Effects of ocean acidification on immune responses of the Pacific oyster Crassostrea gigas

Elucidating responses of the intertidal clam Ruditapes philippinarum to compound extreme oceanic events

Ocean acidification and heatwaves caused by rising CO2 affect bivalves and other coastal organisms. Intertidal bivalves are vital to benthic ecosystems, but their physiological and metabolic responses to compound catastrophic climate events are unknown. Here, we examined Manila clam (Ruditapes philippinarum) responses to low pH and heatwaves. Biochemical and gene expression demonstrated that pH and heatwaves greatly affect physiological energy enzymes and genes expression. In the presence of heatwaves, Manila clams expressed more enzymes and genes involved in physiological energetics regardless of acidity, even more so than in the presence of both. In this study, calcifying organisms' biochemical and molecular reactions are more susceptible to temperature rises than acidity. Acclimation under harsh weather conditions was consistent with thermal stress increase at lower biological organization levels. These substantial temporal biochemical and molecular patterns illuminate clam tipping points. This study helps us understand how compound extreme weather and climate events affect coastal bivalves for future conservation efforts.

Transcriptomic responses in the nervous system and correlated behavioural changes of a cephalopod exposed to ocean acidification

BACKGROUND

The nervous system is central to coordinating behavioural responses to environmental change, likely including ocean acidification (OA). However, a clear understanding of neurobiological responses to OA is lacking, especially for marine invertebrates.

RESULTS

We evaluated the transcriptomic response of the central nervous system (CNS) and eyes of the two-toned pygmy squid (Idiosepius pygmaeus) to OA conditions, using a de novo transcriptome assembly created with long read PacBio ISO-sequencing data. We then correlated patterns of gene expression with CO2 treatment levels and OA-affected behaviours in the same individuals. OA induced transcriptomic responses within the nervous system related to various different types of neurotransmission, neuroplasticity, immune function and oxidative stress. These molecular changes may contribute to OA-induced behavioural changes, as suggested by correlations among gene expression profiles, CO2 treatment and OA-affected behaviours.

CONCLUSIONS

This study provides the first molecular insights into the neurobiological effects of OA on a cephalopod and correlates molecular changes with whole animal behavioural responses, helping to bridge the gaps in our knowledge between environmental change and animal responses.

How water acidification influences the organism antioxidant capacity and gill structure of Mediterranean mussel (Mytilus galloprovincialis, Lamarck, 1819) at normoxia and hypoxia

The effect of water acidification in combination with normoxia or hypoxia on the antioxidant capacity and oxidative stress markers in gills and hemolymph of the Mediterranean mussel (Mytilus galloprovincialis), as well as on gill microstructure, has been evaluated through an in vivo experiment. Mussels were exposed to a low pH (7.3) under normal dissolved oxygen (DO) conditions (8 mg/L), and hypoxia (2 mg/L) for 8 days, and samples were collected on days 1, 3, 6, and 8 to evaluate dynamic changes of physiological responses. Cytoplasmic concentrations of reactive oxygen species (ROS) and levels of DNA damage were measured in hemocytes, while the activity of catalase (CAT) and superoxide dismutase (SOD) and histopathological changes were assessed in gills. The results revealed that while water acidification did not significantly affect the activity of SOD and CAT in gills under normoxic and hypoxic conditions, there was a trend towards suppression of CAT activity at the end of the experimental period (day 8). Similarly, we did not observe increased formation of ROS in hemocytes or changes in the levels of DNA damage during the experimental period. These results strongly suggest that the oxidative stress response system in mussels is relatively stable to experimental conditions of acidification and hypoxia. Experimental acidification under normoxia and hypoxia caused changes to the structure of the gills, leading to various histopathological alterations, including dilation, hemocyte infiltration into the hemal sinuses, intercellular edema, vacuolization of epithelial cells in gill filaments, lipofuscin accumulation, changes in the shape and adjacent gill filaments, hyperplasia, exfoliation of the epithelial layer, necrosis, swelling, and destruction of chitinous layers (chitinous rods). Most of these alterations were reversible, non-specific changes that represent a general inflammatory response and changes in the morphology of the gill filaments. The dynamics of histopathological alterations suggests an active adaptive response of gills to environmental stresses. Taken together, our data indicate that Mediterranean mussels have a relative tolerance to water acidification and hypoxia at tissue and cellular levels.

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.

Influencing intertidal food web: Implications of ocean acidification on the physiological energetics of key species the 'wedge' clam Donax faba

Ocean acidification has become increasingly severe in coastal areas. It poses emerging threats to coastal organisms and influences ecological functioning. Donax faba, a dominant clam in the intertidal zone of the Bay of Bengal, plays an important role in the coastal food web. This clam has been widely consumed by the local communities and also acts as a staple diet for shorebirds and crustaceans. In this paper, we investigated how acidified conditions will influence the physiology, biochemical constituents, and energetics of Donax faba. Upon incubation for 2 months in lowered pH 7.7 ± 0.05 and control 8.1 ± 0.05 conditions, we found a delayed growth in the acidified conditions followed by decrease in calcium ions in the clam shell. Although not significant, we found the digestive enzymes showed a downward trend. Total antioxidant was significantly increased in the acidified condition compared to the control. Though not significant, the expression level of MDA and antioxidant enzymes (SOD, CAT, GST, GPX, and APX) showed increasing trend in acidified samples. Among nutrients such as amino acids and fatty acids, there was no significant difference between treatments, however, showed a downward trend in the acidified conditions compared to control. Among the minerals, iron and zinc showed significant increase in the acidified conditions. The above results suggest that the clam growth, and physiological energetics may have deleterious effects if exposed for longer durations at lowered pH condition thereby affecting the organisms involved in the coastal food web.

Effects of ocean acidification and polystyrene microplastics on the oysters Crassostrea gigas: An integrated biomarker and metabolomic approach

The adverse impacts of microplastics (MPs) or ocean acidification (OA) on mollusks have been widely reported, however, little is known about their combined effects on mollusks. The oysters Crassostrea gigas were exposed to two sizes of polystyrene MPs with 1 × 104 particles/L (small polystyrene MPs (SPS-MPs): 6 μm, large polystyrene MPs (LPS-MPs): 50-60 μm) at two pH levels (7.7 and 8.1) for 14 days. The antagonistic effects between MPs and OA on oysters were mainly observed. Single SPS-MPs exposure can induce CAT enzyme activity and LPO level in gills, while LPS-MPs exposure alone can increase PGK and PEPCK gene expression in digestive glands. Ocean acidification can increase clearance rate and inhibit antioxidant enzyme activity, whereas combined exposure of OA and SPS-MPs can affect the metabolomic profile of digestive glands. This study emphasized that the potential toxic effects of MPs under the scene of climate change should be concerned.

