Ocean Acidification

The ocean is a major sink of anthropogenic carbon, absorbing 20~30% of anthropogenic CO₂ emissions across the air–sea interface. Intense weather systems, such as tropical cyclones, can strongly perturb the upper ocean and thus critically influence this carbon transfer. Here we develop an approach of synthesizing various observations to quantify the role of tropical cyclones in the global carbon cycle. Two primary, but competing effects are: (1) CO₂ efflux (from the ocean to atmosphere) during tropical cyclone passage and (2) CO₂ influx after tropical cyclones, associated with disturbed carbon disequilibrium by mixing upwards of colder water (cold wakes). The CO₂ efflux more than offsets the influx, resulting in net ocean carbon outgassing to the atmosphere. Annual tropical cyclone-induced carbon outgassing has decreased over the past three decades, from 0.09 ± 0.02 PgC in the 1990s to 0.05 ± 0.01 PgC in the 2010s. A strengthening of the vertical temperature gradient in the upper ocean due to anthropogenic climate warming leads to cold wakes with more intense surface cooling and increased carbon uptake after tropical cyclones. This has implications as the vertical temperature gradient continues to grow under high-emissions scenarios, tropical cyclones will cause net increasing ocean carbon uptake and more severe ocean acidification.

The Chicxulub asteroid impact at the Cretaceous–Paleogene (K–Pg) boundary (66 Ma) is thought to have caused the extinction of around 75% of species in the fossil record by triggering catastrophic environmental changes¹. However, despite decades of research, the mechanisms linking the environmental changes to the selective extinction patterns observed in the marine fossil record remain unresolved. Here we use a global trait-based ecosystem model^2,3 to establish this causality for the marine plankton community beyond the fossilized groups. Our model simulates diversity dynamics during the initial 100 years after the K–Pg boundary and represents explicitly extinction based on biomass thresholds that scales with body size. Under K–Pg climatic forcings, the model reproduces successfully key observed extinction patterns, including the high vulnerability of planktic foraminifera and other zooplankton, the survival of small mixotrophs⁴ and phytoplankton^5,6, and potential for reduced diversity loss in high-latitude settings⁷. Our analysis suggests that impact-driven darkness and body-size-dependent extinction thresholds drove most of the observed extinction patterns. These results suggest that plankton ecologies enhance survival through differences in energy demand and acquisition. Our study bridges the gap between fossil evidence of extinction patterns and the K–Pg impact winter hypothesis, highlighting the value of trait-based models for understanding past biodiversity crises.

Marine ecosystems are increasingly vulnerable to multiple stressors associated with climate change, resulting in significant ecological impact including ocean acidification. A 30-day experiment was conducted to investigate the effect of acidified seawater on the growth performance, nutritional status and free neuromast of olfactory organ condition of early larval stage of Asian seabass (Lates calcarifer) larvae. In this experiment, carbon dioxide (CO₂) gas was introduced to lower seawater pH, and a timer system was installed to maintain the pH within specific ranges (5.5, 6.0, 6.5, and 7.0) while, a control treatment (pH fluctuating from 7.8 to 8.5) was also set, mimicking the current pH value of the seawater. Asian seabass larvae (initial total length: 2.13 ± 0.23 mm) were stocked at 30 individual/L in a 7L experimental aquarium in triplicate. The highest survival rate was obtained by Asian seabass larvae reared in control treatment 30.9±8.6% %, while total mortality was observed in pH 5.5 as early as day 1, followed by pH 6.0 and 6.5 at day 2 and 7.0 at day 5, respectively. The larvae in control group showed significantly better growth (14.25±1.02 mm) with excellent nutritional condition. Meanwhile, exposure to acidified seawater significantly reduced the length and density of larval olfactory neuromast hair cells compared to the control. It was concluded that acidified seawater induced mortality at early stage and triggered poor morphological development, resulting from inadequate nutritional condition and impaired sensory function.

