometry to look at the different proteins that they produced, and then compare the zoea that were resistant with the vulnerable ones. We identified a group of 33 proteins that were different between the two groups,” said Holmes, who is now a graduate student in Love’s lab. “We’ve only analyzed zoea from one of the three locations so far. They had three females per location, so the sample size is pretty small, but we’re interested in seeing if that trend holds true as we analyze the rest of the samples.”
A global research network assessed how ocean acidification affects seafood to help countries build climate resilience and secure future food supplies.
The spotted rose snapper (Lutjanus guttatus), locally known as pargo mancha or pargo lunarejo, is a commercially and culturally valuable fish species in Costa Rica. (Photo: C. Sanchez-Noguera/University of Costa Rica)
Global seafood security depends on the health of our ocean. However, this vital resource is currently under threat from the ‘invisible’ process of ocean acidification.
As the ocean absorbs more carbon dioxide from the atmosphere, its chemistry changes, making it increasingly difficult for many marine species, including shellfish and fish that billions of people rely on for protein, to grow and survive.
To address this challenge, the IAEA launched a five-year Coordinated Research Project (CRP) in August 2019 to evaluate how changing ocean chemistry affects seafood and to explore adaptation strategies for the aquaculture and fisheries industries.
A unified scientific protocol for a global assessment
The project applied a novel, collaborative approach in which researchers from 14 countries across five continents used a common experimental protocol, allowing results to be compared and integrated into a single dataset.
The project enabled scientists to study how locally important seafood species respond to the complex physiological stress of acidifying waters, including impacts on growth and mortality, as well as seafood quality, such as taste and texture. By focusing on species of high socio-economic importance such as oysters, mussels, shrimp and fish, the research provides a direct link between marine chemistry changes and the livelihoods of coastal communities.
Using similar methodologies and research kits provided by the IAEA, the consortium successfully co-designed a comprehensive experimental framework. This turnkey scientific model allowed laboratories with varying levels of experience to produce high-quality data to inform policy making.
“This project allowed us to move beyond isolated observations to a more global understanding,” said Florence Descroix-Comanducci, Director of the IAEA Marine Environment Laboratories. “By providing countries with a unified scientific framework to study ocean acidification impacts, we have empowered them to produce data that is high-quality, comparable and ready to inform national policy. We are no longer looking at individual pieces of a puzzle; we are seeing a more complete picture of ocean change.”
The team from the University of Costa Rica organizing a public seafood tasting of the spotted rose snapper (Lutjanus guttatus) previously subjected to ocean acidification, to assess potential impacts on taste and texture. (Photo: C. Sanchez-Noguera/University of Costa Rica)
Strengthening infrastructure and informing policy
The impact of the project extends beyond the laboratory, strengthening research infrastructure and informing policy frameworks of the participating countries.
The initiative led to the establishment of specialized laboratories for ocean acidification research in Türkiye and Cuba, as well as new monitoring stations in Argentina.
Furthermore, the project mentored more than 30 students and early-career scientists, helping to build a skilled workforce capable of tackling future marine challenges. These enhanced capabilities have already translated into real-world policy changes. In Ecuador, for example, data generated through the project was shared with policymakers to inform articles of the National Environmental Law, which now explicitly addresses ocean acidification.
This collaborative effort also brought science to local communities, engaging with aquaculture managers and the public through surveys and seafood-tasting events.
“The collaborative nature of this project helped us bring the topic of ocean acidification to the attention of decision-makers in a way we couldn’t have done alone,” said Betina Lomovasky, a researcher from Argentina.
The project has provided a solid basis for long-term food security and the sustainable management of marine resources in a changing climate.
Marc Metian, IAEA Department of Nuclear Sciences and Applications, 16 June 2026. Article.