The molecular response of Mytilus coruscus mantle to shell damage under acute acidified sea water revealed by iTRAQ based quantitative proteomic analysis

Mytilus coruscus is an economically important marine bivalve that lives in estuarine sea areas with seasonal coastal acidification and frequently suffers shell injury in the natural environment. However, the molecular responses and biochemical properties of Mytilus under these conditions are not fully understood. In the present study, we employed tandem mass spectrometry combined with isobaric tagging to identify differentially expressed proteins in the mantle tissue of M. coruscus under different short-term treatments, including shell-complete mussels raised in normal seawater (pH 8.1), shell-damaged mussels raised in normal seawater (pH 8.1), and acidified seawater (pH 7.4). A total of 2694 proteins were identified in the mantle, and analysis of their relative abundance from the three different treatments revealed alterations in the proteins involved in immune regulation, oxidation-reduction processes, protein folding and processing, energy provision, and cytoskeleton. The results obtained by quantitative proteomic analysis of the mantle allowed us to delineate the molecular strategies adopted by M. coruscus in the shell repair process in acidified environments, including an increase in proteins involved in oxidation-reduction processes, protein processing, and cell growth at the expense of proteins involved in immune capacity and energy metabolism. SIGNIFICANCE: The impact of global ocean acidification on calcifying organisms has become a major ecological and environmental problem in the world. Mytilus coruscus is an economically important marine bivalve living in estuary sea area with seasonal coastal acidification, and frequently suffering shell injury in natural environment. Molecular responses of M coruscus under the shell damage and acute acidification is still largely unknown. For this reason, iTRAQ based quantitative proteomic and histological analysis of the mantle from M. coruscus under shell damage and acute acidification were performed, for revealing the proteomic response and possible adaptation mechanism of Mytilus under combined shell damage and acidified sea water, and understanding how the mussel mantle implement a shell-repair process under acidified sea water. Our study provides important data for understanding the shell repair process and proteomic response of Mytilus under ocean acidification, and providing insights into potential adaptation of mussels to future global change.

Transcriptomics, proteomics, and physiological assays reveal immunosuppression in the eastern oyster Crassostrea virginica exposed to acidification stress

Ocean acidification (OA) is recognized as a major stressor for a broad range of marine organisms, particularly shell-building invertebrates. OA can cause alterations in various physiological processes such as growth and metabolism, although its effect on host-pathogen interactions remains largely unexplored. In this study, we used transcriptomics, proteomics, and physiological assays to evaluate changes in immunity of the eastern oyster Crassostrea virginica exposed to OA conditions (pH = 7.5 vs pH = 7.9) at various life stages. The susceptibility of oyster larvae to Vibrio infection increased significantly (131 % increase in mortality) under OA conditions, and was associated with significant changes in their transcriptomes. The significantly higher mortality of larvae exposed to pathogens and acidification stress could be the outcome of an increased metabolic demand to cope with acidification stress (as seen by upregulation of metabolic genes) at the cost of immune function (downregulation of immune genes). While larvae were particularly vulnerable, juveniles appeared more robust to the stressors and there were no differences in mortality after pathogen (Aliiroseovarius crassostrea and Vibrio spp.) exposure. Proteomic investigations in adult oysters revealed that acidification stress resulted in a significant downregulation of mucosal immune proteins including those involved in pathogen recognition and microbe neutralization, suggesting weakened mucosal immunity. Hemocyte function in adults was also impaired by high pCO2, with a marked reduction in phagocytosis (67 % decrease in phagocytosis) in OA conditions. Together, results suggest that OA impairs immune function in the eastern oyster making them more susceptible to pathogen-induced mortality outbreaks. Understanding the effect of multiple stressors such as OA and disease is important for accurate predictions of how oysters will respond to future climate regimes.

Intrinsic and extrinsic pathways of apoptosis induced by multiple antibiotics residues and ocean acidification in hemocytes of scallop Argopecten irradians irradians: An interactionist perspective

The increasing prevalence of antibiotics in seawater across global coastal areas, coupled with the ocean acidification induced by climate change, present a multifaceted challenge to marine ecosystems, particularly impacting the key physiological processes of marine organisms. Apoptosis is a critical adaptive response essential for maintaining cellular homeostasis and defending against environmental threats. In this study, bay scallops Argopecten irradians irradians were exposed to multiple antibiotics (sulfamethoxazole, tetracycline, oxytetracycline, norfloxacin, and erythromycin, each at a concentration of 1 μg/L) combined with/without acidic seawater (pH 7.6) for 35 days. The single and interactive effects of the two stressors on apoptosis and the underlying mechanisms in hemocytes of A. irradians irradians were determined through flow cytometry analysis, comet assay, oxidative stress biomarkers analysis, and transcriptome analysis. Results showed that apoptosis could be triggered by either AM exposure or OA exposure, but through different pathways. Exposure to AM leads to mitochondrial dysfunction and oxidative damage, which in turn triggers apoptosis via a series of cellular events in both intrinsic and extrinsic pathways. Conversely, while OA exposure similarly induced apoptosis, its effects are comparatively subdued and are predominantly mediated through the intrinsic pathway. Additionally, the synergistic effects of AM and OA exposure induced pronounced mitochondrial dysfunction and oxidative damages in the hemocytes of A. irradians irradians. Despite the evident cellular distress and the potential initiation of apoptotic pathways, the actual execution of apoptosis appears to be restrained, which might be attributed to an energy deficit within the hemocytes. Our findings underscore the constrained tolerance capacity of A. irradians irradians when faced with multiple environmental stressors, and shed light on the ecotoxicity of antibiotic pollution in the ocean under prospective climate change scenarios.

Effects of pH and salinity on survival, growth, and enzyme activities in juveniles of the sunray surf clam (Mactra chinensis Philippi)

The study investigated the impact of salinity and pH changes on the survival, growth, and antioxidant enzyme activity in Mactra chinensis Philippi (1.00 ± 0.10 cm shell length, 0.75 ± 0.04 cm shell height), a marine clam species. Juveniles were exposed to various pH levels (5.4 - 9.6) and salinities (5 - 35 psu) for up to 20 days at 19 ± 0.5 ˚C. The individual effect of salinity and pH on juveniles were evaluated under pH 8.0 and salinity 30 psu, respectively. The results indicated that the highest survival rates were observed at pH 8.0 (85%, salinity = 30 psu) and salinity 30 psu (95%, pH = 8.0). The survival rates were significantly reduced at extreme pH (≤ 7.2; ≥ 8.4) and salinities (≤ 15; 35 psu). Additionally, oxidative stress was observed in clams exposed to low pH and salinity as indicated by the decreased activities of the antioxidant enzymes catalase (CAT) and superoxide dismutase (SOD). Notably, no significant difference in relative growth rates was observed between salinity 25 and 30 psu, between pH 7.8/8.4 and pH 8.0. Our results provide information on potential impact of pH and salinity changes on economically important bivalve species and may be used to optimize pH and salinity in aquaculture.

Molecular responses to ocean acidification in an Antarctic bivalve and an ascidian

Southern Ocean organisms are considered particularly vulnerable to Ocean acidification (OA), as they inhabit cold waters where calcite-aragonite saturation states are naturally low. It is also generally assumed that OA would affect calcifying animals more than non-calcifying animals. In this context, we aimed to study the impact of reduced pH on both types of species: the ascidian Cnemidocarpa verrucosa sp. A, and the bivalve Aequiyoldia eightsii, from an Antarctic fjord. We used gene expression profiling and enzyme activity to study the responses of these two Antarctic benthic species to OA. We report the results of an experiment lasting 66 days, comparing the molecular mechanisms underlying responses under two pCO2 treatments (ambient and elevated pCO2). We observed 224 up-regulated and 111 down-regulated genes (FC ≥ 2; p-value ≤ 0.05) in the ascidian. In particular, the decrease in pH caused an upregulation of genes involved in the immune system and antioxidant response. While fewer differentially expressed (DE) genes were observed in the infaunal bivalve, 34 genes were up-regulated, and 69 genes were downregulated (FC ≥ 2; p-value ≤ 0.05) in response to OA. We found downregulated genes involved in the oxidoreductase pathway (such as glucose dehydrogenase and trimethyl lysine dioxygenase), while the heat shock protein 70 was up-regulated. This work addresses the effect of OA in two common, widely distributed Antarctic species, showing striking results. Our major finding highlights the impact of OA on the non-calcifying species, a result that differ from the general trend, which describes a higher impact on calcifying species. This calls for discussion of potential effects on non-calcifying species, such as ascidians, a diverse and abundant group that form extended three-dimensional clusters in shallow waters and shelf areas in the Southern Ocean.

