Mixotrophic protists capable of both photosynthesis and phagotrophy are key members of marine plankton communities. Yet, little is known about their responses to the combination of ocean acidification and warming.
A marine mixotrophic chrysophyte, Ochromonas CCMP2951, was subjected to two levels of pCO2 (300 and 800 ppm, resulting in pH of 8.2 and 7.8) and temperature (21°C and 26°C) in a factorial design.
Enhanced growth rates were observed in both the high CO2 and high temperature treatments, while cell size significantly decreased with temperature. Strongly decreased cellular phosphorus content led to increased N : P and C : P ratios of Ochromonas with temperature. Furthermore, warming increased grazing rates, while elevated CO2 reduced the Chl content but increased photosynthetic carbon acquisition, albeit only at low temperature. The combination of warming and elevated CO2 had antagonistic effects on the balance between autotrophic and heterotrophic carbon acquisition, keeping the net role of this mixotroph in the marine carbon cycle stable.
Altogether, both the direct stimulation of growth and the indirect effects of altered stoichiometry may favor mixotrophs under future ocean conditions. However, their contribution to future carbon cycling in complex natural communities will need further study.
Graphical Abstract
Grazing rates of Ochromonas CCMP2951 expressed as carbon obtained per mean cell volume (a) and Caut/het-photosynthetically fixed carbon divided by carbon obtained through grazing (b) in the four experimental treatments.
Low pH conditions at CO₂ vents increase THg bioavailability and fish exposure.
Functional traits explain THg patterns better than species identity.
Under low pH, less-mobile benthic fish accumulate more THg than highly-mobile fish.
Trophic position drives stronger THg biomagnification at low pH sites.
Abstract
Ocean acidification is not an isolated climate-related threat to marine organisms, as it may act in combination with other stressors such as trace element contamination. Here, we took advantage of naturally acidified conditions and mercury spill-out from the shallow CO2 vent of Vulcano Island (Italy) to test whether fish functional traits explain contaminant dynamics better than species identity under ocean acidification scenarios. Specifically, we investigated if trophic group and mobility influence total mercury (THg) bioaccumulation and trophic transfer in fish inhabiting Cymodocea nodosa seagrass meadows under low and ambient pH conditions. Low-mobility benthic fish, mainly represented by invertebrate feeders and small piscivores, exhibited higher THg concentration at the vent site than at reference sites, while highly mobile zooplanktivorous fish showed no significant differences, highlighting the importance of fish mobility and local trophic benthic pathways in shaping contaminant exposure and bioaccumulation. Moreover, the relationship between trophic position and THg concentration in fish was stronger in the low pH site, indicating enhanced trophic transfer and biomagnification when Hg availability is higher. In contrast, weak or absent biomagnification in the reference sites suggests that Hg transfer depends more on contaminant availability and food web structure than trophic position alone. Overall, these findings indicate that functional traits better predict Hg bioaccumulation patterns than species identity, supporting trait-based approach to assess contaminant dynamics in future ocean acidification scenarios.
Monitoring pH variations in coastal waters is essential for maintaining marine life and developing strategies to combat ocean acidification caused by climate change. The present study examines pH changes on interannual and seasonal scales along the northwest (NW), southwest (SW), southeast (SE), and northeast (NE) coasts of India over 30 years (1993–2022) using satellite data. Results reveal a significant decline in pH levels (R 2 = 0.727) across Indian coastal waters, with notable regional differences. The northeastern coast showed the greatest pH stability and the smallest decrease (R 2 = 0.798), while the southwestern coast experienced the highest variability (R 2 = 0.490). These regional pH fluctuations suggest varying resilience to ocean acidification. The NE and SW coasts, with higher variability, may be more susceptible to environmental changes, underscoring the importance of targeted monitoring and mitigation measures. In contrast, the more stable trends along the SE and NW coasts present opportunities to explore long-term resilience mechanisms. Seasonal fluctuations were evident everywhere, with winter consistently showing higher pH values and monsoon seasons the lowest. Principal component analysis indicated that the first two components accounted for 85.7% of the variance, highlighting factors such as seasonal river inflows, biological activity, and monsoonal freshwater input. Overall, these findings emphasise the importance of region-specific coastal management strategies to address climate change impacts and human pressures, while also providing a baseline for tracking future ocean acidification trends and assessing their effects on marine biodiversity and ecosystem services.