Body size is a fundamental characteristic of all living organisms that determines physiological functions and life-history traits. Ecological theory predicts that ocean acidification can cause body size reductions, confirmed by several studies reporting miniaturization in ectotherms. Based on this prediction, we would expect a broad suite of species to show similar plastic body-size responses to elevated CO2. Using four natural climate change analogues of ocean acidification across the northern and southern hemispheres, we quantified body size alterations across 18 marine invertebrate and fish taxa to test for climate-driven miniaturization. Only three species consistently showed body-size reductions under ocean acidification: one urchin and two fish species. In contrast, 15 other species, ranging from highly calcified to non-calcified, displayed unchanged or increased body sizes or inconsistent miniaturization. If body-size miniaturization responses were consistently reproducible across taxa we would have observed it more frequently, suggesting that species responses to ocean acidification are more variable than previously thought and likely vary depending on a species’ physiology and life history. Thus, rather than entire communities undergoing miniaturization, species are likely to display a spectrum of responses, with some exhibiting size reductions, others demonstrating physiological resistance to elevated CO2, and others potentially benefiting from the indirect effects of ocean acidification.
First study on pteropod response to ocean acidification in the eastern Arabian Sea.
High pteropod abundance during fall inter monsoon season due to food availability.
pH in the Arabian Sea was low during south west monsoon with pHT upto 7.75
Pteropod shell dissolution was observed under acidified conditions
Protrusions through the pteropod shell were observed under acidified conditions
Abstract
The rapid rise in atmospheric CO2 and its subsequent uptake by the oceans has led to ocean acidification and other associated changes in the marine ecosystem. The recent reports of the shoaling of the aragonite saturation horizon in the northern Indian Ocean are particularly alarming, as they pose a serious threat to the survival of calcareous organisms. Pteropods, also known as sea-butterflies, are believed to be highly susceptible to ocean acidification due to their thin aragonite shell. In our study in the eastern Arabian Sea, we found low pH conditions with surface pHT as low as 7.751 during late South-west monsoon (SWM). The pteropod abundance is high during the fall inter-monsoon (FIM), suggesting that the system continues to sustain productivity even after the cessation of peak monsoon activity. This also implies that the food availability regulates pteropod abundance in the eastern Arabian Sea. As pteropods are key components of food sources for many marine species, such as fish, any changes in their abundance can have cascading effects on the marine food web. To show how pteropods will be affected in futuristic elevated CO2 conditions, a CO2 manipulation experiment was conducted in the eastern Arabian Sea during December 2024. Pteropods belonging to Creseis acicula from the eastern Arabian Sea were subjected to pHT = 7.470, and pCO2 = 1734 μatm under controlled conditions. Our findings suggest that acidification led to the dissolution of pteropod shells. Acidification also led to protrusion through the shells, and these protrusions varied in length up to 88 μm. These structural alterations represent an acute response of pteropod shells to reduced pH, highlighting their rapid vulnerability to acidification stress. These observed protrusions need to be assessed further to determine if they provide any competitive advantage in combating or minimizing the impact of ocean acidification.
Seafood provides an essential source of macro- and micronutrients for coastal communities worldwide. Climate change is a key threat to seafood security, altering the sizes, abundances, distributions, physiology and ecological interactions of fisheries species, and increasingly, there is evidence of impacts to seafood nutritional quality. In a 12-week mesocosm experiment, we tested the influence of projected ocean warming and acidification scenarios on the growth and lipid quality of juvenile sand whiting (Sillago ciliata), a popular fisheries species in eastern Australia. The growth of S. ciliata significantly increased (by 61% body weight) under elevated temperature (+3°C) but was not affected by acidification treatment levels. Lipidomic analysis revealed no influence of temperature or acidification on total lipid content or the composition and total proportions of lipid classes and subclasses. However, elevated temperatures significantly impacted the overall composition of fatty acids, including a shift toward higher saturation and a decline in important omega-3 fatty acids. Fish exposed to elevated temperature treatments had more saturated fatty acids than those at control temperatures, along with reduced levels of the valuable omega-3 eicosapentaenoic (C20:5) and docosahexaenoic (C22:6) fatty acids. Despite impacting fatty acid composition in S. ciliata, the increased growth of the juvenile whiting, if sustained into adulthood, under elevated temperatures, may help compensate for the overall availability of essential polyunsaturated fatty acids to support consumer nutritional requirements. These findings contribute to the growing body of evidence on variable climate resilience in nearshore species to future environmental conditions and the implications for the trophic transfer of nutrients in estuarine ecosystems.