Combination of RNAseq and RADseq to Identify Physiological and Adaptive Responses to Acidification in the Eastern Oyster (Crassostrea virginica)

Ocean acidification (OA) is a major stressor threatening marine calcifiers, including the eastern oyster (Crassostrea virginica). In this paper, we provide insight into the molecular mechanisms associated with resilience to OA, with the dual intentions of probing both acclimation and adaptation potential in this species. C. virginica were spawned, and larvae were reared in control or acidified conditions immediately after fertilization. RNA samples were collected from larvae and juveniles, and DNA samples were collected from juveniles after undergoing OA-induced mortality and used to contrast gene expression (RNAseq) and SNP (ddRADseq) profiles from animals reared under both conditions. Results showed convergence of evidence from both approaches, particularly in genes involved in biomineralization that displayed significant changes in variant frequencies and gene expression levels among juveniles that survived acidification as compared to controls. Downregulated genes were related to immune processes, supporting previous studies demonstrating a reduction in immunity from exposure to OA. Acclimation to OA via regulation of gene expression might confer short-term resilience to immediate threats; however, the costs may not be sustainable, underscoring the importance of selection of resilient genotypes. Here, we identified SNPs associated with survival under OA conditions, suggesting that this commercially and ecologically important species might have the genetic variation needed for adaptation to future acidification. The identification of genetic features associated with OA resilience is a highly-needed step for the development of marker-assisted selection of oyster stocks for aquaculture and restoration activities.

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.

Effect of elevated temperature, sea water acidification, and phenanthrene on the expression of genes involved in the shell and pearl formation of economic pearl oyster (Pinctada radiata)

Our study aims to examine the effect of some stressors on the gene expression levels of shell matrix proteins in a pearl oyster. Oysters were exposed to the different combinations of the temperature, pH, and phenanthrene concentration is currently measured in the Persian Gulf and the predicted ocean warming and acidification for 28 days. The expression of all the studied genes was significantly downregulated. Time and temperature had the greatest effects on the decreases in n19 and n16 genes expression, respectively. Aspein and msi60 genes expression were highly influenced by pH. Pearlin was affected by double interaction temperature and phenanthrene. Moreover, a correlation was observed among the expression levels of studied genes. This study represents basic data on the relationship between mRNA transcription genes involved in the shell and pearl formation and climate changes in pollutant presence conditions and acclimatizing mechanism of the oyster to the future scenario as well.

Transcriptomic response of Mytilus coruscus mantle to acute sea water acidification and shell damage

Mytilus coruscus is an economically important marine calcifier living in the Yangtze River estuary sea area, where seasonal fluctuations in natural pH occur owing to freshwater input, resulting in a rapid reduction in seawater pH. In addition, Mytilus constantly suffers from shell fracture or injury in the natural environment, and the shell repair mechanisms in mussels have evolved to counteract shell injury. Therefore, we utilized shell-complete and shell-damaged Mytilus coruscus in this study and performed transcriptomic analysis of the mantle to investigate whether the expression of mantle-specific genes can be induced by acute seawater acidification and how the mantle responds to acute acidification during the shell repair process. We found that acute acidification induced more differentially expressed genes than shell damage in the mantle, and the biomineralization-related Gene Ontology terms and KEGG pathways were significantly enriched by these DEGs. Most DEGs were upregulated in enriched pathways, indicating the activation of biomineralization-related processes in the mussel mantle under acute acidification. The expression levels of some shell matrix proteins and antimicrobial peptides increased under acute acidification and/or shell damage, suggesting the molecular modulation of the mantle for the preparation and activation of the shell repairing and anti-infection under adverse environmental conditions. In addition, morphological and microstructural analyses were performed for the mantle edge and shell cross-section, and changes in the mantle secretory capacity and shell inner film system induced by the two stressors were observed. Our findings highlight the adaptation of M. coruscus in estuarine areas with dramatic fluctuations in pH and may prove instrumental in its ability to survive ocean acidification.

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.

Transgenerational exposure to ocean acidification impacts the hepatic transcriptome of European sea bass (Dicentrarchus labrax)

Physiological effects of ocean acidification associated with elevated CO2 concentrations in seawater is the subject of numerous studies in teleost fish. While the short time within-generation impact of ocean acidification (OA) on acid-base exchange and energy metabolism is relatively well described, the effects associated with transgenerational exposure to OA are much less known. Yet, the impacts of OA can vary in time with the potential for acclimation or adaptation of a species. Previous studies in our lab demonstrated that transgenerational exposure to OA had extensive effects on the transcriptome of the olfactory epithelium of European sea bass (Dicentrarchus labrax), especially on genes related to ion balance, energy metabolism, immune system, synaptic plasticity, neuron excitability and wiring. In the present study, we complete the previous work by investigating the effect of transgenerational exposure to OA on the hepatic transcriptome of European sea bass. Differential gene expression analysis was performed by RNAseq technology on RNA extracted from the liver of two groups of 18 months F2 juveniles that had been exposed since spawning to the same AO conditions as their parents (F1) to either actual pH or end-of-century predicted pH levels (IPCC RCP8.5), respectively. Here we show that transgenerational exposure to OA significantly impacts the expression of 236 hepatic transcripts including genes mainly involved in inflammatory/immune responses but also in carbohydrate metabolism and cellular homeostasis. Even if this transcriptomic impact is relatively limited compared to what was shown in the olfactory system, this work confirmed that fish transgenerationally exposed to OA exhibit molecular regulation of processes related to metabolism and inflammation. Also, our data expand the up-regulation of a key gene involved in different physiological pathways including calcium homeostasis (i.e. pthr1), which we already observed in the olfactory epithelium, to the liver. Even if our experimental design does not allow to discriminate direct within F2 generation effects from transgenerational plasticity, these results offer the perspective of more functional analyses to determine the potential physiological impact of OA exposure on fish physiology with ecological relevance.

Behavioral and physiological effects of ocean acidification and warming on larvae of a continental shelf bivalve

The negative impacts of ocean warming and acidification on bivalve fisheries are well documented but few studies investigate parameters relevant to energy budgets and larval dispersal. This study used laboratory experiments to assess developmental, physiological and behavioral responses to projected climate change scenarios using larval Atlantic surfclams Spisula solidissima solidissima, found in northwest Atlantic Ocean continental shelf waters. Ocean warming increased feeding, scope for growth, and biomineralization, but decreased swimming speed and pelagic larval duration. Ocean acidification increased respiration but reduced immune performance and biomineralization. Growth increased under ocean warming only, but decreased under combined ocean warming and acidification. These results suggest that ocean warming increases metabolic activity and affects larval behavior, while ocean acidification negatively impacts development and physiology. Additionally, principal component analysis demonstrated that growth and biomineralization showed similar response profiles, but inverse response profiles to respiration and swimming speed, suggesting alterations in energy allocation under climate change.

Acid times in physiology: A systematic review of the effects of ocean acidification on calcifying invertebrates

The reduction in seawater pH from rising levels of carbon dioxide (CO₂) in the oceans has been recognized as an important force shaping the future of marine ecosystems. Therefore, numerous studies have reported the effects of ocean acidification (OA) in different compartments of important animal groups, based on field and/or laboratory observations. Calcifying invertebrates have received considerable attention in recent years. In the present systematic review, we have summarized the physiological responses to OA in coral, echinoderm, mollusk, and crustacean species exposed to predicted ocean acidification conditions in the near future. The Scopus, Web of Science, and PubMed databases were used for the literature search, and 75 articles were obtained based on the inclusion criteria. Six main physiological responses have been reported after exposure to low pH. Growth (21.6%), metabolism (20.8%), and acid-base balance (17.6%) were the most frequent among the phyla, while calcification and growth were the physiological responses most affected by OA (>40%). Studies show that the reduction of pH in the aquatic environment, in general, supports the maintenance of metabolic parameters in invertebrates, with redistribution of energy to biological functions, generating limitations to calcification, which can have severe consequences for the health and survival of these organisms. It should be noted that the OA results are variable, with inter and/or intraspecific differences. In summary, this systematic review offers important scientific evidence for establishing paradigms in the physiology of climate change in addition to gathering valuable information on the subject and future research perspectives.

