Impact of Ocean Acidification on the Energy Metabolism and Antioxidant Responses of the Yesso Scallop (Patinopecten yessoensis)

Combined effects of toxic Microcystis aeruginosa and high pH on antioxidant responses, immune responses, and apoptosis of the edible freshwater bivalve Corbicula fluminea

Due to increasing anthropogenic perturbation and water eutrophication, cyanobacterial blooms (CYBs) have become a global ecological and environmental problem. Toxic CYBs and elevated pH are considered to be the two key stressors associated with eutrophication in natural waters, particularly in the event of CO2 depletion induced by dense blooms. However, previous research has been focused on investigating the impacts of toxic CYBs or pH changes in isolation, whereas the interactive effects of such stressors on edible bivalves that inhabit CYB waters still lack information. In this study, the combined effects of toxic Microcystis aeruginosa and pH shifts on the antioxidant responses, immune responses, and apoptosis of the edible freshwater bivalve Corbicula fluminea were explored. The results showed that the activity of antioxidant enzymes was significantly impacted by the interactive effects between toxic M. aeruginosa exposure and time course, yet pH shifts showed no significant effects on the activities of these antioxidant enzymes, implying that the antioxidant response in C. fluminea was mainly triggered by toxic M. aeruginosa exposure. Toxic M. aeruginosa also induced an increased production of reactive oxygen species and malondialdehyde in treated clams, particularly under high pH settings. The elevated lysosomal enzyme activity helped C. fluminea defend against toxic M. aeruginosa exposure under high pH conditions. The principal component analysis (PCA) and the integrated biomarker response (IBR) results suggested that the treated clams were subjected to the elevated toxicity of toxic M. aeruginosa in conditions of high pH. The heat shock proteins-related genes might be triggered to resist the oxidative damage in treated clams. Moreover, the upregulation of TNF and casp8 genes indicated the potential activation of the caspase8-mediated apoptotic pathway through TNF receptor interaction, potentially resulting in apoptosis. The TUNEL assay results further confirmed that apoptosis appeared in treated clams. These findings improve our understanding of the combined toxicological effects of harmful algae and pH shifts on bivalves, which will provide insights into a comprehensive ecological risk assessment of toxic CYBs to edible bivalve species.

Living under natural conditions of ocean acidification entails energy expenditure and oxidative stress in a mussel species

We investigated the health conditions of the Mediterranean mussel Mytilus galloprovincialis recruited in the CO2 vents system of Castello Aragonese at Ischia Island (Mediterranean Sea). Individuals of M. galloprovincialis were sampled in three sites along the pH gradient (8.10, 7.7 and up to <7.4). Untargeted metabolomics and biochemical endpoints related to energetic metabolism, oxidative stress/damage, neurotoxicity and immune defense were analyzed. Corrosion of the valves occurred at low pH. A separation of the metabolome was observed along the pH gradient. Metabolites belonging to amino acids, nucleosides, lipids and organic osmolytes were significantly reduced in the organisms from the most acidified sites. The content of reactive oxygen species and the activity of glutathione peroxidase were reduced in organisms from the acidified sites compared to ambient pH, and no oxidative damage was induced. Overall results suggested the presence of an energy cost underpinning long-term survival in acidified conditions for this species.

Sex-specific responses of Ruditapes philippinarum to ocean acidification following gonadal maturation

Ocean acidification (OA) can seriously affect marine bivalves at different levels of biological organization, generating widespread consequences on progeny recruitment and population maintenance. Yet, few effort has been devoted to elucidating whether female and male bivalves respond differentially to OA in their reproductive seasons. Here, we estimated differences in physiological responses of female and male Manila clams (Ruditapes philippinarum) to OA during gonadal maturation. In comparison to OA-stressed male clams, females significantly depressed activities in enzymes related to energy metabolism (NKA, T-ATP), antioxidant defence (SOD and MDA), and non-specific immune function (ACP), and downregulated expression of AMPK that plays a key role in cellular metabolism, indicating that sex did significantly affect responses of R. philippinarum to OA. Such sex-based differences can be likely couched in energetic terms, given the much more energetically expensive cost of egg production than that of sperms. These results indicate that sex-specific responses to OA during reproductive seasons do exist in marine bivalves, and therefore accounting for such sex specificity is of paramount importance when projecting population sustainability and formulating conservation strategies in an acidifying ocean.

Metabolic profiling of Mytilus coruscus mantle in response of shell repairing under acute acidification

Mytilus coruscus is an economically important marine bivalve mollusk found in the Yangtze River estuary, which experiences dramatic pH fluctuations due to seasonal freshwater input and suffer from shell fracture or injury in the natural environment. In this study, we used intact-shell and damaged-shell M. coruscus and performed metabolomic analysis, free amino acids analysis, calcium-positive staining, and intracellular calcium level tests in the mantle to investigate whether the mantle-specific metabolites can be induced by acute sea-water acidification and understand how the mantle responds to acute acidification during the shell repair process. We observed that both shell damage and acute acidification induced alterations in phospholipids, amino acids, nucleotides, organic acids, benzenoids, and their analogs and derivatives. Glycylproline, spicamycin, and 2-aminoheptanoic acid (2-AHA) are explicitly induced by shell damage. Betaine, aspartate, and oxidized glutathione are specifically induced by acute acidification. Our results show different metabolic patterns in the mussel mantle in response to different stressors, which can help elucidate the shell repair process under ocean acidification. furthermore, metabolic processes related to energy supply, cell function, signal transduction, and amino acid synthesis are disturbed by shell damage and/or acute acidification, indicating that both shell damage and acute acidification increased energy consumption, and disturb phospholipid synthesis, osmotic regulation, and redox balance. Free amino acid analysis and enzymatic activity assays partially confirmed our findings, highlighting the adaptation of M. coruscus to dramatic pH fluctuations in the Yangtze River estuary.