A. viridis tentacle microbiomes were studied under changing natural pH conditions.
Notable shifts in the abundance of specific taxa emerged in the acidified sites.
Differences in seawater emphasized the host’s unique microbial signature.
Rickettsiales predominance suggested a specialized ecological role in symbiosis.
Further research is needed to discern the role of microbes for host resilience.
Abstract
Marine hydrothermal vents are extreme environments that naturally select for organisms with strong resistance and the ability to cope with special conditions of acidification. Sea anemones are an interesting example that are able to buffer intracellular pH conditions. In this study, the influence of a natural pH gradient on microbial communities associated with Anemonia viridis (Cnidaria, Anthozoa) tentacles was investigated. We hypothesized that exposure to a natural pH gradient would be associated with changes in the structure and activity of A. viridis-associated microbial communities, potentially contributing to the host’s resilience in hydrothermal environments. Microbial enzymatic activities within anemones’ tentacles were investigated by incubation with fluorogenic compounds. The leucine amino peptidase activity was highest in the tentacles of specimens living in more acidified sites. A microbial biodiversity loss was observed in bacterial symbionts from less acidified to more acidified sites, with a reduction of relative abundance in certain groups (i.e., Planctomycetota, Firmicutes, and Desulfobacterota). Results obtained by a metabarcoding approach provided interesting insights into the taxonomic shifts of the A. viridis holobiont system in naturally acidified environments.
The carbon flux estimated from biogeochemical Argo float data indicates a lower annual carbon uptake by the Southern Ocean compared to fluxes derived from other observations (e.g., ship and aircraft measurements). The root cause of this discrepancy remains controversial, with growing evidence suggesting that potential biases in float-derived pCO2 may be a plausible explanation. Here, we perform a multi-variable comparison of vertical profiles between float- and ship based-data and reveal consistent discrepancies in pH, pCO2 and dissolved inorganic carbon, which are not found in other variables such as dissolved oxygen, nitrate and total alkalinity. Our findings are consistent with a previously unrecognized negative bias in float pH driving a positive offset in float-derived pCO2. The float-derived surface pCO2 is, on average, biased high by 15 ± 3 µatm compared to ship data, representing a larger magnitude of bias than previously recognized. Biases exist in both surface and deep waters, including old deep waters containing minimal anthropogenic carbon. A more sophisticated adjustment for float pH, involving multiple cross-reference depths, may be required for accurate estimation of air-sea CO2 exchange in the Southern Ocean.
In recent years, the East China Sea (ECS) has experienced frequent harmful algal blooms (HABs), driven by the complex interplay of climate change—specifically ocean warming and acidification—and eutrophication-induced light attenuation. Despite their ecological significance, the interactive effects of these environmental stressors on the competitive dynamics between bloom-forming microalgae remain poorly understood. This study aimed to elucidate how warming, reduced light, and elevated CO2 influence the competition between two dominant diatoms. We conducted controlled monoculture and mixed-culture experiments using two key species: Skeletonema costatum and Chaetoceros curvisetus. The experimental design incorporated varying levels of CO2, temperature, and light intensity to simulate future coastal scenarios. Growth rates, peak cell densities, and successional patterns were monitored to assess competitive outcomes under multiple stressors. Monoculture results indicated that high temperature and low light intensity promoted the growth of both species. However, in mixed cultures, these conditions significantly accelerated the time to reach peak density and induced a definitive successional shift from S. costatum to C. curvisetus. Notably, while the general successional pattern was consistent, elevated CO2 further enhanced the competitive advantage of C. curvisetus, particularly when combined with high-temperature and low-light scenarios. These findings suggest that the synergy of future warming, declining light availability, and intensified ocean acidification in the ECS will likely favor C. curvisetus over S. costatum. This shift may increase the frequency of HAB events dominated by C. curvisetus, driving significant climate-related restructuring of phytoplankton communities in coastal ecosystems.