Molecular Features Associated with Resilience to Ocean Acidification in the Northern Quahog, Mercenaria mercenaria

The increasing concentration of CO₂ in the atmosphere and resulting flux into the oceans will further exacerbate acidification already threatening coastal marine ecosystems. The subsequent alterations in carbonate chemistry can have deleterious impacts on many economically and ecologically important species including the northern quahog (Mercenaria mercenaria). The accelerated pace of these changes requires an understanding of how or if species and populations will be able to acclimate or adapt to such swift environmental alterations. Thus far, studies have primarily focused on the physiological effects of ocean acidification (OA) on M. mercenaria, including reductions in growth and survival. However, the molecular mechanisms of resilience to OA in this species remains unclear. Clam gametes were fertilized under normal pCO₂ and reared under acidified (pH ~ 7.5, pCO₂ ~ 1200 ppm) or control (pH ~ 7.9, pCO₂ ~ 600 ppm) conditions before sampled at 2 days (larvae), 32 days (postsets), 5 and 10 months (juveniles) and submitted to RNA and DNA sequencing to evaluate alterations in gene expression and genetic variations. Results showed significant shift in gene expression profiles among clams reared in acidified conditions as compared to their respective controls. At 10 months of exposure, significant shifts in allele frequency of single nucleotide polymorphisms (SNPs) were identified. Both approaches highlighted genes coding for proteins related to shell formation, bicarbonate transport, cytoskeleton, immunity/stress, and metabolism, illustrating the role these pathways play in resilience to OA.

pCO2-driven seawater acidification affects aqueous-phase copper toxicity in juvenile flounder Paralichthys olivaceus: Metal accumulation, antioxidant defenses and detoxification in livers

Ocean acidification potentially influences the biotoxicity of metals and the antioxidant defense systems of marine organisms. This study investigated how pCO₂-driven seawater acidification (SA) affected aqueous-phase copper (Cu) toxicity in the juvenile flounder Paralichthys olivaceus from the perspective of hepatic oxidative stress and damage to better understand the mechanisms underlying the biological effects produced by the two stressors. Fish were exposed to aqueous-phase Cu at relevant ambient and polluted concentrations (0, 5, 10, 50, 100 and 200 μg L^-1) at different pH levels (no SA: pH 8.10; moderate SA: pH 7.70, pCO₂ ∼1353.89 μatm; extreme SA: pH 7.30, pCO₂ ∼3471.27 μatm) for 28 days. A battery of biomarkers in the livers was examined to investigate their roles in antioxidant defense and detoxification in response to coexposure. Hepatic Cu accumulation (30.22-184.90 mg kg^-1) was positively correlated with Cu concentrations. The biomarkers responded adaptively to different redox states following SA and Cu exposure. In unacidified seawater, increases in Cu concentrations significantly induced hepatic lipid peroxidation (LPO, by up to 27.03 %), although compensatory responses in antioxidant defenses and detoxification were activated. Moderate SA helped maintain hepatic redox homeostasis and alleviated LPO through different defense strategies, depending on Cu concentrations. Under extreme SA, antioxidant-based defenses were activated to cope with oxidative stress at ambient-low Cu concentrations but failed to defend against Cu toxicity at polluted Cu levels, and LPO (by up to 63.90 %) was significantly induced. Additionally, thiols (GSH and MT) responded actively to cope with Cu toxicity under SA. SOD, CAT, EROD, and GST were also sensitively involved in defending against hepatic oxidative stress during coexposure. These findings highlight the notable interactive effects of SA and Cu and provide a basis for understanding antioxidant-based defenses in marine fish confronting environmental challenges.

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.

Pathogens transported by plastic debris: does this vector pose a risk to aquatic organisms?

Microplastics are small (<5 mm) plastic particles of varying shapes and polymer types that are now widespread global contaminants of marine and freshwater ecosystems. Various estimates suggest that several trillions of microplastic particles are present in our global oceanic system, and that these are readily ingested by a wide range of marine and freshwater species across feeding modes and ecological niches. Here, we present some of the key and pressing issues associated with these globally important contaminants from a microbiological perspective. We discuss the potential mechanisms of pathogen attachment to plastic surfaces. We then describe the ability of pathogens (both human and animal) to form biofilms on microplastics, as well as dispersal of these bacteria, which might lead to their uptake into aquatic species ingesting microplastic particles. Finally, we discuss the role of a changing oceanic system on the potential of microplastic-associated pathogens to cause various disease outcomes using numerous case studies. We set out some key and imperative research questions regarding this globally important issue and present a methodological framework to study how and why plastic-associated pathogens should be addressed.

Microplastics can aggravate the impact of ocean acidification on the health of mussels: Insights from physiological performance, immunity and byssus properties

Ocean acidification may increase the risk of disease outbreaks that would challenge the future persistence of marine organisms if their immune system and capacity to produce vital structures for survival (e.g., byssus threads produced by bivalves) are compromised by acidified seawater. These potential adverse effects may be exacerbated by microplastic pollution, which is forecast to co-occur with ocean acidification in the future. Thus, we evaluated the impact of ocean acidification and microplastics on the health of a mussel species (Mytilus coruscus) by assessing its physiological performance, immunity and byssus properties. We found that ocean acidification and microplastics not only reduced hemocyte concentration and viability due to elevated oxidative stress, but also undermined phagocytic activity of hemocytes due to lowered energy budget of mussels, which was in turn caused by the reduced feeding performance and energy assimilation. Byssus quality (strength and extensibility) and production were also reduced by ocean acidification and microplastics. To increase the chance of survival with these stressors, the mussels prioritized the synthesis of some byssus proteins (Mfp-4 and Mfp-5) to help maintain adhesion to substrata. Nevertheless, our findings suggest that co-occurrence of ocean acidification and microplastic pollution would increase the susceptibility of bivalves to infectious diseases and dislodgement risk, thereby threatening their survival and undermining their ecological contributions to the community.

Molecular basis of ocean acidification sensitivity and adaptation in Mytilus galloprovincialis

Predicting the potential for species adaption to climate change is challenged by the need to identify the physiological mechanisms that underpin species vulnerability. Here, we investigated the sensitivity to ocean acidification in marine mussels during early development, and specifically the trochophore stage. Using RNA and DNA sequencing and in situ RNA hybridization, we identified developmental processes associated with abnormal development and rapid adaptation to low pH. Trochophores exposed to low pH seawater exhibited 43 differentially expressed genes. Gene annotation and in situ hybridization of differentially expressed genes point to pH sensitivity of (1) shell field development and (2) cellular stress response. Five genes within these two processes exhibited shifts in allele frequencies indicative of a potential for rapid adaptation. This case study contributes direct evidence that protecting species’ existing genetic diversity is a critical management action to facilitate species resilience to climate change.