The intestinal bacterial community over seasons and its relationship with physiological status of Yesso scallop Patinopecten yessoensis

Emerging evidence indicates that the intestinal bacterial communities associated with eukaryotes play critical roles in the physiological activities and health of their hosts. Yesso scallop Patinopecten yessoensis, one of the cold-water aquaculture species in the North Yellow Sea of China, has suffered from massive mortality in recent years. In the present study, P. yessoensis were collected from Zhangzi Island, Dalian from March 2021 to January 2022 to investigate the intestinal bacterial community and physiological indices. 16S rRNA gene sequencing data revealed that the diversity of intestinal bacteria changed significantly over seasons, with the highest Chao1 (237.42) and Shannon (6.13) indices detected in January and the lowest Chao1 (115.44) and Shannon (2.73) indices detected in July. Tenericutes, Proteobacteria and Firmicutes were dominant phyla in the intestinal bacteria of P. yessoensis, among which Firmicutes and Proteobacteria significantly enriched in August and January, respectively. Mycoplasma was the most abundant genus during the sampling period, which exhibited the highest abundance in October (75.26%) and lowest abundance in August (13.15%). The functional profiles of intestinal bacteria also exhibited seasonal variation, with the pathways related to pentose phosphate and deoxyribonucleotides biosynthesis enriched in August while the glycogen biosynthesis pathway enriched in October. Redundancy analysis showed that seawater pH, dissolved inorganic nitrogen and silicate were major environmental factors driving the temporal succession of scallop intestinal bacteria. Correlation clustering analysis suggested that the relative abundances of Endozoicomonas and Vibrio in the intestine were positively correlated with superoxide dismutase activity in hepatopancreas while negatively correlated with malondialdehyde content in hepatopancreas and glycogen content in adductor muscle. All the results revealed that the intestine harbored a lower bacterial diversity and a higher abundance of Vibrio in August, compared to January, which were closely related to the oxidative stress status of scallop in summer. These findings will advance our understanding of the relationship between seasonal alteration in the intestinal bacteria and the physiological status of scallops.

Effect of seawater acidification on physiological and energy metabolism responses of the common Cockle (Anadara antiquata) of Gazi Bay, Kenya

Ocean acidification (OA) is becoming a potential threat to marine organisms, especially in calcifying marine invertebrates. So far, along the Kenya Coast, there has been little research on the impact of OA on cockle (Anadara antiquata), particularly on their physiological impacts induced by exposure to acidified seawater. Hence, this study aimed to investigate the physiological and biochemical responses of Anadara antiquata under present and future predicted seawater pH. In this study, the Anadara antiquata was exposed to three pH treatments (pH 7.90, 7.60, and 7.30) for 8 weeks to mimic future OA and to understand the physiological and biochemical effects on the organisms. Condition index, energy reserves (glycogen and protein), and cellular damage (e.g., lipid peroxidation level) were measured. Condition index (CI) showed no significant difference at different pH treatments (pH 7.90, 7.60, and 7.30), whereas the survival Anadara antiquata was slightly reduced after 8 weeks of exposure to pH 7.30. Glycogen and protein content were not affected at reduced pH (7.60 and 7.30). However, after 8 weeks of exposure to pH 7.60 and 7.30, Anadara antiquata showed a slight decrease in lipid peroxidation, an indication of cellular damage. The physiological and biochemical parameters analyzed (glycogen and protein content; lipid peroxidation levels) showed useful biomarkers to assess ocean acidification impacts in cockle.

CO2-Induced Ocean Acidification Alters the Burrowing Behavior of Manila Clam Ruditapes philippinarum by Reversing GABAA Receptor Function

Biological burrowing behavior is an important driver shaping ecosystems that is being threatened by CO₂-induced ocean acidification; however, the effects of ocean acidification on burrowing behavior and its neurological mechanism remain unclear. This study showed that elevated pCO₂ significantly affected the burrowing behaviors of the Manila clam Ruditapes philippinarum, such as increased foot contraction, burrowing time, and intrabottom movement and decreased burrowing depth. Delving deeper into the mechanism, exposure to elevated pCO₂ significantly decreased extracellular pH and increased [HCO₃^-]. Moreover, an indicator GABA_A receptor, a neuroinhibitor for movement, was found to be closely associated with behavioral changes. In situ hybridization confirmed that the GABA_A receptor was widely distributed in ganglia and foot muscles, and elevated pCO₂ significantly increased the mRNA level and GABA concentration. However, the increase in GABA_A receptor and its ligand did not suppress the foot movement, but rather sent "excitatory" signals for foot contraction. The destabilization of acid-base homeostasis was demonstrated to induce an increase in the reversal potential for GABA_A receptor and an alteration in GABA_A receptor function under elevated pCO₂. This study revealed that elevated pCO₂ affects the burrowing behavior of Manila clams by altering GABA_A receptor function from inhibitory to excitatory.

Effect of heat and hypoxia stress on mitochondrion and energy metabolism in the gill of hard clam

Aquatic animals suffer from heat and hypoxia stress more frequently due to global climate change and other anthropogenic activities. Heat and hypoxia stress can significantly affect mitochondrial function and energy metabolism. Here, the response and adaptation characteristics of mitochondria and energy metabolism in the gill of the hard clam Mercenaria mercenaria under heat (35 °C), hypoxia (0.2 mg/L), and heat plus hypoxia stress (35 °C, 0.2 mg/L) after 48 h exposure were investigated. Mitochondrial membrane potentials were depolarized under environmental stress. Mitochondrial fusion, fission and mitophagy played a key role in maintain mitochondrion function. The AMPK subunits showed different expression under environmental stress. Acceleration of enzyme activities (phosphofructokinase, pyruvate kinase and lactic dehydrogenase) and accumulation of anaerobic metabolites in glycolysis and TCA cycle implied that the anaerobic metabolism might play a key role in providing energy. Accumulation of amino acids might help to increase tolerance under heat and heat combined hypoxia stress. In addition, urea cycle played a key role in amino acid metabolism to prevent ammonia/nitrogen toxicity. This study improved our understanding of the mitochondrial and energy metabolism responses of marine bivalves exposed to environmental stress.