Anthropogenic CO2 emissions are altering marine ecosystems through two interconnected processes: climate change and ocean acidification, both of which are particularly affecting the Southern Ocean. However, the molecular responses of cold-water corals to these stressors remain poorly understood. In this study, we investigated the transcriptomic response of the Antarctic coral Malacobelemnon daytoni exposed to ocean acidification (pH ~7.7; LpH) and elevated temperatures (+2°C and +4°C above ambient; T1 and T2, respectively) under controlled laboratory conditions. Using high-throughput RNA sequencing (RNA-Seq), we compared gene expression profiles across six treatments, including a control (pH ~8.1, CpH; 0–1°C; CT), acidification, warming, and their combinations. Principal component analysis revealed treatment-specific clustering and greater transcriptomic dispersion under LpH conditions. Differential expression analysis identified between 475 and 767 differentially expressed genes (DEGs), with the strongest transcriptional responses observed under combined stress conditions (LpH + T1 and LpH + T2). Combined exposure to ocean acidification and warming elicited the largest gene expression response, suggesting enhanced, potentially non-additive effects of multiple stressors. Functional enrichment analyses revealed differential regulation of genes associated with protein folding, signal transduction, energy metabolism, oxidative stress, and regulation of cell death. Our findings demonstrate that Malacobelemnon daytoni exhibits complex, treatment-dependent gene expression responses to environmental stressors. These results highlight the capacity of this Antarctic octocoral to mount molecular responses to projected future ocean conditions while also emphasizing its potential vulnerability to the interacting effects of ocean warming and acidification.
Organic carbon mineralization is generally recognized as the primary source of dissolved inorganic carbon (DIC) released from sediments in coastal seas. The CO2 accumulation or the formation of corrosive microenvironment induced by organic carbon degradation can promote the dissolution of calcium carbonate (CaCO3) in sediments, complicating the efficiency of carbon burial and total alkalinity (TA) inputs to aquatic environments. However, quantitative assessments of sediment CaCO3 dissolution and its impacts on the seawater carbonate remain poorly constrained. In this study, we selected typical high-productivity regions, mariculture farms, and applied the 224Ra–228Th disequilibrium approach to quantify the effluxes of DIC and TA across the sediment-water interface. Stable carbon isotopes of DIC (δ13CDIC) were employed to trace DIC sources in porewater. The results showed that CaCO3 dissolution in sediments accounted for 27–56 % of the benthic DIC efflux. Notably, a high contribution of CaCO3 dissolution did not coincide with strong organic carbon degradation across sites, suggesting that dynamic disturbance on sediments, which weakened the metabolic CO2 accumulation in porewater, was also a crucial factor affecting carbonate dissolution. According to the evaluation of the influence that benthic DIC and TA efflux exerted on the seawater CO2 content, the TA supplied by the CaCO3 dissolution was identified to enhance the carbonate buffering capacity of seawater and counteracted the acidification driven by organic matter remineralization. This indicates that CaCO3 dissolution in sediments should be involved in coastal carbon cycling and assessments on coastal ecosystem resilience under the risk of CO2 elevation.
Terrestrial inputs can alter total alkalinity (TA) and dissolved inorganic carbon (DIC) of the coastal ocean and modify seawater pH. Here, we first characterize the carbonate system in river and groundwater draining to the Baltic Sea using observations of TA, DIC, δ13C-DIC, and major ions across 6 countries and 17 beaches. We then assess whether submarine groundwater discharge (SGD) may impact coastal acidification. TA and DIC concentrations were about 2 times greater in groundwater than river water. 84% of the groundwater and 72% of river samples showed potential to acidify receiving Baltic Sea waters and degas CO2 due to low TA/DIC ratios. Mixing plots revealed non-conservative production of TA and DIC in subterranean estuaries. δ13C-DIC values imply that organic matter respiration was a main source of DIC to northern catchments, while calcium carbonate (CaCO3) dissolution was more important along the southeastern coast. Fresh SGD contributed only < 2% of TA and DIC, and 5–7% of Ca, Mg, and SO4 fluxes compared to river discharge when extrapolated to the entire Baltic Sea. However, unquantified total SGD (fresh groundwater plus recirculated seawater) water and chemical fluxes are likely higher. Overall, SGD can locally acidify the Baltic Sea and should be considered in regional carbon budgets.