NOAA Ship Ronald H. Brown during the 2021 West Coast Ocean Acidification research cruise with a NOAA mooring measuring ocean chemistry in the foreground. Credit: NOAA
This June, NOAA’s Ocean Acidification Program (OAP) launches two major research missions at sea to track how changing ocean chemistry is affecting marine life along both the East and West coasts of the United States.
OAP’s East Coast (ECOA-4) and West Coast (WCOA 2026) Ocean Acidification research cruises collect the highest quality information that serve as vital benchmarks for research, monitoring and modeling in each region. By coupling ocean chemistry, biology and physics, researchers are able to better understand how ocean acidification impacts marine life. Each coastwide cruise occurs every four years on average. Data collected during these cruises “serve as a vital back-bone to NOAA’s ocean acidification observing enterprise allowing us to document the primary drivers and risks of acidification along much of the nation’s coastal waters,” says OAP Acting Director Dwight Gledhill.
Tracking potential El Niño effects
This year’s missions are particularly timely as a predicted El Niño builds during the missions. This concurrence provides a unique opportunity to assess how these conditions affect ocean chemistry and marine ecosystems. El Niño is a natural variation in sea temperature that occurs when weaker than normal trade winds occur. Warmer conditions can shift where marine species occur – and where people need to fish – and significantly alter ecosystems and fisheries. If conditions develop as predicted, ECOA-4 and WCOA 2026 will both capture how El Niño conditions affect ocean acidification and impacts to marine resources.
Delivering critical information on two coasts
The ECOA-4 research cruise will survey the Atlantic seaboard from Florida to Canadian waters and launches first in early June for a 50-day journey. WCOA 2026 departs from San Diego, CA and heads north to Washington over a month of sampling. Both coastal research cruises collect coastwide data of ocean biogeochemical and physical conditions and how conditions are affecting marine resources. Research cruises are needed to obtain the quality and breadth of measurements required to infer long-term changes and to see how marine life responds to ocean acidification.
Already, each coast has experienced the effects of ocean acidification on fisheries, aquaculture and ecosystems. The data collected by ECOA-4 and WCOA 2026 are fundamental to validating ocean models and forecast changes in ocean acidification and other conditions like hypoxia and warming to better prepare for future impacts to valued fisheries and ecosystems.
East Coast Atlantic sea scallop fishermen are working together with researchers to develop research and adaptive management strategies addressing the impacts of ocean acidification and warming. The information also validates ocean and other models such as the Chesapeake Bay Environmental Forecast System (CBEFS) used by resource managers, shellfish growers and fishermen. The West Coast, which first saw devastating impacts to oyster farming, now produces forecasts through J-SCOPE that incorporate measures of ocean acidification and other ocean conditions into integrated ecosystem assessments and fisheries management. Research of economically, ecologically and culturally valuable species like Dungeness crab, krill and oysters also benefit from the data produced by these coast-wide research missions.
In the face of a rapidly changing climate, assessing organismal responses to future stressors in the context of current, natural exposure to stress could provide key insights to understanding marine ecosystem resilience. I used Balanophyllia elegans, a cold-water, solitary, azooxanthellate coral as a model to better understand how varying oceanographic conditions across its geographic range have shaped its ability to tolerate and potentially adapt to current and future ocean acidification conditions. I collected B. elegans individuals from four sites across 2,500km of their range and subjected them to two pH treatments to investigate site-specific protein expression in response to low pH. Using proteomic analysis, I found that corals from each site responded differentially to low pH, mainly through changes in regulation of metabolism, calcification, and homeostasis-related proteins. Additionally, health condition varied significantly between sites after exposure to low pH, providing further evidence of site-specific responses. These results demonstrate site-specific variation in responses and tolerance to low pH, a pattern that could inform future investigations into environmental-driven adaptive expression. Such site-specific responses highlight the importance of multi-source studies for predicting a species’ ability to navigate future climate changes.