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Marine mussel larval development and genetic adaptation in low pH seawater

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RNA and DNA responses reveal impacts on shell field development and cell stress

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Five genes exhibited both physiological sensitivity and long-term adaptive potential

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Conserving standing genetic variation could bolster resilience to global change

The extensive transgenerational transcriptomic effects of ocean acidification on the olfactory epithelium of a marine fish are associated with a better viral resistance

Background

Progressive CO₂-induced ocean acidification (OA) impacts marine life in ways that are difficult to predict but are likely to become exacerbated over generations. Although marine fishes can balance acid–base homeostasis efficiently, indirect ionic regulation that alter neurosensory systems can result in behavioural abnormalities. In marine invertebrates, OA can also affect immune system function, but whether this is the case in marine fishes is not fully understood. Farmed fish are highly susceptible to disease outbreak, yet strategies for overcoming such threats in the wake of OA are wanting. Here, we exposed two generations of the European sea bass (Dicentrarchus labrax) to end-of-century predicted pH levels (IPCC RCP8.5), with parents (F1) being exposed for four years and their offspring (F2) for 18 months. Our design included a transcriptomic analysis of the olfactory rosette (collected from the F2) and a viral challenge (exposing F2 to betanodavirus) where we assessed survival rates.

Results

We discovered transcriptomic trade-offs in both sensory and immune systems after long-term transgenerational exposure to OA. Specifically, RNA-Seq analysis of the olfactory rosette, the peripheral olfactory organ, from 18-months-old F2 revealed extensive regulation in genes involved in ion transport and neuronal signalling, including GABAergic signalling. We also detected OA-induced up-regulation of genes associated with odour transduction, synaptic plasticity, neuron excitability and wiring and down-regulation of genes involved in energy metabolism. Furthermore, OA-exposure induced up-regulation of genes involved in innate antiviral immunity (pathogen recognition receptors and interferon-stimulated genes) in combination with down-regulation of the protein biosynthetic machinery. Consistently, OA-exposed F2 challenged with betanodavirus, which causes damage to the nervous system of marine fish, had acquired improved resistance.

Conclusion

F2 exposed to long-term transgenerational OA acclimation showed superior viral resistance, though as their metabolic and odour transduction programs were altered, odour-mediated behaviours might be consequently impacted. Although it is difficult to unveil how long-term OA impacts propagated between generations, our results reveal that, across generations, trade-offs in plastic responses is a core feature of the olfactory epithelium transcriptome in OA-exposed F2 offspring, and will have important consequences for how cultured and wild fish interacts with its environment.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08647-w.

Identification, characterization and expression analyses of PC4 genes in Yesso scallop (Patinopecten yessoensis) reveal functional differentiations in response to ocean acidification

Transcriptional coactivator p15 (PC4), considered a multifunctional chromosome associated protein, is actively involved in transcription regulation, DNA replication, damage repair and chromosome formation. Although studies have reported significant effects of PC4 in most vertebrates and some invertebrates, the complete PC4 gene members are less systematically identified and characterized in scallops. In this study, seven PC4 genes (PyPC4s) were identified in the Yesso scallop Patinopecten yessoensis using whole-genome scanning via bioinformatic analyses. Phylogenetic and protein structural analyses were performed to determine the identities and evolutionary relationships of the seven genes. Expression profiles of PyPC4s were further investigated in embryos/larvae at all developmental stages, healthy adult tissues, and mantles that were exposed to low pH stress (pH 6.5 and 7.5) with different time durations (3, 6, 12 and 24 h). Spatiotemporal expression patterns indicated the functional roles of PyPC4s at all development stages and in healthy adult tissues, with PY-3235.33 demonstrating remarkably high constitutive expressions. Expression regulations (up- and down-regulation) of PyPC4s under low pH stress levels demonstrated a time-dependent pattern with functional complementation and/or enhancement, revealing that PyPC4s exhibited differentiated functions in response to ocean acidification (OA). Collectively, our data offer a novel perspective stating that low pH is a potential inducer leading to functional differentiation of PyPC4s in scallops. The results provide preliminary information on the versatile roles of PC4(s) in bivalves in response to OA.

Immunotoxicity of Perfluorooctanoic Acid to the Marine Bivalve Species Ruditapes philippinarum

Polyfluorinated alkylated substances are recognized as an important class of pollutants in marine environments. Bivalves are good model organisms for evaluating the toxicity of pollutants and monitoring marine environments. In the present study, immunotoxicity of perfluorooctanoic acid (PFOA) was investigated by measuring biomarkers of the immune profile of Ruditapes philippinarum. In bivalves, hemocytes are an important component of the immune system. Thus, hemocyte proliferation, phagocytosis, cell viability, and immune enzyme activities, which have been applied as marine pollution bioindicators, were identified and observed for changes after exposure to PFOA in R. philippinarum. Based on the integrated biomarker responses method, we selected five biomarkers to evaluate PFOA risk at the multibiomarker level. In addition, the histopathological alterations of hemocytes in bivalves were used as indexes of the response to environmental stress. The subcellular structure of the hemocytes in R. philippinarum changed significantly with PFOA exposure, including hemocyte and nucleus morphological changes, organelle dissolution, cytomembrane and karyotheca swelling, and cytoplasm vacuolization. The present study verifies PFOA immunotoxicity to R. philippinarum at different levels and the integrated assessment of stress levels caused by PFOA in marine environment. Our results will provide new insights into evaluating adverse effects of PFOA and monitoring marine ecosystem. Environ Toxicol Chem 2022;41:426-436. © 2021 SETAC.

Acute acidification stress weakens the head kidney immune function of juvenile Lates calcarifer

Acidized water environment can impact many physiological processes of aquatic animals. The response of the head kidney to acidification, especially the immune response, is of great significance to health. This study analyzed the histological and transcriptional changes under different acidification levels (C group, pH 8.1; P group, pH 7.4; E group, pH 3.5) in the short term (12 h, 36 h and 60 h) in the head kidney of juvenile L. calcarifer. The results showed that the acidification of the water environment caused tissue damage to the head kidney of L. calcarifer, and the damage appeared earlier and was stronger in the extreme pH group. The transcriptional response of L. calcarifer head kidney increased with the increase of acidification level. The two treatments transcriptional responses showed different trends in terms of time. After KEGG function enrichment, with the increase of stimulation time, the proportion of down-regulated pathways was increasing, and the types of pathway enrichment at different acidification levels were quite different at the initial stage. At 12 h, the first category in the P group with the most significant number of pathways was 'Metabolism', and the first category in the E group with the largest number of pathways was 'Human Diseases'. At 60 h, the enrichment pathways of the two groups were highly overlapping in immune-related pathways, which contained 26 common DEGs. They had a dominant expression pattern. In the P group, the expression level decreased with time. In the E group, the down-regulation degree of expression level at 12 h reached the level of the P group at 60 h, and the expression level remained low until 60 h. Through the correlation network, interferon regulatory factor 7 (IRF7), Tripartite motif containing-21 (TRIM21), Signal transducer and activator of transcription 1 (STAT1) and Signal transducer and activator of transcription 3 (STAT3) were found to have the most correlation with other genes. In this study, juvenile L. calcarifer showed different coping strategies to different levels of acute acidification stress, but all of them resulted in the extensive weakening of head kidney immune function.