The process of decreasing seawater pH due to increasing atmospheric carbon dioxide known as ocean acidification is a global phenomenon with very strong regional effects. We report the first seasonal and interannual carbonate chemistry variabilities in the Philippines determined using carbonate boron systematics. Coral δ11B-pHsw from two contrasting site conditions showed more positive trends than basin-wide averages, suggesting that reef waters around the Philippines represented by these sites may be offsetting the general pH decline in global seawater. This pattern is consistent with δ11B-pHsw studies in neighboring Palau and Taiwan and can be explained by decreases in net ecosystem calcification and/or increases in net ecosystem productivity. Other potential site-specific local drivers of δ11B-pHsw changes include vent-driven input of nutrient- and CO2-rich groundwater that alters seawater dissolved inorganic carbon (DIC). The El Niño Southern Oscillation that influences water mass movement along the Philippine Pacific seaboard, and seasonal zonal current reversal in the Verde Island Passage (VIP) that switches source waters within the VIP, may also contribute to the recorded changes in δ11B-pHsw. Differences between the coral internal fluid chemistry and modelled regional seawater pH trends highlight the importance of localized monitoring of acidification for accurate management strategies of reef habitats.
The ocean is the foundation of life on Earth. But its health is at grave risk as ecosystems and habitats approach or surpass critical tipping points. In the search for solutions, ocean science is indispensable. The third WorldOceanAssessment offers the most comprehensive evaluation to date, providing Governments, institutions and the public with the evidence needed to shape coordinated, effective action.
Five years ago, the previous Assessment found persistent degradation of marine ecosystems. This third Assessment documents a deepening crisis, as climate change, pollution, overfishing and biodiversity loss put ocean systems under severe strain. Its findings demand urgent action, through stronger multilateral cooperation, greater ambition and decisions grounded in the best available science.
This Assessment comes midway through the United Nations Decade of Ocean Science for Sustainable Development and the United Nations Decade on Ecosystem Restoration. It takes stock of what we have learned, how far we have come and what more is needed to secure a healthy, resilient ocean for current and future generations.
The entry into force of the Agreement under the United Nations Convention on the Law of the Sea on the Conservation and Sustainable Use of Marine Biological Diversity of Areas Beyond National Jurisdiction in January 2026 marks a historic milestone for ocean stewardship and multilateral cooperation. Now we must translate that momentum, and the latest science, into decisive action for a healthy and resilient ocean.
May this Assessment strengthen our resolve to protect the ocean and, in doing so, help safeguard our climate, our food systems, our prosperity and our future.
António Guterres, Secretary-General of the United Nations
Professor Pablo Sánchez Jerez is part of the international team that has produced the World Ocean Assessment
The University of Alicante is participating in the largest scientific assessment driven to date by the United Nations to analyse the health of the world’s oceans. This international project brings together hundreds of specialists from across the globe and currently serves as the primary global benchmark for guiding marine conservation policies internationally.
The Alicante institution’s involvement is delivered through the work of Professor Pablo Sánchez Jerez, from the Department of Marine Sciences and Applied Biology. He is part of the international team of authors for the third edition of the World Ocean Assessment, the most ambitious report compiled so far on the current state of seas and oceans, coordinated by the United Nations.
According to Sánchez Jerez, the ocean sustains processes fundamental to life on Earth, and we need to understand how its ecosystems are changing with the utmost scientific rigour in order to make effective decisions. Participating in this assessment means contributing directly to the global knowledge required to protect one of the most vital systems for the future of humanity.
The oceans constitute Earth’s primary life-support system. Covering more than 70% of the planet’s surface, they produce approximately half of the oxygen we breathe thanks to phytoplankton activity, regulate the global climate by transporting heat between continents, absorb around a third of the carbon dioxide generated by human activities, and host an extraordinary biodiversity that sustains complex ecological networks and numerous economic activities.