Increased Food Resources Help Eastern Oyster Mitigate the Negative Impacts of Coastal Acidification

Oceanic absorption of atmospheric CO2 results in alterations of carbonate chemistry, a process coined ocean acidification (OA). The economically and ecologically important eastern oyster (Crassostrea virginica) is vulnerable to these changes because low pH hampers CaCO3 precipitation needed for shell formation. Organisms have a range of physiological mechanisms to cope with altered carbonate chemistry; however, these processes can be energetically expensive and necessitate energy reallocation. Here, the hypothesis that resilience to low pH is related to energy resources was tested. In laboratory experiments, oysters were reared or maintained at ambient (400 ppm) and elevated (1300 ppm) pCO2 levels during larval and adult stages, respectively, before the effect of acidification on metabolism was evaluated. Results showed that oysters exposed to elevated pCO2 had significantly greater respiration. Subsequent experiments evaluated if food abundance influences oyster response to elevated pCO2. Under high food and elevated pCO2 conditions, oysters had less mortality and grew larger, suggesting that food can offset adverse impacts of elevated pCO2, while low food exacerbates the negative effects. Results also demonstrated that OA induced an increase in oyster ability to select their food particles, likely representing an adaptive strategy to enhance energy gains. While oysters appeared to have mechanisms conferring resilience to elevated pCO2, these came at the cost of depleting energy stores, which can limit the available energy for other physiological processes. Taken together, these results show that resilience to OA is at least partially dependent on energy availability, and oysters can enhance their tolerance to adverse conditions under optimal feeding regimes.

Differential gene expression analysis in the scallop Argopecten purpuratus exposed to altered pH and temperature conditions in an upwelling-influenced farming area

Increased carbon dioxide in the atmosphere and its absorption across the ocean surface will alter natural variations in pH and temperature levels, occurring in coastal upwelling ecosystems. The scallop Argopecten purpuratus, one of the most economically important species farmed in northern Chile, has been shown to be vulnerable to these environmental drivers. However, the regulatory responses at the gene-level of scallops to these climate stressors remain almost unknown. Consequently, we used an orthogonal experimental design and RNAseq approach to analyze the acute effects of variability in pH and temperature on gene expression in the muscle tissue of A. purpuratus. In respect to control conditions (pH ~ 8.0/ 14 °C), the influence of low pH (~ 7.7) and temperature (14 °C) induced the activation of several genes associated with apoptotic signaling pathways and protein localization to plasma membrane. Elevated temperature (18 °C) and pH (~8.0) conditions increased the expression of transcripts associated with the activation of muscle contraction, regulation, and sarcomere organization effects on muscle tissue. In scallops exposed to low pH and elevated temperature, the genes expressed were differentially associated with the oxidation-reduction process, signal translation, and positive regulation of GTPase activity. These results indicated that the differentially expressed genes under the experimental conditions tested are mainly related to the mitigation of cellular damage and homeostasis control. Our results add knowledge about the function of the adductor muscle in response to stressors in scallops. Furthermore, these results could help in the identification of molecular biomarkers of stress necessary to be integrated into the aquaculture programs for the mitigation of climate change.

Effects of one-year exposure to ocean acidification on two species of abalone

Ocean acidification (OA) resulting from the absorption of excess atmospheric CO₂ by the ocean threatens the survival of marine calcareous organisms, including mollusks. This study investigated the effects of OA on adults of two abalone species (Haliotis diversicolor, a subtropical species, and Haliotis discus hannai, a temperate species). Abalone were exposed to three pCO₂ conditions for 1 year (ambient, ~ 880, and ~ 1600 μatm), and parameters, including mortality, physiology, immune system, biochemistry, and carry-over effects, were measured. Survival decreased significantly at ~ 800 μatm pCO₂ for H. diversicolor, while H. discus hannai survival was negatively affected only at a higher OA level (~ 1600 μatm pCO₂). H. diversicolor exhibited depressed metabolic and excretion rates and a higher O:N ratio under OA, indicating a shift to lipids as a metabolism substrate, while these physiological parameters in H. discus hannai were robust to OA. Both abalone failed to compensate for the pH decrease of their internal fluids because of the lowered hemolymph pH under OA. However, the reduced hemolymph pH did not affect total hemocyte counts or tested biomarkers. Additionally, H. discus hannai increased its hemolymph protein content under OA, which could indicate enhanced immunity. Larvae produced by adults exposed to the three pCO₂ levels were cultured in the same pCO₂ conditions and larval deformation and shell length were measured to observe carry-over effects. Enhanced OA tolerance was observed for H. discus hannai exposed under both of the OA treatments, while that was only observed following parental pCO₂ ~ 880 μatm exposure for H. diversicolor. Following pCO₂ ~ 1600 μatm parental exposure, H. diversicolor offspring exhibited higher deformation and lower shell growth in all pCO₂ treatments. In general, H. diversicolor were more susceptible to OA compared with H. discus hannai, suggesting that H. diversicolor could be unable to adapt to acidified oceans in the future.