Regulation of apoptosis by Pacific oyster Crassostrea gigas reveals acclimation strategy to CO2 driven acidification

Ocean acidification (OA) has posed formidable threats to marine calcifiers. In response to elevated CO₂ levels, marine calcifiers have developed multiple strategies to survive, such as taking advantage of apoptosis, but its regulation mechanism remains largely unknown. Here, we used the Pacific oyster Crassostrea gigas as model to understand the apoptotic responses and regulation mechanism at short- (7 d) to long-term (56 d) CO₂ exposure (pH = 7.50). The apoptosis of hemocytes was significantly induced after short-term treatment (7-21 d) but was suppressed under long-term CO₂ exposure (42-56 d). Similarly, caspase-3 and caspase-9 were also increased post short-term exposure and fell back to normal levels after long-term exposure. These data together indicated diverse regulation mechanisms of apoptosis through different exposure periods. Through analysis of the B-cell lymphoma 2 (Bcl-2) family mitochondrial apoptosis regulators, we showed that only CgBcl-XL's expression kept at high levels after 42- and 56-day CO₂ exposure. CgBcl-XL shared sequence, and structural similarity with its mammalian counterpart, and knockdown of CgBcl-XL in hemocytes via RNA interference promoted apoptosis. The protein level of CgBcl-XL was significantly increased after long-term CO₂ exposure (28-56 d), and its distribution in hemocytes became more concentrated and dense. Therefore, CgBcl-XL serves as an essential anti-apoptotic protein for tipping the balance of cell apoptosis, which may play a key role in survival under long-term CO₂ exposure. These results reveal a potential adaptation strategy of oysters towards OA and the variable environment changes through the modulation of apoptosis.

Transcriptional changes revealed water acidification leads to the immune response and ovary maturation delay in the Chinese mitten crab Eriocheir sinensis

Nowadays, due to increasing carbon dioxide released, water acidification poses a series of serious impacts on aquatic organisms. To evaluate the effects of water acidification on crustaceans, we focused on the Chinese mitten crab Eriocheir sinensis, which is a spawning migration and farmed species in China. Based on histological and oocyte transparent liquid observation, we found that the acidified environment significantly delayed the ovarian maturation of E. sinensis. Moreover, RNA-seq was applied to obtain gene expression profile from the crab's gills and ovaries in response to acidified environment. Compared with control groups, a total of 5471 differentially expressed genes (DEGs) were identified in acidified gills and 485 DEGs were identified in acidified ovaries. Enrichment analysis indicated that some pathways also responded to the acidified environment, such as PI3K-Akt signaling pathway, Chemokine signaling pathway, apoptosis, and toll-like receptor signaling pathway. Subsequently, some DEGs involved in immune response (ALF, Cathepsin A, HSP70, HSP90, and catalase) and ovarian maturation (Cyclin B, Fem-1a, Fem-1b, and Fem-1c) were selected to further validate the influence of water acidification on gene expression by qRT-PCR. The results showed that the expression level of immune-related genes was significantly increased to response to the water acidification, while the ovarian maturation-related genes were significantly decreased. Overall, our data suggested that E. sinensis was sensitive to the reduced pH. This comparative transcriptome also provides valuable molecular information on the mechanisms of the crustaceans responding to acidified environment.

Impact on Fertility Rate and Embryo-Larval Development Due to the Association Acidification, Ocean Warming and Lead Contamination of a Sea Urchin Echinometra lucunter (Echinodermata: Echinoidea)

Ocean warming and acidification can cause deleterious effects on marine biota, which may be potentialized when associated with metal pollution. Thus, the aim of this work was to evaluate the effects of pH decrease, temperature increase and lead contamination on fertility rate and embryo-larval development of Echinometra lucunter. Gametes and embryos were exposed at pH 8.2 (control) and 7.5; at 26°C (control) and 28°C; and at lead concentrations of 0 (control), 125, 250 and 500 μg/L. These conditions were tested individually and in combination. The fertilization rate of E. lucunter was only significantly reduced in the treatments where temperature was increased and in the treatment where pH decreased. However, the development rate of the pluteus larvae was significantly affected in the majority of treatments: metal contamination in the higher concentration; decreased pH in all metal concentrations; increased temperature in the highest metal concentration; decreased pH and increased temperature and all variables combined, which is decreased pH, increased temperature and metal contamination in relation to the control group (C). The development test was shown to be more sensitive than the fertilization test in all the studied scenarios. In general, the present study suggests that pH decrease, temperature increase and metal pollution may have a significant impact on E. lucunter reproductive cycle.

Nano zero valent iron (nZVI) particles for the removal of heavy metals (Cd2+, Cu2+ and Pb2+) from aqueous solutions

For the past 15 years, nanoscale metallic iron (nZVI) has been investigated as a new tool for the treatment of heavy metal contaminated water. The removal mechanisms depend on the type of heavy metals and their thermodynamic properties. A metal whose redox potential is more negative or close to the reduction potential of Fe(0) is removed by the reduction process, while the others will be mediated by precipitation, complexation or other sorption processes. This review summarises our contemporary knowledge of nZVI aqueous chemistry, synthesis methods, mechanisms and actions (practical experiences) of heavy metal (Cd, Cu and Pb) removal and challenges of nZVI practical applications. Its inner core (iron(0)) has reducing ability towards pollutants, while the iron oxide (FeO) outer shell provides reaction sites for chemisorption and electrostatic interactions with heavy metals. Emerging studies highlighted that nZVI surfaces will have negatively charged species at higher pH and have good affinity for the removal of positively charged species such as heavy metals. Different sizes, shapes and properties of nZVI have been produced using various methods. Ferric salt reduction methods are the most common methods to produce stable and fine graded nZVI. Higher uptake of copper(ii), lead(ii) and cadmium(ii) has also been reported by various scholars. Practical pilot tests have been conducted to remove heavy metals, which gave highly satisfactory results. Challenges such as agglomeration, sedimentation, magnetic susceptibility, sorption to other fine materials in aqueous solution and toxicity of microbiomes have been reported. Emerging studies have highlighted the prospects of industrial level application of nano zero valent particles for the remediation of heavy metals and other pollutants from various industries.

Tissue comparison of transcriptional response to acute acidification stress of barramundi Lates calcarifer in coastal and estuarine areas

In order to explore the common and unique physiological changes in tissues of juvenile barramundi Lates calcarifer in acidified water environment, RNA sequence analysis was used to analyze the molecular responses of liver, head kidney, and gill of juvenile barramundi in pH 7.4 and pH 8.1 seawater environment. The number of differential expression genes identified in liver, head kidney and gill were 860, 388 and 1792, respectively. Through functional enrichment analysis, the differential expression genes common to the three tissues were all related to immunity. Among the unique differential genes in the liver, pathways related to digestion, endocrine, and metabolism were enriched. Among the unique differential expression genes in gill, pathways related to genetic information processing, immunity and metabolism were enriched. The findings of the present study uncover the transcriptional changes in fish correspond to environmental pH change, and provide a better understanding on the biological process at molecular level to environmental pH adapting. This work highlights that assessments for the potential of estuarine fishes to cope with environmental pH change to develop the future conservation strategies.

Magnitude and Predictability of pH Fluctuations Shape Plastic Responses to Ocean Acidification

AbstractPhenotypic plasticity is expected to facilitate the persistence of natural populations as global change progresses. The attributes of fluctuating environments that favor the evolution of plasticity have received extensive theoretical investigation, yet empirical validation of these findings is still in its infancy. Here, we combine high-resolution environmental data with a laboratory-based experiment to explore the influence of habitat pH fluctuation dynamics on the plasticity of gene expression in two populations of the Mediterranean mussel, Mytilus galloprovincialis. We linked differences in the magnitude and predictability of pH fluctuations in two habitats to population-specific gene expression profiles in ambient and stressful pH treatments. Our results demonstrate population-based differentiation in gene expression plasticity, whereby mussels native to a habitat exhibiting a large magnitude of pH fluctuations with low predictability display reduced phenotypic plasticity between experimentally imposed pH treatments. This work validates recent theoretical findings on evolution in fluctuating environments, suggesting that the predictability of fluctuating selection pressures may play a predominant role in shaping the phenotypic variation observed across natural populations.