The health of these ecosystems is closely linked to human well-being. Activities such as fishing, aquaculture, coastal tourism, maritime transport, and numerous industrial sectors depend directly on the good condition of marine ecosystems. However, these natural systems currently face unprecedented challenges stemming from climate change, pollution, habitat loss, the overexploitation of resources, and growing human pressure on coastal areas.
Against this backdrop, the international scientific community agrees that the conservation and restoration of marine ecosystems is one of the major environmental priorities of the 21st century. Among the most effective tools are marine reserves and marine protected areas (MPAs)—spaces capable of recovering exploited species populations, conserving habitats of high ecological value, and increasing ecosystem resilience against global disturbances such as planetary warming.
Numerous scientific studies have shown that these protected areas function as genuine havens for biodiversity. They generate benefits that transcend their own boundaries by exporting larvae, juveniles, and adults to adjacent zones, thereby supporting both conservation and the sustainability of fisheries. It is precisely this need for robust scientific information to guide marine management and conservation that has driven the production of this third edition of the World Ocean Assessment. This global evaluation provides a comprehensive overview of the changes oceans are undergoing, the risks they face, and the potential solutions needed to guarantee their future sustainability.
The assessment is conclusive: the oceans are under increasing pressure from climate change, pollution, resource overexploitation, and the progressive degradation of marine ecosystems. Among the most worrying conclusions are the accelerated warming of ocean waters, marine acidification, rising sea levels, the expansion of oxygen-depleted zones, and the widespread accumulation of plastic and microplastic waste in practically every oceanic ecosystem on the planet.
The report further warns that the loss of marine biodiversity represents a direct threat to ecological stability and to numerous essential ecosystem services that sustain society. In this context, expanding and improving the management of marine protected areas emerges as one of the main international recommendations to strengthen ocean resilience against global change.
Acuicultura marina
Within this international assessment, Professor Pablo Sánchez Jerez has specifically participated as an author of the chapter dedicated to medium- and large-scale marine aquaculture. This is a strategic sector called upon to play a decisive role in sustainable food production within a context marked by global population growth, an increased demand for marine protein, and the need to reduce pressure on wild fish stocks. “The future of global food security will largely depend on developing increasingly sustainable marine production systems. Aquaculture must grow, but it must do so by relying on technological innovation, environmental sustainability, and a balanced coexistence with natural ecosystems,” the researcher notes.
The chapter analyses the evolution of marine aquaculture on a global scale and addresses strategic aspects such as technological innovation, the digitalisation of production systems, the environmental sustainability of farms, the social acceptance of the activity, and the sector’s adaptation to the effects of climate change. Furthermore, it identifies significant knowledge gaps and establishes research priorities necessary to advance towards more sustainable production models compatible with marine conservation.
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.
We are thrilled to invite you to attend the International Conference on Coastal Ocean Acidification and Ecosystem Effects – (ICCA2E-26) conference, taking place on 14th – 15th August 2026 at Dallas, USA. The event will bring together leading experts, thought-provoking discussions, and a diverse group of attendees from around the world. With over 500 participants expected, the conference will provide ample opportunities for networking and professional growth.
The conference will feature a wide range of sessions, including keynote lectures, oral presentations, poster sessions, symposia, and workshops. Our panel of speakers includes academics, students, researchers, and industry professionals, making for a dynamic and engaging environment. Attendees will have the opportunity to actively participate in discussions, connect with others in their field, learn about the latest research and discoveries, and explore solutions to ongoing challenges.
In addition, the conference will bridge the gap between theoretical studies and practical applications, providing attendees with valuable insights and knowledge. Join us for the (ICCA2E-26) and take the next step in your professional journey. Don’t miss out on this opportunity to expand your knowledge, make meaningful connections, and explore new areas of the field.