Climate change and cetacean health: impacts and future directions

Climate change directly impacts the foraging opportunities of cetaceans (e.g. lower prey availability), leads to habitat loss, and forces cetaceans to move to other feeding grounds. The rise in ocean temperature, low prey availability and loss of habitat can have severe consequences for cetacean survival, particularly those species that are already threatened or those with a limited habitat range. In addition, it is predicted that the concentration of contaminants in aquatic environments will increase owing to Arctic meltwater and increased rainfall events leading to higher rates of land-based runoff in downstream coastal areas. These persistent and mobile contaminants can bioaccumulate in the ecosystem, and lead to ecotoxicity with potentially severe consequences on the reproductive organs, immune system and metabolism of marine mammals. There is a need to measure and assess the cumulative impact of multiple stressors, given that climate change, habitat alteration, low prey availability and contaminants do not act in isolation. Human-caused perturbations to cetacean foraging abilities are becoming a pervasive and prevalent threat to many cetacean species on top of climate change-associated stressors. We need to move to a greater understanding of how multiple stressors impact the metabolism of cetaceans and ultimately their population trajectory. This article is part of the theme issue 'Nurturing resilient marine ecosystems'.

Responses of Ruditapes philippinarum to contamination by pharmaceutical drugs under ocean acidification scenario

In coastal systems, organisms are exposed to a multitude of stressors whose interactions and effects are poorly studied. Pharmaceutical drugs and Climate Change consequences, such as lowered pH, are examples of stressors affecting marine organisms, as bivalves. Although a vast literature is available for the effects of these stressors when acting individually, very limited information exists on the impacts that the combination of both can have on marine bivalves. For this reason, this study aimed to evaluate the impacts of a simulated ocean acidification scenario (control pH, 8.0; lowered pH, pH 7.6) on the effects of the antiepileptic carbamazepine (CBZ, 1 μg/L) and the antihistamine cetirizine (CTZ, 0.6 μg/L), when acting individually and combined (CBZ + CTZ), on the edible clam Ruditapes philippinarum. After 28 days of exposure, drug concentrations, bioconcentration factors and biochemical parameters related to the clams' metabolic capacity and oxidative stress were evaluated. The results showed that R. philippinarum clams responded differently to pharmaceutical drugs depending on the pH tested, influencing both bioconcentration and biological responses. In general, drug combined treatments showed fewer impacts than drugs acting alone, and acidification seemed to activate at a higher extension the elimination processes that were not activated under control pH. Also, lowered pH per se exerted negative impacts (e.g., cellular damage) on R. philippinarum and the combination with pharmaceutical drugs did not enhance the toxicity.

Assessment of Paracetamol Toxic Effects under Varying Seawater pH Conditions on the Marine Polychaete Hediste diversicolor Using Biochemical Endpoints

Simple Summary

Context of climate change is being widely studied, nevertheless its effects in the toxicity of other contaminants have been poorly study. Particularly, the effects of ocean acidification on the modulation of pharmaceutical absorption and consequent effects, have not been extensively addressed before. In this study, we aimed to assess the effects of ocean acidification (specifically pH values of 8.2, 7.9, and 7.6) combined with paracetamol exposure (0, 30, 60, and 120 µg/L) on the polychaeta Hediste diversicolor. To do so, specific biomarkers were measured namely (CAT), glutathione S-transferases (GSTs), acetylcholinesterase (AChE), and cyclooxygenase (COX) activities, as well as thiobarbituric acid reactive substance (TBARS), were quantified to serve as ecotoxicological endpoints. Alterations of CAT, and GSTs activities, and TBARS levels indicate an alteration in redox balances. Differences in exposed pH levels indicate the possible modulation of the absorption of this pharmaceutical in ocean acidifications scenarios. Alterations in AChE were only observed following paracetamol exposure, not being altered by media pH. Hereby obtained results suggest that seawater acidification is detrimental to marine wildlife, since it may enhance toxic effects caused by environmental realistic concentrations of pharmaceuticals. This work is crucial to understand the potential effects of pharmaceuticals in a climate change scenario.

Abstract

Increasing atmospheric carbon dioxide (CO₂) levels are likely to lower ocean pH values, after its dissolution in seawater. Additionally, pharmaceuticals drugs are environmental stressors due to their intrinsic properties and worldwide occurrence. It is thus of the utmost importance to assess the combined effects of pH decreases and pharmaceutical contamination, considering that their absorption (and effects) are likely to be strongly affected by changes in oceanic pH. To attain this goal, individuals of the marine polychaete Hediste diversicolor were exposed to distinct pH levels (8.2, 7.9, and 7.6) and environmentally relevant concentrations of the acidic drug paracetamol (PAR: 0, 30, 60, and 120 µg/L). Biomarkers such as catalase (CAT), glutathione S-transferases (GSTs), acetylcholinesterase (AChE), and cyclooxygenase (COX) activities, as well as peroxidative damage (through thiobarbituric acid reactive substance (TBARS) quantification), were quantified to serve as ecotoxicological endpoints. Data showed a general increase in CAT and a decrease in GST activities (with significant fluctuations according to the tested conditions of PAR and pH). These changes are likely to be associated with alterations of the redox cycle driven by PAR exposure. In addition, pH levels seemed to condition the toxicity caused by PAR, suggesting that the toxic effects of this drug were in some cases enhanced by more acidic conditions. An inhibition of AChE was observed in animals exposed to the highest concentration of PAR, regardless of the pH value. Moreover, no lipid peroxidation was observed in most individuals, although a significant increase in TBARS levels was observed for polychaetes exposed to the lowest pH. Finally, no alterations of COX activities were recorded on polychaetes exposed to PAR, regardless of the pH level. The obtained results suggest that seawater acidification is detrimental to marine wildlife, since it may enhance toxic effects caused by environmental realistic concentrations of acidic drugs, such as PAR. This work was crucial to evidence that ocean acidification, in the context of a global change scenario of increased levels of both atmospheric and oceanic CO₂, is a key factor in understanding the putative enhanced toxicity of most pharmaceutical drugs that are of an acidic nature.