Climate change alters the haemolymph microbiome of oysters

The wellbeing of marine organisms is connected to their microbiome. Oysters are a vital food source and provide ecological services, yet little is known about how climate change such as ocean acidification and warming will affect their microbiome. We exposed the Sydney rock oyster, Saccostrea glomerata, to orthogonal combinations of temperature (24, 28 °C) and pCO₂ (400 and 1000 μatm) for eight weeks and used amplicon sequencing of the 16S rRNA (V3-V4) gene to characterise the bacterial community in haemolymph. Overall, elevated pCO₂ and temperature interacted to alter the microbiome of oysters, with a clear partitioning of treatments in CAP ordinations. Elevated pCO₂ was the strongest driver of species diversity and richness and elevated temperature also increased species richness. Climate change, both ocean acidification and warming, will alter the microbiome of S. glomerata which may increase the susceptibility of oysters to disease.

The effects of co-exposure of graphene oxide and copper under different pH conditions in Manila clam Ruditapes philippinarum

Carbon nanomaterials (CNM), such as graphene oxide (GO), have been the focus of study in several areas of science mostly due to their physical-chemical properties. However, data concerning the potential toxic effects of these CNM in bivalves are still scarce. When present in the aquatic systems, the combination with other contaminants, as well as pH environmental variations, can influence the behavior of these nanomaterials and, consequently, their toxicity. Thus, the main goal of this study was to evaluate the effect of exposure of clam Ruditapes philippinarum to GO when acting alone and in the combination with copper (Cu), under two pH levels (control 7.8 and 7.3). A 28-day exposure was performed and metabolism and oxidative stress-related parameters were evaluated. The effects caused by GO and Cu exposures, either isolated or co-exposed, showed a direct and dependent relationship with the pH in which the organisms were exposed. In clams maintained at control pH (7.8), Cu and GO + Cu treatments showed lower lipid peroxidation (LPO) and lower electron transport system (ETS) activity, respectively. In clams maintained at low pH, glutathione-S-transferases (GSTs) activities were increased in Cu and Cu + GO treatments, whereas reduced glutathione (GSH) levels were increased in Cu treatment and ETS activity was higher in GO + Cu. Thus, it can be observed that clams responses to Cu and GO were strongly modulated by pH in terms of their defense system and energy production, although this does not result into higher LPO levels.

Haplosporidian host:parasite interactions

The host:parasite interactions of the 3 serious haplosporidian pathogens of oysters, on which most information exists, are reviewed. They are Bonamia ostreae in Ostrea spp. and Crassostrea gigas; Bonamia exitiosa in Ostrea spp.; and Haplosporidium nelsoni in Crassostrea spp. Understanding the haemocytic response to pathogens is constrained by lack of information on haematopoiesis, haemocyte identity and development. Basal haplospridians in spot prawns are probably facultative parasites. H. nelsoni and a species infecting Haliotis iris in New Zealand (NZAP), which have large extracellular plasmodia that eject haplosporosomes or their contents, lyse surrounding cells and are essentially extracellular parasites. Bonamia spp. have small plasmodia that are phagocytosed, haplosporosomes are not ejected and they are intracellular obligate parasites. Phagocytosis by haemocytes is followed by formation of a parasitophorous vacuole, blocking of haemocyte lysosomal enzymes and the endolysosomal pathway. Reactive oxygen species (ROS) are blocked by antioxidants, and host cell apoptosis may occur. Unlike susceptible O. edulis, the destruction of B. ostreae by C. gigas may be due to higher haemolymph proteins, higher rates of granulocyte binding and phagocytosis, production of ROS, the presence of plasma β-glucosidase, antimicrobial peptides and higher levels of haemolymph and haemocyte enzymes. In B.exitiosa infection of Ostrea chilensis, cytoplasmic lipid bodies (LBs) containing lysosomal enzymes accumulate in host granulocytes and in B. exitiosa following phagocytosis. Their genesis and role in innate immunity and inflammation appears to be the same as in vertebrate granulocytes and macrophages, and other invertebrates. If so, they are probably the site of eicosanoid synthesis from arachidonic acid, and elevated numbers of LBs are probably indicative of haemocyte activation. It is probable that the molecular interaction, and role of LBs in the synthesis and storage of eicosanoids from arachidonic acid, is conserved in innate immunity in vertebrates and invertebrates. However, it seems likely that haplosporidians are more diverse than realized, and that there are many variations in host parasite interactions and life cycles.

Diel-cycling seawater acidification and hypoxia impair the physiological and growth performance of marine mussels

Ocean acidification and hypoxia are concurrent in some coastal waters due to anthropogenic activities in the past decades. In the natural environment, pH and dissolved oxygen (DO) may fluctuate and follow diel-cycling patterns, but such effects on marine animals have not been comprehensively studied compared to their constant effects. In order to study the effects of diel-cycling seawater acidification and hypoxia on the fitness of marine bivalves, the thick shell mussels Mytilus coruscus were exposed to two constant levels of dissolved oxygen (2 mg/L, 8 mg/L) under two pH treatments (7.3, 8.1), as well as single diel fluctuating pH or DO, and the combined diel fluctuating of pH and DO for three weeks. The experimental results showed that constant acidification and hypoxia significantly reduced the extracellular pH (pH_e) and condition index (CI) of mussels, and significantly increased HCO₃^-, pCO₂ and standard metabolic rate (SMR). Diel fluctuating hypoxia and acidification also significantly reduced the pH_e and CI, and significantly increased pCO₂ and SMR, but had no significant effects on HCO₃^-. However, the diel-cycling acidification and hypoxia resulted in a higher CI compared to continuous exposure. In general, continuous and intermittent stress negatively impact the hemolymph and growth performance of mussels. However, mussels possess a little stronger resistance to diel-cycling seawater acidification and hypoxia than sustained stress.

Effects of pH and nitrogen form on Nitzschia closterium growth by linking dynamic with enzyme activity

In this study, Nitzschia closterium was incubated in seawater at different pH values (8.10, 7.71, and 7.45) and using different nitrogen forms (NO₃-N and NH₄-N) in the laboratory. The results showed that the growth of N. closterium was inhibited by ocean acidification, with individuals under lower pH levels showing lower growth rates and lower nitrogen uptake rates for both nitrogen forms. The V_max/K_s ratio decreased with decreasing pH, indicating the inhibition of nitrogen uptake, whereas the ratios for NH₄-N cultures were higher than those for NO₃-N cultures, implying the highly competitive position of NH₄-N. Acidification might induce reactive oxygen species based on the result that the maximum enzyme activities of SuperOxide Dismutase (SOD) and CATalase (CAT) increased under lower pH levels. The SOD and CAT activities for the NO₃-N cultures were higher than those for NH₄-N cultures at the low pH level, indicating that acidification might cause more oxidative stress for NO₃-N cultures than for NH₄-N cultures. Thus, ocean acidification might have a more detrimental effect on the growth of N. closterium under NO₃-N conditions than NH₄-N conditions, with a lower ratio (γ) of the maximum growth rate to the maximum nutrient uptake rate, and a drop in nitrate reductase activity under lower pH levels.

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.