Objective
The primary objective of the (ICCA2E-26) is to bring together a diverse group of individuals, including academics, scientists, professionals, and practitioners, to discuss current trends, advancements, and new developments within the field of Coastal Ocean Acidification and Ecosystem Effects. The event aims to provide a clear understanding of the current state of the field across different countries and to encourage collaboration and progress in the field. Additionally, the event aims to bridge the gap between theory and practical applications by providing attendees with opportunities to explore various fields, learn about the latest research and discoveries, and discuss ideas for implementation The event will also provide opportunities for academic research, with the publication of original research papers, review papers, surveys, case studies, technical and management reports, viewpoints, conference publications, theses, book reviews, notes, commentaries, and research papers. These publications will be subject to peer review, providing young researchers with valuable feedback and helping to enhance their research efforts. The event will also foster engagement and discussion of creative ideas, providing opportunities for professional and personal growth.
Climate change models predict future ocean-wide decreases in pH and increases in temperature, posing a serious threat to calcifying marine invertebrates. In Hawaiʻi, local seawater temperature and pH are predicted to change even more rapidly. The Hawaiian oyster Dendostrea sandvichensis, an endemic species, remains understudied despite its ecological roles in reef- building, biofiltration, and as a food source. While previous studies have revealed the alarming impacts of ocean acidification and warming on bivalves, little is known about how the planktonic larvae of D. sandvichensis will respond to projected climate conditions.
To investigate these effects, the larvae of D. Sandvichensis were reared under present-day conditions in Pearl Harbor, HI (pH 8.1, 26.5 °C) and projected future conditions (pH 7.7, 30.0°C) for one week. Shell growth, density, and degradation were then measured using micro-CT, confocal, and scanning electron microscopy. Additionally, two larval husbandry methods were evaluated, with a static-flow system paired with UV water treatment yielding approximately 11- fold higher survival than an open-flow system. Elevated temperature reduced larval survival (~67%) compared to ambient conditions. Reduced pH significantly decreased shell length (~11.8μm), while both stressors reduced shell density in Experiment 3. Shell density decreased by ~12% under elevated temperature, ~18% under reduced pH, and ~43% under the combined stressors, and was accompanied by increased shell degradation and abnormalities.
These findings indicate that elevated temperature and reduced pH impact larval oysters through multiple pathways, including reduced survival and compromised shell integrity. This study provides new insights into the vulnerability of an endemic Hawaiian species and essential data for predicting its resilience under future climate change scenarios.
Ocean acidification (OA) remains a major and underexplored threat to marine fishes, particularly regarding reproductive physiology and early life stages (ELS). Although research over the past 15 years has documented diverse OA effects, substantial knowledge gaps persist. Most studies focussed on a limited set of species from North America and Europe, leaving broad uncertainty across phylogenetic groups, geographic regions and multi-stressor conditions. In adult fish, especially females, elevated pCO2 can shift energy allocation to prioritise reproductive output at the expense of egg or clutch size. While adult and juvenile fish have well-developed acid–base balancing systems, embryos and larvae possess only rudimentary mechanisms, making them more vulnerable to OA. This article stresses the importance of understanding these physiological and mechanistic responses to predict the future of fish stocks and ecosystem health as OA intensifies due to ongoing CO2 emissions. Our results highlight that OA responses in fish are highly variable and often specific to life stage and species, with acute and sometimes stage-specific effects not fully documented. Lastly, our recommendations on targeted research and funding are necessary to address the remaining knowledge gaps, including broadening taxonomic and geographic sampling, exploring multi-stressor scenarios and improving understanding of the downstream effects of OA on fish reproduction and development. Maintaining robust fish populations is vital for food security, employment and ecosystem functioning, making continued investigation into OA’s impacts a scientific and societal priority.
Polar oceans play a major role in the global carbon cycle, absorbing a substantial fraction of human-emitted carbon dioxide and helping regulate Earth’s climate. Extreme conditions and seasonal sea-ice limit in situ observations, leaving major uncertainties in how carbon exchange varies across these regions. Consequently, the processes controlling surface ocean carbon at high latitudes remain poorly understood. Here we demonstrate that polar oceans exhibit a pronounced hemispheric asymmetry in the drivers of surface carbon variability. By combining machine learning with a data-driven regionalization of biogeochemical provinces, we reconstruct surface carbon patterns across both polar oceans over the period 1998-2022 and identify their dominant controls. Variability in the Southern Ocean is primarily governed by non-thermal processes linked to biological activity and wind-driven mixing, whereas in the Arctic Ocean thermodynamic forcing dominates in open waters and freshwater-driven stratification shapes the central basin. Polar oceans therefore do not operate as a single carbon regime. Instead, distinct mechanisms governing carbon cycling in each hemisphere are associated with opposing long-term pCO₂ trajectories, with weak or negative trends across much of the Southern Ocean but widespread increases throughout the Arctic.