This study comprehensively assesses the long-term variability of temperature, ocean acidity changes, and their implications on sound absorption and acoustic propagation in the Bay of Bengal. The analysis reveals a persistent warming trend in the Indian Ocean over the past 50 years, with a significant increase in temperature observed during the Sagar Maitri cruise in 2019. Thermal structure analysis using HadleySST EN4 data indicates warming in the upper 50m but a cooling trend in the 100-200m depth range. Oceanic Heat Content analysis highlights an increasing tendency of heat storage in the upper 50m, indicative of global warming.

In the context of surface ducted propagation, Sonic Layer Depth (SLD) and gradients in the Sound Speed Profile (SSP) were crucial factors influencing acoustic energy behavior. The study revealed a decreasing trend in in-layer gradient (Gr_SL) since 1990, intensifying after that period. The below-layer gradient (Gr_BL) also exhibited a decreasing trend, implying complex dynamics in the sonic layer with potential implications for sound propagation in the surface duct.

The investigation into pH changes spanning 65 years demonstrates a declining trend, particularly since the 1990s, attributed to increased atmospheric CO₂ dissolution. The study linked this decrease to anthropogenic activities, aligning with global trends. The analysis of sound absorption illustrated a nonlinear relationship between absorption, frequency, and pH, emphasizing a significant impact of ocean acidification on sound absorption in the Bay of Bengal. The acoustic propagation modeling further highlighted a decrease in transmission loss with reducing pH, leading to increased sound travel and potentially noisier oceans. Salinity variations play a more significant role than temperature in influencing sound absorption.

Coral reefs are among the most extraordinary ecosystems on Earth. They are living structures built by countless tiny polyps, yet they rival tropical rainforests in biodiversity, productivity, and ecological importance. They are subject to global, well-known, nonhuman disturbances, such as intense ocean currents, storm impacts, extreme weather events, climatic variations, disease, and predator outbreaks. They are recognized by the global human society for their care and preservation in a variety of Protected Areas. Coral reefs are also affected by the deleterious effects of diverse human activities, including local activities such as fisheries and tourism, and regional activities such as deforestation – illustrated by the unexpected impact of large logs on the coral crest – agriculture, the oil industry, coastal urban development, river outflow quality and quantity, nutrients, and contaminants. These factors collectively cause a harmful synergistic effect. Additionally, coral reefs are vulnerable to the long-term effects of climate change, including sea level rise, acidification, and high temperatures. Over evolutionary timescales, several forces have shaped coral reefs. For instance, Hamilton et al. [1] note that deforestation on tropical islands releases sediments that travel through rivers into the ocean. These sediments settle into reef crevices, effectively “suffocating” the habitat. Furthermore, this research emphasizes that water quality is degraded not only by land-based runoff (sedimentation) but also by the transport of agricultural nutrients. These nutrients promote macroalgal blooms, which directly compete with coral for space and sunlight. Knowledge of volcanic activity today still focuses on human risk to infrastructure and human life, while attention to potential effects on natural resources remains minimal. For example, Loughlin et al. [2] discuss how risk is calculated based on “Exposure” and “Vulnerability,” traditionally measured by human population density and capital assets.