Climate change effects on the ecophysiology and ecological functioning of an offshore wind farm artificial hard substrate community

In the effort towards a decarbonised future, the local effects of a proliferating offshore wind farm (OWF) industry add to and interact with the global effects of marine climate change. This study aimed to quantify potential ecophysiological effects of ocean warming and acidification and to estimate and compare the cumulative clearance potential of suspended food items by OWF epifauna under current and future climate conditions. To this end, this study combined ecophysiological responses to ocean warming and acidification of three dominant colonising species on OWF artificial hard substrates (the blue mussel Mytilus edulis, the tube-building amphipod Jassa herdmani and the plumose anemone Metridium senile). In general, mortality, respiration rate and clearance rate increased during 3- to 6-week experimental exposures across all three species, except for M. senile, who exhibited a lower clearance rate in the warmed treatments (+3 °C) and an insensitivity to lowered pH (-0.3 pH units) in terms of survival and respiration rate. Ocean warming and acidification affected growth antagonistically, with elevated temperature being beneficial for M. edulis and lowered pH being beneficial for M. senile. The seawater volume potentially cleared from suspended food particles by this AHS colonising community increased significantly, extending the affected distance around an OWF foundation by 9.2% in a future climate scenario. By using an experimental multi-stressor approach, this study thus demonstrates how ecophysiology underpins functional responses to climate change in these environments, highlighting for the first time the integrated, cascading potential effects of OWFs and climate change on the marine ecosystem.

Physiological responses of the symbiotic shrimp Ancylocaris brevicarpalis and its host sea anemone Stichodactyla haddoni to ocean acidification

In this study, the physiology of symbiotic 'peacock-tail' shrimp Ancylocaris brevicarpalis and its host 'Haddon's carpet' sea anemone Stichodactyla haddoni were tested under lowered pH (7.7) and control (8.1) conditions. The biochemical responses such as digestive enzyme (AP), organic acids (lactate and succinate), oxidative damages (MDA), antioxidants metabolites/enzymes (ASC, GSH, SOD, CAT, APX, GPX, GR, POX, and PHOX), and detoxification enzyme (GST) were measured. The AP showed insignificantly reduced values in both the organisms in lowered pH conditions compared to control indicating the effect of abiotic stress. The hierarchical clustering analysis indicated low MDA in sea anemone can be explained by higher POX, APX, GR, ASC, and GSH levels compared to shrimps. However, the detoxification enzyme GST showed less activity in sea anemones compared to shrimps. The results suggest that A. brevicarpalis and sea anemone S. haddoni may have deleterious effects when exposed to short-term acidification stress.

Cellular level response of the bivalve Limecola balthica to seawater acidification due to potential CO2 leakage from a sub-seabed storage site in the southern Baltic Sea: TiTank experiment at representative hydrostatic pressure

Understanding of biological responses of marine fauna to seawater acidification due to potential CO₂ leakage from sub-seabed storage sites has improved recently, providing support to CCS environmental risk assessment. Physiological responses of benthic organisms to ambient hypercapnia have been previously investigated but rarely at the cellular level, particularly in areas of less common geochemical and ecological conditions such as brackish water and/or reduced oxygen levels. In this study, CO₂-related responses of oxygen-dependent, antioxidant and detoxification systems as well as markers of neurotoxicity and acid-base balance in the Baltic clam Limecola balthica from the Baltic Sea were quantified in 50-day experiments. Experimental conditions included CO₂ addition producing pH levels of 7.7, 7.0 and 6.3, respectively and hydrostatic pressure 900 kPa, simulating realistic seawater acidities following a CO₂ seepage accident at the potential CO₂-storage site in the Baltic. Reduced pH interfered with most biomarkers studied, and modifications to lactate dehydrogenase and malate dehydrogenase indicate that aerobiosis was a dominant energy production pathway. Hypercapnic stress was most evident in bivalves exposed to moderately acidic seawater environment (pH 7.0), showing a decrease of glutathione peroxidase activity, activation of catalase and suppression of glutathione S-transferase activity likely in response to enhanced free radical production. The clams subjected to pH 7.0 also demonstrated acetylcholinesterase activation that might be linked to prolonged impact of contaminants released from sediment. The most acidified conditions (pH 6.3) stimulated glutathione and malondialdehyde concentration in the bivalve tissue suggesting potential cell damage. Temporal variations of most biomarkers imply that after a 10-to-15-day initial phase of an acute disturbance, the metabolic and antioxidant defence systems recovered their capacities.

Genome-wide association study reveals genetic variations associated with ocean acidification resilience in Yesso scallop Patinopecten yessoensis

Ocean acidification (OA), which refers to a gradual decrease in seawater pH due to the absorption of atmospheric carbon dioxide, profoundly affects the growth, development and survival of bivalves. Relatively limited studies have assessed the resilience of bivalve to OA. In the present study, Patinopecten yessoensis, an economically and ecologically significant species, were exposed to low pH (pH = 7.5) for 4 weeks. Forty-seven scallops that died in the first week were considered pH-sensitive population, and 20 that were alive at the end of the experiment were considered pH-tolerant population. A genome-wide association study was conducted to identify the genomic loci associated the resilience of P. yessoensis to OA. Twenty-one single nucleotide polymorphisms were significantly associated with resilience, which were distributed in 11 linkage groups. Within the linkage disequilibrium block region (± 300 kb) surrounding the 21 SNPs, 193 candidate genes were successfully identified. Particularly, five associated SNPs were directly located on five genes, including SP24, CFDH, 5HTR3, HSDL1 and ZFP346. The GO enrichment and KEGG pathway analyses showed that the molecular response of P. yessoensis to OA mainly involved neural signal transmission, energy metabolism and redox reaction. Candidate genes were expressed during larval development and in adult tissues. Furthermore, the expression of 30 candidate genes changed significantly under low pH stress in the mantle. Our results reveal certain SNPs and candidate genes that could elucidate the different responses of P. yessoensis to OA. The genetic variations indicated molecular resilience in P. yessoensis populations, which may enable adaptation to future acidification stress.