Exposure to Decreased pH and Caffeine Affects Hemocyte Parameters in the Mussel Mytilus galloprovincialis

Combined effects of reduced pH, as predicted under climate change scenarios, and the most popular and widely used stimulant caffeine were assessed in hemocyte parameters of the mussel Mytilus galloprovincialis, being hemocytes involved in immune defense. Bivalves were exposed for one week to natural pH (8.1) and two reduced pH values (pH −0.4 units and pH −0.7 units). Exposure continued for additional two weeks, both in the absence and in the presence of environmentally relevant concentrations of caffeine (0.05 and 0.5 µg/L). Hemocyte parameters (total hemocyte count, hemocyte volume and diameter, neutral red uptake and hemocyte proliferation) were measured after 7 days of exposure to pH only, and after 14 (T1) and 21 (T2) days of exposure to the various pH*caffeine combinations. At all sampling times, pH significantly affected all the biological variables considered, whereas caffeine exhibited a significant influence at T2 only. Among the various hemocyte parameters, caffeine caused a significant increase in total hemocyte count at T2, and in hemocyte volume and diameter at both T1 and T2, when a significant interaction between pH and caffeine was also found. Overall, results demonstrated that hemocyte functionality was strongly influenced by the experimental conditions tested. Further studies are needed to assess combined effects of climate changes and emerging contaminants on bivalve immune system when challenged with environmental pathogens.

Genome-wide identification, characterization of RLR genes in Yesso scallop (Patinopecten yessoensis) and functional regulations in responses to ocean acidification

Retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs), are crucial sensors with a conserved structure in cytoplasm, inducing the production of cytokines, chemokines and host restriction factors which mediate a variety of intracellular activities to interfere with distinct PAMPs (pathogen-associated molecular patterns) for eliminating pathogens in innate immune system. Although RLR genes have been investigated in most vertebrates and some invertebrates, the systematic identification and characterization of RLR genes have not been reported in scallops. In this study, four RLR genes (PY-10413.4, PY-10413.5, PY-443.7 and PY-443.8, designated PyRLRs) were identified in Yesso scallop (Patinopecten yessoensis) through whole-genome scanning through in silico analysis, including two pairs of tandem duplicate genes located on the same scaffold (PY-10413.4 and PY-10413.5, PY-443.7 and PY-443.8, respectively). Phylogenetic and protein structural analyses were performed to determine the identities and evolutionary relationships of these genes. The expression profiles of PyRLRs were determined in all developmental stages, in healthy adult tissues, and in mantles that simulated ocean acidification (OA) exposure (pH = 6.5 and 7.5) at different time points (3, 6, 12 and 24 h). Spatiotemporal expression patterns suggested the functional roles of PyRLRs in all stages of development and growth of the scallop. Regulation expressions revealed PY-10413.4 and PY-10413.5 with one or two CARD(s) (caspase activation and recruitment domain) were up-regulated expressed at most time points, whereas PY-443.8 and PY-10413.4 without CARD were significantly down-regulated at each time points, suggesting functional differentiations in the two pairs of PyRLRs based on the structural differences in response to OA. Collectively, this study demonstrated gene duplication of RLR family genes and provide primary analysis for versatile roles in the response of the bivalve innate immune system to OA challenge.

Immunosuppression of aquatic organisms exposed to elevated levels of manganese: From global to molecular perspective

Manganese (Mn) is an essential trace metal for all organisms. However, in excess it causes toxic effects but the impact on aquatic environments has so far been highly overlooked. Manganese is abundant both in costal and deep sea sediments and becomes bioavailable (Mn²⁺) during redox conditions. This is an increasing phenomenon due to eutrophication-induced hypoxia and aggravated through the ongoing climate change. Intracellular accumulation of Mn²⁺ causes oxidative stress and activates evolutionary conserved pathways inducing apoptosis and cell cycle arrest. Here, studies are compiled on how excess of dissolved Mn suppresses the immune system of various aquatic organisms by adversely affecting both renewal of immunocytes and their functionality, such as phagocytosis and activation of pro-phenoloxidase. These impairments decrease the animal's bacteriostatic capacity, indicating higher susceptibility to infections. Increased distribution of pathogens, which is believed to accompany climate change, requires preserved immune sentinel functions and Mn can be crucial for the outcome of host-pathogen interactions.

Experimental evaluation of survival of Vibrio parahaemolyticus in fertilized cold-water sediment

AIMS

This experimental study focuses on survival and consistence of Vibrio parahaemolyticus in cold-water sediments and how increasing temperature and nutritional availability can affect growth.

METHODS AND RESULTS

A pathogenic strain of V. parahaemolyticus was inoculated in seawater microcosms containing bottom sediment. Gradually, during 14 days, the temperature was upregulated from 8 to 21°C. Culturable V. parahaemolyticus was only found in the sediment but declined over time and did not recover even after another 2 days at 37°C. Numbers of culturable bacteria matched the amount found by q-PCR indicating that they did not enter a dormant state, contrary to those in the water layer. After adding decaying phytoplankton as fertilizer to the microcosms of 8 and 21°C for 7 and 14 days, the culturability of the bacteria increased significantly in the sediments at both temperatures and durations of exposure.

CONCLUSION

The study showed that V. parahaemolyticus can stay viable in cold-water sediment and growth was stimulated by fertilizers rather than by temperature.

SIGNIFICANCE AND IMPACT OF THE STUDY

Vibrio parahaemolyticus is a common cause of seafood-borne gastroenteritis and is today recognized in connection to increasing ocean temperature. The results indicate that this pathogen should be considered a risk in well-fertilized environments, such as aquacultures, even during cold periods.

Experimental acidification increases susceptibility of Mercenaria mercenaria to infection by Vibrio species

Ocean acidification alters seawater carbonate chemistry, which can have detrimental impacts for calcifying organisms such as bivalves. This study investigated the physiological cost of resilience to acidification in Mercenaria mercenaria, with a focus on overall immune performance following exposure to Vibrio spp. Larval and juvenile clams reared in seawater with high pCO₂ (~1200 ppm) displayed an enhanced susceptibility to bacterial pathogens. Higher susceptibility to infection in clams grown under acidified conditions was derived from a lower immunity to infection more so than an increase in growth of bacteria under high pCO₂. A reciprocal transplant of juvenile clams demonstrated the highest mortality amongst animals transplanted from low pCO₂/high pH to high pCO₂/low pH conditions and then exposed to bacterial pathogens. Collectively, these results suggest that increased pCO₂ will result in immunocompromised larvae and juveniles, which could have complex and pernicious effects on hard clam populations.

Vulnerability of glutathione-depleted Crassostrea gigas oysters to Vibrio species

Glutathione (GSH) is a major cellular antioxidant molecule participating in several biological processes, including immune function. In this study, we investigated the importance of GSH to oysters Crassostrea gigas immune response. Oysters were treated with the GSH-synthesis inhibitor buthionine sulfoximine (BSO), and the function of immune cells and mortality were evaluated after a bacterial challenge with different Vibrio species. BSO caused a moderate decrease (20-40%) in GSH levels in the gills, digestive gland, and hemocytes. As expected, lower GSH decreased survival to peroxide exposure. Hemocyte function was preserved after BSO treatment, however, oysters became more susceptible to challenges with Vibrio anguillarum, V. alginolyticus, or V. harveyi, but not with V. parahaemolyticus and V. vulnificus, indicating a species-specific vulnerability. Our study indicates that in natural habitats or in mariculture farms, disturbances in GSH metabolism may pre-dispose oysters to bacterial infection, decreasing survival.

Vibrio harveyi: A brief survey of general characteristics and recent epidemiological traits associated with climate change

Here we briefly review the major characteristics of the emerging pathogen Vibrio harveyi and discuss survival strategies and adaptation mechanisms underlying the capacity of this marine bacterium to thrive in natural and artificial aquatic settings. Recent studies suggest that some adaptation mechanisms can easily be acquired by V. harveyi and other members of the Vibrionaceae family owing to efficient horizontal gene transfer and elevated mutation rate. While discussing the main factors in charge of the expansion of Vibrio spp. habitats and concomitant spread of Vibrio-associated diseases under climate change, this review highlights the need for future studies able to address the joint impact of environmental and anthropogenic factors on the long-term dynamics and virulence of V. harveyi populations at the global scale.