Oysters (Crassostrea virginica) are critical foundation species in estuaries, providing numerous ecological and economic benefits. However, oyster populations have diminished worldwide. Effective oyster restoration and aquaculture require a mechanistic understanding of the physiological and environmental factors that govern oyster feeding, growth, and resilience under changing coastal conditions. We investigated how oyster ploidy and environmental conditions influenced oyster feeding and investigated how changes in abiotic conditions affected behavioral performance of oyster drills (Stramonita spp.), a key oyster predator. To better understand feeding responses and behaviors of both predator and prey we 1) used in-situ filter feeding assays to determine feeding differences existed amongst diploid and triploid oysters, 2) gathered a baseline for growth and in-situ feeding rates of oysters across Mississippi Sound in the Northern Gulf in the Spring, Summer, and Fall, 3) simulated present-day and projected future pH conditions (7.0-8.8) to analyze oyster feeding responses, and 4) introduced oyster drills to acidified conditions (7.0-8.8) to monitor behavior and foraging rates. Diploid oysters exhibited higher overall feeding rates, yet equivalent absorption efficiency between ploidies demonstrates a difference in energy allocation which might be the key to triploids’ ability to grow quickly. These findings highlight the role of intrinsic genetic and physiological traits in shaping oyster performance and provide a baseline for interpreting responses to environmental variability. Across spatial and seasonal variation in oyster in-situ feeding and growth across three contrasting sites in Mobile Bay and Mississippi Sound, in the Northern Gulf on the western border of Alabama and Mississippi, results revealed strong spatial and seasonal variability in feeding and growth. This was driven primarily by seston composition and salinity. Under present-day and projected future ocean acidification conditions, overall oyster feeding rates declined with lower pH’s, absorption efficiency remained stable, suggesting partial physiological compensation. These results indicate that pH can impose sublethal constraints on energy acquisition and growth, with individual variability at extreme pH highlighting potential acclimation or tolerance thresholds. When subjecting the oyster’s predator, the oyster drill, to similar pH conditions (7.0-8.8) experimental results indicate that decreased pH may increase drill foraging times. Behaviors like inactivity and climbing out of the water indicate a stress response under both high and low pH, demonstrating the complexity of predicting predator-prey outcomes under more acidic conditions. Collectively, these chapters demonstrate that oyster feeding, growth, and survival are shaped by both intrinsic traits, such as ploidy, and extrinsic factors including environmental variability and ocean acidification. Understanding the interplay between physiological plasticity, seston quality, and predator-prey interactions is essential for informing restoration and aquaculture strategies that sustain ecological function and the ecosystem services oysters provide.

Microalgae are an important feedstock in aquaculture with significant economic potential in generating a diversity of bioproducts. To facilitate expansion of microalgal cultivation, a continuous automated bioreactor that uses waste acid to increase carbon bioavailability in seawater for enhanced biomass production was designed and tested with Tetraselmis suecica UTEX2286. Carbon bioavailability was inferred from culture pH and bioreactor headspace CO₂ concentration measurements and controlled via acidification and seawater dilution. Operating over a period of several days, the culture exhibited greater biomass productivity at a pH setpoint of 7-7.5. Outside of this range, algal activity slowed, accompanied by greater CO₂ released to the headspace and lower pH during incubation. Increasing the carbon introduced to the bioreactor by increasing the dilution factor did not significantly increase the algal productivity. Importantly, acidification led to statistically significant gains in biomass productivity. Preliminary cost analysis showed while seawater is inexpensive, the acid cost drives the overall cost of the designed bioreactor system. Thus, the designed bioreactor and control scheme supports algal cultivation but requires low-cost acid to be economical, which may be achieved by strategically integrating microalgae cultivation with other coastal industries.

This data paper describes bacterial and fungal communities associated with the sponge Chondrilla nucula collected from two Eastern Mediterranean populations (North and South Aegean Sea) and maintained under controlled common-garden conditions simulating present and projected climate scenarios over a period of 3 months. Microbial composition was characterised using two complementary ribosomal marker approaches: Illumina (MiSeq) sequencing of the 16S rRNA gene for Bacteria and Oxford Nanopore (MinION) sequencing of a long 18S-ITS-28S rRNA fragment for Fungi. A total of 24 sponge libraries (3 climate conditions x 2 populations x 4 biological replicates) along with six control libraries (water from three experimental tanks, extraction and PCR blanks) were constructed for each group of microsymbionts. The resulting reads were processed using custom and publicly available bioinformatic pipelines and databases, followed by initial taxonomic assignment. This dataset represents the first fungal community associated with C. nucula and the first bacterial community for this species from the Aegean Sea.