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.
The GOOD-OARS International Summer School 2025 aimed to equip the next generation of ocean scientists with the multidisciplinary research skills to enhance our understanding of how marine ecosystems will respond over the coming decades and support efforts to address harmful impacts on the ocean.
OA reduces calcification, compromising the UV shield and exacerbating photoinhibition in C. officinalis.
OA amplifies PSII donor-side damage, disrupts electron flow to PSI, and suppresses PSI function.
OA shifts PSII damage-repair balance toward irreversible photoinhibition by increasing damage and suppressing repair.
Abstract
Ocean acidification (OA) exerts diverse effects on marine macroalgae, with calcified species being particularly vulnerable. Due to calcified skeletons can contribute to physical screening against solar ultraviolet radiation (UVR), OA-driven calcification loss may increase exposure of the photosynthetic apparatus to UVR. Here, we cultured Corallina officinalis under ambient CO₂ (∼420 μatm) or elevated CO₂ (∼1000 μatm), with or without UVR, under natural solar radiation. Our results confirmed that OA reduced calcification and, under UVR, enhanced donor-side impairment of photosystem II (PSII), as evidenced by an increase in the relative K-step (Wk, an indicator of OEC damage) and a decrease in the maximum quantum yield of PSII (Fᵥ/Fₘ). This donor-side injury was accompanied by a reconfiguration of energy fluxes per PSII reaction center, particularly under combined OA and UVR. These impairments further extended to intersystem electron transport and limited the linear electron flow from PSII to the intersystem chain. Photosystem I (PSI) related electron transport was also functionally constrained, as evidenced by the reduced electron transfer probability and terminal reduction yield. This, together with the reduction of cyclic electron transport around PSI, resulted in over-reduction of the intersystem chain and making PSI the limiting photosystem. Together, these results indicate that OA amplified UVR-induced net photodamage and weakened PSII repair capacity in C. officinalis, while also constraining PSI-related electron transport. These findings highlight the potential vulnerability of calcified red algae under future high-CO₂, high-UVR coastal oceans.
Ocean acidification (OA) refers to the increase in the partial pressure of CO2 and decrease in the pH of seawater resulting from the absorption of atmospheric CO2 by the ocean. This study investigated OA impacts across multiple generations (F0-F3) of the copepod Acartia tonsa which were exposed to four pH conditions (8.1, 7.8, 7.6 and 7.1). Key reproductive and developmental traits were evaluated, including egg production, hatchability, development time, fecal pellet production, and sex ratio. Results showed that pH 7.1 significantly reduced egg production, hatchability and fecal pellet production, while prolonging the N–C time; N-A time remained unaffected by pH across all groups. For sex ratio, a downward trend was observed with increasing generations and decreasing pH: in F3, female proportion in pH 7.8, 7.6 and 7.1 groups was significantly lower than in the control group, and pH 7.1 and 7.6 groups had lower F3 female proportion than in the F1 generation. A. tonsa showed some tolerance under short-term acidification conditions at pH 7.1, but its physiological functions were further reduced after long-term exposure, suggesting a limited adaptive capacity. Ecologically, A. tonsa is a dominant coastal zooplankton species that links phytoplankton to higher trophic levels (e.g., fish larvae) and mediates energy flow and biogeochemical cycling. These results highlight the risks to key zooplankton populations and associated ecosystem functioning. Future studies should focus on the long-term effects of acidification on A. tonsa and combine multi-species experiments with field surveys to assess the ecological risks of OA.