Experimental Studies on the Impact of the Projected Ocean Acidification on Fish Survival, Health, Growth, and Meat Quality; Black Sea Bream (Acanthopagrus schlegelii), Physiological and Histological Studies

Simple Summary

This study’s data suggest that under the projected scenarios of ocean acidification by 2100 and beyond, significant negative impacts on growth, health, and meat quality are expected, particularly on black sea bream, and will be susceptible to the scientifically approved fish having a weaker resistance to diseases and environmental changes if CO₂ emissions in the atmosphere are not curbed. Knowing the expected consequences, mitigation measures are urgently needed.

Abstract

Acidification (OA), a global threat to the world’s oceans, is projected to significantly grow if CO₂ continues to be emitted into the atmosphere at high levels. This will result in a slight decrease in pH. Since the latter is a logarithmic scale of acidity, the higher acidic seawater is expected to have a tremendous impact on marine living resources in the long-term. An 8-week laboratory experiment was designed to assess the impact of the projected pH in 2100 and beyond on fish survival, health, growth, and fish meat quality. Two projected scenarios were simulated with the control treatment, in triplicates. The control treatment had a pH of 8.10, corresponding to a pCO₂ of 321.37 ± 11.48 µatm. The two projected scenarios, named Predict_A and Predict_B, had pH values of 7.80-pCO₂ = 749.12 ± 27.03 and 7.40-pCO₂ = 321.37 ± 11.48 µatm, respectively. The experiment was preceded by 2 weeks of acclimation. After the acclimation, 20 juvenile black sea breams (Acanthopagrus schlegelii) of 2.72 ± 0.01 g were used per tank. This species has been selected mainly due to its very high resistance to diseases and environmental changes, assuming that a weaker fish resistance will also be susceptibly affected. In all tanks, the fish were fed with the same commercial diet. The seawater’s physicochemical parameters were measured daily. Fish samples were subjected to physiological, histological, and biochemical analyses. Fish growth, feeding efficiency, protein efficiency ratio, and crude protein content were significantly decreased with a lower pH. Scanning electron microscopy revealed multiple atrophies of microvilli throughout the small intestine’s brush border in samples from Predict_A and Predict_B. This significantly reduced nutrient absorption, resulting in significantly lower feed efficiency, lower fish growth, and lower meat quality. As a result of an elevated pCO₂ in seawater, the fish eat more than normal but grow less than normal. Liver observation showed blood congestion, hemorrhage, necrosis, vacuolation of hepatocytes, and an increased number of Kupffer cells, which characterize liver damage. Transmission electron microscopy revealed an elongated and angular shape of the mitochondrion in the liver cell, with an abundance of peroxisomes, symptomatic of metabolic acidosis.

The effects of low pH on the taste and amino acid composition of tiger shrimp

Recent research has revealed that shrimp sensory quality may be affected by ocean acidification but we do not exactly know why. Here we conducted controlled pH exposure experiments on adult tiger shrimp, which were kept in 1000-L tanks continuously supplied with coastal seawater. We compared survival rate, carapace properties and flesh sensory properties and amino acid composition of shrimp exposed to pH 7.5 and pH 8.0 treatments for 28 days. Shrimp reared at pH 7.5 had a lower amino acid content (17.6% w/w) than those reared at pH 8.0 (19.5% w/w). Interestingly, the amino acids responsible for the umami taste, i.e. glutamate and aspartic acid, were present at significantly lower levels in the pH 7.5 than the pH 8.0 shrimp, and the pH 7.5 shrimp were also rated as less desirable in a blind quality test by 40 volunteer assessors. These results indicate that tiger shrimp may become less palatable in the future due to a lower production of some amino acids. Finally, tiger shrimp also had a lower survival rate over 28 days at pH 7.5 than at pH 8.0 (73% vs. 81%) suggesting that ocean acidification may affect both the quality and quantity of future shrimp resources.

Molecular mechanisms underlying responses of the Antarctic coral Malacobelemnon daytoni to ocean acidification

Benthic organisms of the Southern Ocean are particularly vulnerable to ocean acidification (OA), as they inhabit cold waters where calcite-aragonite saturation states are naturally low. OA most strongly affects animals with calcium carbonate skeletons or shells, such as corals and mollusks. We exposed the abundant cold-water coral Malacobelemnon daytoni from an Antarctic fjord to low pH seawater (LpH) (7.68 ± 0.17) to test its physiological responses to OA, at the level of gene expression (RT-PCR) and enzyme activity. Corals were exposed in short- (3 days) and long-term (54 days) experiments to two pCO₂ conditions (ambient and elevated pCO₂ equaling RCP 8.5, IPCC 2019, approximately 372.53 and 956.78 μatm, respectively). Of the eleven genes studied through RT-PCR, six were significantly upregulated compared with control in the short-term in the LpH condition, including the antioxidant enzyme superoxide dismutase (SOD), Heat Shock Protein 70 (HSP70), Toll-like receptor (TLR), galaxin and ferritin. After long-term exposure to low pH conditions, RT-PCR analysis showed seven genes were upregulated. These include the mannose-binding C-Lectin and HSP90. Also, the expression of TLR and galaxin, among others, continued to be upregulated after long-term exposure to LpH. Expression of carbonic anhydrase (CA), a key enzyme involved in calcification, was also significantly upregulated after long-term exposure. Our results indicated that, after two months, M. daytoni is not acclimatized to this experimental LpH condition. Gene expression profiles revealed molecular impacts that were not evident at the enzyme activity level. Consequently, understanding the molecular mechanisms behind the physiological processes in the response of a coral to LpH is critical to understanding the ability of polar species to cope with future environmental changes. Approaches integrating molecular tools into Antarctic ecological and/or conservation research make an essential contribution given the current ongoing OA processes.