Diatoms are of significance in the marine ecosystem, playing a pivotal role in the sustenance of marine life and the transfer of carbon from the surface ocean to deeper waters. Although numerous studies have investigated the effects of elevated carbon dioxide (CO₂) on marine diatoms across both short- and long-term adaptation scales, the molecular mechanisms governing chitin metabolism in response to ocean acidification remain poorly understood. In this study, we employed an integrated approach combining transcriptomic, metabolomic, and physiological analyses to examine the marine diatom Thalassiosira weissflogii following 40-day acclimation to high-CO₂ conditions. Physiological studies have demonstrated that ocean acidification has the capacity to result in an augmentation of the C/N ratio, chitin content, maximum PSII quantum yield (F_v/F_m), and photosynthetic pigment content of T. weissflogii. Analysis of chlorophyll fluorescence dynamics further demonstrated enhanced primary photochemical efficiency of PSII in the acidified treatment group. Consistent with this, the transcriptome results also showed that the photosynthesis-related pathways were upregulated to meet the increased material and energy requirements after adaptation to elevated CO₂ levels. More importantly, it was determined that acidification treatment resulted in the upregulation of chitin synthesis and the downregulation of chitin degradation in T. weissflogii, consequently leading to an augmentation in chitin content. These findings indicate that ocean acidification (high CO₂, low pH) prompts T. weissflogii to prioritize the allocation of carbon resources to the synthesis of chitin. The synthesis of chitin may reinforce cell wall formation as an adaptive response to ocean acidification. Our research provides new insights into the marine acidification adaptation strategies of T. weissflogii.

Coral reefs consist of diverse benthic habitats that influence seawater CO₂ chemistry variability on multiple spatial and temporal scales. Understanding the present-day seawater CO₂ chemistry variability across both habitat-specific and reef-wide scales is critical to accurately predict the effects of future environmental change. Here, we utilize autonomous sensors and discrete seawater samples across diverse habitats at multiple scales ranging from habitat-specific (inner lagoon, patch reefs and seagrass beds; 0.02–0.72km²) to reef-wide scales at Dongsha Atoll (250km²) and Taiping Island (20km²) to characterize seawater chemistry. Across all habitats, daily mean pH ranged from 7.79–8.60 with mean diel variability ranging from 0.19–0.91. Spatially, pH variability ranged from 0.08 (patch reef) to 1.29 (inner lagoon). Biogeochemical modification of seawater chemistry was dominated by organic carbon cycling at individual habitat scales, whereas inorganic carbon cycling dominated at the scale of Dongsha Atoll. The largest alkalinity depletion (net calcification) was associated with patch reef habitats, whereas the highest alkalinity repletion was associated with a semi-enclosed lagoon. Under two climate change scenarios (linear dissolved inorganic carbon increase derived from historical observations and the CMIP6 SSP5-8.5 pathway), pH and/or aragonite saturation state (Ω_Ar) observations across all habitats in this study are projected to be below proposed thresholds for net reef accretion (pH < 7.7: inner lagoon ~ 10–13%; seagrass beds ~ 21–44%; patch reefs ~ 0–100%; atoll-wide ~ 4–98% of observations) or net dissolution (Ω_Ar < 2.92: inner lagoon ~ 10–18%; seagrass beds ~ 44–75%; patch reefs ~ 77–100%; atoll-wide ~ 94–100% of observations) by the year 2100. The results highlight the importance of habitat-specific and scale-conscious assessments of future coral reef environmental conditions.

Highlights

• Combined acidification and hypoxia trigger significant mortality in Magallana gigas.
• Seawater acidification suppresses the nonspecific immune response in Magallana gigas.
• Gill histopathology and immunosuppression are most pronounced with combined exposure.