Did you know that the ocean plays a vital role in controlling the planet’s temperature, as well as providing food, minerals, and half of the oxygen we breathe? Another important role is balancing the amount of carbon dioxide gas in the atmosphere. Unfortunately, due to large emissions of carbon dioxide over the past few centuries, the ocean is becoming acidic. This process is called ocean acidification. Ocean acidification affects many sea organisms, especially those with shells, like mussels and corals, but it can also affect other animals’ sense of orientation, like fish. Scientists are discovering more about the potential impacts of ocean acidification on the marine system and, consequently, on humans and other life forms who depend on or explore the ocean. In this article, we explain why ocean acidification started, where it has been detected, its present and future impacts on marine life, and how we can help correct it.
Climate change explains ∼40% of biological stress variance in Mediterranean mussels.
Ocean acidification drives 67% of metal bioavailability changes via pH-metal speciation.
64% of climate effects on biomarkers operate indirectly through metal bioaccumulation.
Multi-method convergence (LMG, GAM, SEM, Bayesian Networks) confirms stressor ranking.
Only SSP1-2.6 keeps biological stress below the chronic-stress threshold through 2050.
Abstract
Marine coastal ecosystems face concurrent pressure from climate change and anthropogenic contamination, yet their relative contributions to biological stress remain poorly quantified. Here we present a decade-long (2014–2023) biomonitoring study on Mytilus galloprovincialis in the Ligurian Sea (NW Mediterranean), integrating quarterly biomarker measurements, heavy metal bioaccumulation data (12 metals), and high-resolution oceanographic records at a control site (Gorgona Island Marine Protected Area) and an offshore regasification terminal.
Biological stress variance was partitioned using five complementary analytical frameworks — Lindeman–Merenda–Gold (LMG) variance decomposition, Hierarchical and Generalised Additive Models (HGAM/GAM), Structural Equation Modelling (SEM), and Bayesian Networks — applied to four biomarkers: DNA damage, lysosomal membrane stability, gill tissue integrity, and immune response. A campaign-specific T0 baseline normalisation isolated environmental signals from initial population variability.
Climate change emerged as the dominant driver, consistently explaining ∼40% of variance across all methods, significantly exceeding metal bioaccumulation (∼30%), terminal influence (∼13%), and seasonal effects (∼2%). Ocean acidification was the primary climate mechanism, influencing 67% of analysed metals. Causal mediation analysis revealed that 64% of the climate effect operates indirectly through enhanced metal bioaccumulation (Climate→Metals→Biomarkers), while 36% acts directly. Climate and biological stress indices co-varied strongly (ρ = 0.78, p < 0.001), with marine heatwaves coinciding with peak biomarker responses.
Under IPCC Shared Socioeconomic Pathway (SSP) scenarios, the Biological Stress Index is projected to cross chronic-stress thresholds by 2035–2040 under the high-emission scenario (SSP5-8.5) and the intermediate-emission scenario (SSP2-4.5), with only the low-emission scenario (SSP1-2.6) maintaining stress below critical levels through 2050.
These findings challenge pollution-centric monitoring paradigms and demonstrate that CO2 mitigation now constitutes the highest-leverage intervention for marine invertebrate health in the Mediterranean.
Graphical abstract
This graphical abstract illustrates the main findings of a decade-long field study (2014–2023) on the effects of climate change and metal contamination on Mytilus galloprovincialis in the Ligurian Sea (NW Mediterranean). Three panels summarise the causal chain from environmental drivers to biological outcomes. The first panel depicts the key oceanographic trends recorded at the study site: ocean warming (+0.41°C/decade), acidification (−0.020 pH units/decade), deoxygenation, and a doubling of marine heatwave frequency after 2018. The second panel shows how pH decline enhances the bioavailability of 67% of the metals analysed, driving a predominantly indirect pathway (64%) from climate stressors to biological stress, mediated by metal bioaccumulation, as revealed by structural equation modelling and Bayesian network analysis. The final panel presents the four biomarkers used to compute the Biological Stress Index (BSI), its strong temporal correlation with the Climate Change Index (ρ = 0.78), and BSI projections to 2050 under three IPCC emission scenarios, showing that only SSP1-2.6 keeps BSI below the chronic-stress threshold throughout the projection period.