Biochemical performance of mussels, cockles and razor shells contaminated by paralytic shellfish toxins

Marine toxins in bivalves pose an important risk to human health, and regulatory authorities throughout the world impose maximum toxicity values. In general, bivalve toxicities due to paralytic shellfish toxins (PSTs) above the regulatory limit occur during short periods, but in some cases, it may be extended from weeks to months. The present study examines whether cockles (Cerastoderme edule), mussels (Mytilus galloprovincialis) and razor shells (Solen marginatus) naturally exposed to a bloom of Gymnodinium catenatum activated or suppressed biochemical responses as result of the presence of PSTs in their soft tissues. Toxins (C1+2, C3+4, GTX5, GTX6, dcSTX, dcGTX2+3 and dcNEO) and a set of biomarkers (ETS, electron transport system activity; GLY, glycogen; PROT, protein; SOD, superoxide dismutase; CAT, catalase; GPx, glutathione peroxidase; GST, glutathione S-transferases; LPO, lipid peroxidation; reduced (GSH) and oxidized (GSSG) glutathione contents and AChE, acetylcholinesterase activity) were determined in the three bivalve species. Specimens were harvested weekly in Aveiro lagoon, Portugal, along thirteen weeks. This period included three weeks in which bivalve toxicity exceeded largely the regulatory limit and the subsequence recovery period of ten weeks. Biochemical performance of the surveyed species clearly indicated that PSTs induce oxidative stress and neurotoxicity, with higher impact on mussels and razor shells than in cockles. The antioxidant enzymes CAT and GPx seemed to be the biomarkers better associated with toxin effects.

Correlation between intermediary metabolism, Hsp gene expression, and oxidative stress-related proteins in long-term thermal-stressed Mytilus galloprovincialis

Long-term exposure of Mytilus galloprovincialis to temperatures beyond 26°C triggers mussel mortality. The present study aimed to integratively illustrate the correlation between intermediary metabolism, hsp gene expression, and oxidative stress-related proteins in long-term thermally stressed Mytilus galloprovincialis and whether they are affected by thermal stress magnitude and duration. We accordingly evaluated the gene expression profiles, in the posterior adductor muscle (PAM) and the mantle, concerning heat shock protein 70 and 90 ( hsp70 and hsp90), and the antioxidant defense indicators Mn-SOD, Cu/Zn-SOD, catalase, glutathione S-transferase, and the metallothioneins mt-10 and mt-20. Moreover, we determined antioxidant enzyme activities, oxidative stress through lipid peroxidation, and activities of intermediary metabolism enzymes. The pattern of changes in relative mRNA expression levels indicate that mussels are able to sense thermal stress even when exposed to 22°C and before mussel mortality is initiated. Data indicate a close correlation between the magnitude and duration of thermal stress with lipid peroxidation levels and changes in the activity of antioxidant enzymes and the enzymes of intermediary metabolism. The gene expression and increase in the activities of antioxidant enzymes support a scenario, according to which exposure to 24°C might trigger reactive oxygen species (ROS) production, which is closely correlated with anaerobic metabolism under hypometabolic conditions. Increase and maintenance of oxidative stress in conjunction with energy balance disturbance seem to trigger mussel mortality after long-term exposure at temperatures beyond 26°C. Eventually, in the context of preparation for oxidative stress, certain hypotheses and models are suggested, integrating the several steps of cellular stress response.

Effects of seawater salinity and pH on cellular metabolism and enzyme activities in biomineralizing tissues of marine bivalves

Molluscan shell formation is a complex energy demanding process sensitive to the shifts in seawater CaCO₃ saturation due to changes in salinity and pH. We studied the effects of salinity and pH on energy demand and enzyme activities of biomineralizing cells of the Pacific oyster (Crassostrea gigas) and the hard-shell clam (Mercenaria mercenaria). Adult animals were exposed for 14 days to high (30), intermediate (18), or low (10) salinity at either high (8.0-8.2) or low (7.8) pH. Basal metabolic cost as well as the energy cost of the biomineralization-related cellular processes were determined in isolated mantle edge cells and hemocytes. The total metabolic rates were similar in the hemocytes of the two studied species, but considerably higher in the mantle cells of C. gigas compared with those of M. mercenaria. Cellular respiration was unaffected by salinity in the clams' cells, while in oysters' cells the highest respiration rate was observed at intermediate salinity (18). In both studied species, low pH suppressed cellular respiration. Low pH led to an upregulation of Na⁺/K⁺ ATPase activity in biomineralizing cells of oysters and clams. Activities of Ca²⁺ ATPase and H⁺ ATPase, as well as the cellular energy costs of Ca²⁺ and H⁺ transport in the biomineralizing cells were insensitive to the variation in salinity and pH in the two studied species. Variability in cellular response to low salinity and pH indicates that the disturbance of shell formation under these conditions has different underlying mechanisms in the two studied species.

Transgenerational acclimation to changes in ocean acidification in marine invertebrates

The rapid pace of increasing oceanic acidity poses a major threat to the fitness of the marine ecosystem, as well as the buffering capacity of the oceans. Disruption in chemical equilibrium in the ocean leads to decreased carbonate ion precipitation, resulting in calcium carbonate saturation. If these trends continue, calcifying invertebrates will experience difficultly maintaining their calcium carbonate exoskeleton and shells. Because malfunction of exoskeleton formation by calcifiers in response to ocean acidification (OA) will have non-canonical biological cascading results in the marine ecosystem, many studies have investigated the direct and indirect consequences of OA on ecosystem- and physiology-related traits of marine invertebrates. Considering that evolutionary adaptation to OA depends on the duration of OA effects, long-term exposure to OA stress over multi-generations may result in adaptive mechanisms that increase the potential fitness of marine invertebrates in response to OA. Transgenerational studies have the potential to elucidate the roles of acclimation, carryover effects, and evolutionary adaptation within and over generations in response to OA. In particular, understanding mechanisms of transgenerational responses (e.g., antioxidant responses, metabolic changes, epigenetic reprogramming) to changes in OA will enhance our understanding of marine invertebrate in response to rapid climate change.