Abstract

Rising atmospheric carbon dioxide leads to oxygen depletion and increased acidification in marine areas worldwide. The combined effects of these two stressors on the health of commercially important bivalves have not been sufficiently studied. We experimentally studied the effects of water acidification in combination with normoxic and hypoxic conditions on the parameters of cellular immunity and the gills microstructure of the Magallana gigas. We evaluated the hemolymph cellular composition, the total number and phagocytosis capacity of hemocytes, and also evaluated the histopathology of oyster gills during an 8-day experimental period. The oysters were exposed to low pH conditions (7.3), either under normoxic conditions (dissolved oxygen concentration of 8.0 mg/L) or hypoxic conditions (dissolved oxygen concentration of 2.0 mg/L). The parameters were assessed at days 1, 3, 6, and 8 of the experiment. It was shown that acidification of the aquatic environment causes significant suppression of oyster immunity in both normoxia and hypoxia, leading to a decrease in phagocytic capacity and ROS production by hemocytes. The combined effect of these factors increased the negative impact, ultimately leading to the oyster death at the end of the experiment. In addition, the effects of acidification caused serious and progressive histopathological damage to the oyster gills, while the most severe and frequent pathologies, such as almost complete expansion of the water chambers and severe dilation of the hemal sinuses, were caused by the combined effects of acidification and hypoxia. Therefore, the synergistic impact of acidification and hypoxia poses a substantial threat to oyster health.

Highlights

• 7 CaM family members and 55 CML family members were identified in Magallana gigas.
• MgCaM and MgCML genes showed tissue-specific and developmental stage-specific expression patterns.
• Distinct expression patterns emerged under heat and acidification stresses.

Abstract

Ca²⁺ is a multifunctional second messenger that can regulate the activities of hormones and environmental signals related to biotic and abiotic stresses, playing a role in a wide range of cellular processes and influencing almost all aspects of life. In organisms, calmodulin (CaM) and calmodulin-like proteins (CML) can sense and decode the regulatory signals of Ca²⁺ through the EF-hand (a helix-loop-helix structure) domain. In this study, 7 CaM family members and 55 CML family members were identified in Magallana gigas. All MgCaM and MgCML genes distributed unevenly on 7 chromosomes, with 90% of the genes located on chromosomes 6 and 5. Furthermore, the expression of MgCaMs and MgCMLs was tissue-specific in M. gigas, and most of the genes expressed highly in gill, labial palp, adductor muscle and female gonad. Through the analysis of transcriptome data, it was found that the MgCaM and MgCML genes showed specific expression patterns in response to abiotic stress. When encountering heat-shock stress, different genes responded at different time points. In response to acidification stimulation, genes in the outer edge of mantle could respond to the stimulus obviously. The expression patterns of five representative genes were validated by RT-qPCR under acidification. This study systematically analyzed the characteristics of oyster CaM and CML gene families, revealing their crucial roles in the environmental adaptation mechanisms of M. gigas.

Ocean acidification (OA) and zinc pollution co-occur in marine ecosystems, yet phytoplankton responses show conflicting patterns. We examined OA (1200 µatm CO₂) and zinc (sub-toxic SZn: 1.20 mg L⁻¹; toxic TZn: 2.40 mgL⁻¹) effects on Thalassiosira weissflogii. OA inhibited growth under SZn but enhanced it under TZn. Under SZn, OA elevated oxidative stress markers—with superoxide dismutase (SOD) activity increasing by 48.12%, catalase (CAT) activity by 114.22%, and malondialdehyde (MDA) content by 76.27% compared to the SZn treatment—and increased cellular Zn content by 49.56% relative to SZn alone. Conversely, under TZn, OA reduced SOD by 61.31%, CAT by 45.45%, MDA by 25.29%, and cellular Zn by 35.96% compared to the TZn treatment and stimulated glycan/protein production. Gene expression revealed OA suppressed photosynthesis, TCA cycle, and oxidative phosphorylation genes under SZn. Under TZn, OA alleviated photosynthetic inhibition and upregulated gluconeogenesis, glycolysis, and oxidative phosphorylation genes. These findings indicate that OA exacerbates the detrimental effects of sub-toxic Zn on T. weissflogii, while mitigating the toxicity of Zn at lethal concentrations. This study provides a potential mechanistic explanation for contradictory reports on microalgal responses to heavy metal stress under OA and significantly advances our understanding of marine organismal adaptation to multiple stressors, offering critical insights into the ecological risks posed by zinc under future OA scenarios.