Marine pufferfishes are globally distributed and ecologically important organism notable for accumulating tetrodotoxin [TTX], a potent neurotoxin with wide ecological ramifica-tions. Unlike many endogenous defences, TTX in pufferfishes is acquired indirectly via microbial and trophic pathways, linking pufferfish toxicity to the dynamics of marine mi-crobial assemblages and food webs. Anthropogenic climate change principally ocean warming, deoxygenation, and acidification is rapidly reshaping marine environments in ways that are likely to intensify and redistribute TTX exposure. Observational and experimental studies indicate that elevated seawater temperatures favour the proliferation of thermophilic, toxin-producing bacteria [e.g., Vibrio spp.], increase the abundance of toxic prey, and raise TTX burdens in pufferfish tissues seasonally and spatially. Concurrently, warming-driven range shifts have promoted poleward expansions of several tropical and subtropical puffer species, producing novel sympatric assemblages, hybridization events, and “cryptic” toxic phenotypes that complicate species identification and risk assess-ment. These biogeographic rearrangements, together with altered prey communities and microbial composition, reconfigure the trophic pathways by which TTX is transferred and concentrated in higher trophic levels. Early evidence also links multistressor conditions elevated temperature combined with hypoxia or acidification to altered developmental success and changes in toxin allocation during reproduction, suggesting potential popu-lation-level consequences. This review synthesizes current global evidence on cli-mate-linked changes in pufferfish TTX dynamics, integrating microbial ecology, trophic transfer, life-history shifts, and biogeography. We highlight [i] mechanistic pathways by which warming and associated ocean changes increase environmental TTX availability, [ii] how shifting species ranges and hybridization alter toxicity patterns across regions, and [iii] key methodological advances [e.g., high-resolution LC-MS/MS, metagenomics] needed to resolve open questions. We identify critical research gaps long-term field moni-toring, integrated microbial–trophic mapping, and multistressor population studies and recommend synthesis strategies that link environmental monitoring to toxin surveillance. Understanding pufferfish toxification as a climate-sensitive ecological process [not a static species trait] is essential to anticipate how marine toxin landscapes will change in the Anthropocene and to develop timely, science-based monitoring frameworks.
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.
Gas hydrates modulate methane and carbon dioxide benthic fluxes into the ocean and usually occur embedded in the sediment. Here we use acoustic surveys alongside optical and geochemical observations from remotely operated vehicles to show that CO2 hydrate mounds are forming directly on the seafloor atop a large liquid CO2 vent field offshore Mayotte Island. The venting, which initiated following volcanic activity in 2018, deleteriously impacts surrounding coral communities due to local acidification.
Cutaneous Mel, unlike cortisol, shows high sensitivity to slight shifts in water pH.
Water pH was regulated by a custom-designed system controlling dissolved CO2 levels.
High skin Mel levels and distinct pH-dependent responses indicate local Mel synthesis.
Abstract
Fish skin functions not only as a passive protective barrier but also as an active site of key physiological processes, including a local stress response system. In fish, this system involves the hormones cortisol and melatonin (Mel), which contribute to counteracting environmental stressors and maintaining homeostasis. In this study, we examined the sensitivity of both components of the cutaneous stress response system (CSRS) in three-spined sticklebacks (Gasterosteus aculeatus) exposed to acidic (pH = 6.54) and basic (pH = 8.70) water conditions, representing the boundary values of the species’ optimal pH range, under either rapid or gradual pH change regimes. Water pH in the aquaria was precisely controlled using a custom-designed gas-exchange system regulating dissolved CO2 levels. Mel concentrations were measured in the skin, brain and eyeball, while cortisol was determined in the skin. Samples were collected during the day. Skin Mel levels were significantly higher under acidification than under basification (P = 0.036; rapid change regime), whereas cortisol remained stable across all conditions. Ocular Mel levels were not affected by treatments. Brain Mel concentrations were generally very low but tended to be slightly higher under basification than under acidification in both regimes (P = 0.05, borderline significance). The marked differences in skin Mel levels between acidic and basic pH water conditions, accompanied by stable cortisol concentrations, indicates that cutaneous Mel, but not cortisol, is highly sensitive to subtle water pH fluctuations even within the species’ optimal range.