CO2-driven ocean acidification repressed the growth of adult sea urchin Strongylocentrotus intermedius by impairing intestine function

Strongylocentrotus intermedius cultured in the northern Yellow Sea in China was utilized to evaluate the effects of chronic CO₂-driven ocean acidification (OA) on adult sea urchins. Based on the projection of the Intergovernmental Panel on Climate Change (IPCC), present natural seawater conditions (pH_NBS = 8.10 ± 0.03) and three laboratory-controlled OA conditions (OA₁, ΔpH_NBS = - 0.3 units; OA₂, ΔpH_NBS = - 0.4 units; OA₃, ΔpH_NBS = - 0.5 units) were employed. After 60-day incubation, our results showed that (1) OA significantly repressed the growth of adult S. intermedius; (2) food consumption tended to be decreased with pH decline; (3) intestinal morphology was changed, and activities of intestinal cellulase and lipase were decreased under acidified conditions; (4) expression levels of two immune-related genes (SiTNF14 and SiTGF-β) were altered; (5) rate-limiting enzyme activities of the glycolytic pathway and tricarboxylic acid cycle (TAC) were changed in all OA treatments compared to those of controls.

CO2-driven ocean acidification weakens mussel shell defense capacity and induces global molecular compensatory responses

Oceanic uptake of atmospheric CO₂ is reducing seawater pH and shifting carbonate chemistry within, a process termed as ocean acidification (OA). Marine mussels are a family of ecologically and economically significant bivalves that are widely distributed along coastal areas worldwide. Studies have demonstrated that OA greatly disrupts mussels' physiological functions. However, the underlying molecular responses (e.g., whether there were any molecular compensation mechanisms) and the extent to which OA affects mussel shell defense capacity remain largely unknown. In this study, the thick shell mussels Mytilus coruscus were exposed to the ambient pH (8.1) or one of two lowered pH levels (7.8 and 7.4) for 40 days. The results suggest that future OA will damage shell structure and weaken shell strength and shell closure strength, ultimately reducing mussel shell defense capacity. In addition, future OA will also disrupt haemolymph pH and Ca²⁺ homeostasis, leading to extracellular acidosis and Ca²⁺ deficiency. Mantle transcriptome analyses indicate that mussels will adopt a series of molecular compensatory responses to mitigate these adverse effects; nevertheless, weakened shell defense capacity will increase mussels' susceptibility to predators, parasites and pathogens, and thereby reduce their fitness. Overall, the findings of this study have significant ecological and economic implications, and will enhance our understanding of the future of the mussel aquaculture industry and coastal ecosystems.

Antioxidant responses of triangle sail mussel Hyriopsis cumingii exposed to harmful algae Microcystis aeruginosa and high pH

In lakes and reservoirs, harmful algal blooms and high pH have been deemed to be two important stressors related to eutrophication, especially in the case of CO₂ depletion caused by dense blooms. However, the effects of these stressors on the economically important shellfish that inhabit these waters are still not well-understood. This study evaluated the combined effects of the harmful algae Microcystis aeruginosa (0%, 50%, and 100% of total dietary dry weight) and high pH (8.0, 8.5 and 9.0) on the antioxidant responses of the triangle sail mussel H. cumingii. The mussels were exposed to algae and high pH for 14 d, followed by a 7-day depuration period. Reactive oxygen species (ROS) in the mussel hemolymph, antioxidant and detoxifying enzymes, such as glutathione-S-transferase (GST), glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and malondialdehyde (MDA) in the digestive glands were analyzed during the experimental period. GST, SOD and GPx activity levels and the content of GSH increased following exposure to toxic M. aeruginosa, whereas CAT activity was inhibited. pH showed no significant effects on the immune defense mechanisms and detoxification processes. However, a high pH could cause increased ROS and MDA levels, resulting in oxidative injury. After a 7-day depuration period, exposure to toxic M. aeruginosa or high pH resulted in latent effects for most of the examined parameters. The treatment group exposed to the highest pH (9.0) displayed an increased oxidation state compared with the other pH treatments (8.0 and 8.5) for the same concentrations of toxic M. aeruginosa. The trends observed for ROS, MDA, GPx, GST, SOD and GSH levels indicated that a high density of toxic algae could result in severe and continuous effects on mussel health.

Integration of Biochemical, Cellular, and Genetic Indicators for Understanding the Aging Process in a Bivalve Mollusk Chlamys farreri

The major causal factors for the irreversible decline in physical vitality during organismal aging are postulated to be a chronic state of cellular redox imbalance, metabolic toxicity, and impaired energy homeostasis. We assessed whether the relevant enzyme activity, oxidative stress, and intracellular ATP might be causally involved in the aging of short-lived Chlamys farreri (life span 4~5 years). A total of eight related biochemical and cellular indicators were chosen for the subsequent analysis. All the indicators were measured in seven different tissues from scallops aged one to four years, and our data support that the aging of C. farreri is associated with attenuated tissue enzyme activity as well as a decreased metabolic rate. Through principal component analysis, we developed an integrated vigor index for each tissue for comprehensive age-related fitness evaluation. Remarkably, all tissue-integrated vigor indexes significantly declined with age, and the kidney was observed to be the most representative tissue. Further transcriptional profiling of the enzymatic genes provided additional detail on the molecular responses that may underlie the corresponding biochemical results. Moreover, these critical molecular responses may be attributed to the conserved hierarchical regulators, e.g., FOXO, AMPKs, mTOR, and IGF1R, which were identified as potentially novel markers for chronic fitness decline with age in bivalves. The present study provides a systematic approach that could potentially benefit the global assessment of the aging process in C. farreri and provide detailed evaluation of the biochemical, cellular, and genetic indicators that might be involved. This information may assist in a better understanding of bivalve adaptability and life span.