Highlights

• Eight CgSLC4 genes family members were identified.
• CgSLC4s exhibited tissue-specific and developmentally variable expression patterns.
• CgSLC4 gene family responds to acidification stress in different mantle folds.
• CgSLC4A10-1 shows marked acidification responsiveness, especially in mantle epithelium cell.

Abstract

The solute carrier 4 (SLC4) family represents a category of integral membrane transporters responsible for bicarbonate mediation, which is vital for numerous fundamental biological functions. In this study, eight SLC4 genes were identified and annotated in Crassostrea gigas genome, comprising one member of Cl⁻/HCO₃⁻ exchanger, five genes coding Na⁺-dependent HCO₃⁻ transporters, and two Na⁺-coupled borate transporter copies, which were located on three chromosomes. In general, the expression of CgSLC4s showed tissue specificity, and differential expression patterns of CgSLC4s was observed at different developmental stages. The CgSLC4 family genes displayed divergent responses to acidification across different mantle folds. Among these family members, CgSLC4A10-1 exhibited the most dramatic and statistically significant expression changes in response to acidification across mantle folds, with fold changes ranging from 0.008-fold down-regulation to 85.95-fold up-regulation. According to the results of RT-qPCR and immunofluorescence, after 14 days of acidification treatment, CgSLC4A10-1 mRNA expression level was significantly increased, immunoblotting signal intensity was also enhanced in the epithelial cells. These results provide a general characterization of the SLC4 gene family in C. gigas, which may provide a systematic overview of the SLC4 gene family in C. gigas, and lay a foundation for future studies to explore its potential involvement in ion homeostasis and acidification adaptation in bivalve mollusks.

Highlights

• Higher salinity and applied voltage accelerated acidification kinetics.
• Buffer capacity (HCO₃⁻) negatively impacted acidification kinetics.
• Non-ideal membrane permselectivity allowed considerable transport of co-ions.
• Natural waters of low conductivity showed lower SEC during acidification.

Abstract

Anthropogenic CO₂ emissions have reached unprecedented levels, where approximately 30% is absorbed by aquatic systems. This study investigated the acidification of natural waters using bipolar membrane electrodialysis (BPMED), a crucial step in the extraction of CO₂ from water, i.e., key Carbon Dioxide Removal (CDR) technology. Natural waters of different salinity and bicarbonate concentration were selected: seawater (51.2 mS/cm, 140 mg/L HCO₃⁻), brackish groundwater (5.4 mS/cm, 290 mg/L HCO₃⁻), and surface freshwater (0.4 mS/cm, 162 mg/L HCO₃⁻), where BPMED shifted the carbonate equilibrium toward H₂CO₃/CO_2(aq), which could be subsequently extracted, thus making the current approach sustainable. The influence of applied voltage, the acidified:basified volume ratio, and hydrochemical composition on acidification kinetics, energy consumption, and ion transport was analyzed. Results showed that higher salinity and applied voltage accelerated acidification kinetics, whereas buffer capacity (HCO₃⁻) slowed down the acidification process. In addition, waters with lower salinity required lower specific energy (1.02 vs 1.86 kWh/kg CO₂ for freshwater and seawater, respectively) to achieve acidification (final pH 4), while increasing the acidified:basified volume ratio led to higher energy consumption. Acidified water exhibited increased Na⁺ and Cl⁻ concentrations, while K⁺ and HCO₃⁻ decreased across all water sources. These findings contribute to the scientific understanding of electrochemically-based water acidification processes and provide insights into sustainable approaches for managing dissolved CO2 in natural waters.