Metabolic and physiological responses in tissues of the long-lived bivalve Arctica islandica to oxygen deficiency

Metabolic adaptation of the clam Ruditapes philippinarum during air exposure and the positive effects of sodium nitroprusside pretreatment

The Manila clam (Ruditapes philippinarum), as one of the shellfish living in the intertidal zone, is known for its strong ability to withstand air exposure. Sodium nitroprusside (SNP), a donor of nitric oxide (NO), has been shown to be useful for antioxidant and immune regulation in aquatic animals. In this study, an untargeted metabolomics (LC-MS/MS) technique was employed for the first time in Manila clam to analyze the metabolic and histological impacts after air exposure and the positive effects of SNP pretreatment. During air exposure, a significant increase in taurine, L-glutamate, and several polyunsaturated fatty acids in clams was detected, which indicates that clams may experience inflammatory reactions, oxidative stress, and an increase in blood ammonia content. When clams were exposed to SNP for 6 h, arginine, spermine, L-glutamic acid, and glutathione content were all upregulated, indicating that the SNP exposure induced NO production and improved antioxidant capacity in clams. When the clams were exposed to air after SNP pretreatment, there were no significant differences in the levels of taurine, L-glutamate, or aliphatic acids between the experimental and control groups. Gill tissue was more severely damaged in clams directly exposed to air than in those that experienced air exposure after SNP pretreatment, especially in clams exposed to air for a long time (72 h). Both metabolomics and tissue section structure indicated that SNP pretreatment decreased the stress responses caused by air exposure in R. philippinarum. These findings provided fresh insights and a theoretical foundation for understanding the tolerance to air exposure and physiological functions of SNP (or NO) in R. philippinarum.

Energy metabolism of juvenile scallops Nodipecten subnodosus under acute increased temperature and low oxygen availability

High temperature increases energy demand in ectotherms, limiting their physiological capability to cope with hypoxic events. The present study aimed to assess the metabolic tolerance of juvenile Nodipecten subnodosus scallops to acute hyperthermia combined with moderate hypoxia. A previous study showed that juveniles exhibited a high upper temperature limit (32 °C), but the responses of juveniles to combined hyperthermia and low dissolved oxygen are unknown. Scallops were exposed to control conditions (treatment C: 22 °C, ∼7.1 mg O₂ L^-1 or PO₂ 156.9 mmHg), acute hyperthermia under normoxia (treatment T: 30 °C, ∼6.0 mg O₂ L^-1 or PO₂ 150.9 mmHg) or acute hyperthermia plus hypoxia (treatment TH: 30 °C, ∼2.5 mg O₂ L^-1 or PO₂ 62.5 mmHg) for 18 h. In T, juveniles exhibited an enhanced oxygen consumption, together with a decrease in adenylate energy charge (AEC) and arginine phosphate (ArgP), and with no changes in metabolic enzyme activity in the muscle. In TH, scallops maintained similar AEC and ArgP levels in muscle as those observed in T treatment. This response occurred along with the accumulation of inosine monophosphate and hypoxanthine. Besides, reduced citrate synthase and pyruvate kinase activities, enhanced hexokinase activity, and a higher octopine dehydrogenase/lactate dehydrogenase ratio in the mantle indicated the onset of anaerobiosis in TH. These responses indicate that juvenile scallops showed tissue-specific compensatory responses regarding their energy balance under moderate hypoxia at high temperatures. Our results give an insight into the tolerance limit of this species to combined hyperthermia and hypoxia in its northern limit of distribution.

Changes in metabolic profiling of whiteleg shrimp (Penaeus vannamei) under hypoxic stress

Hypoxia is a common concern in shrimp aquaculture, affecting growth and survival. Although recent studies have revealed important insights into hypoxia in shrimp and crustaceans, knowledge gaps remain regarding this stressor at the molecular level. In the present study, a gas chromatography-mass spectrometry (GC-MS)-based metabolomics approach was employed to characterize the metabolic signatures and pathways underlying responses of Pacific white shrimp (Penaeus vannamei) to hypoxia and to identify associated candidate biomarkers. We compared metabolite profiles of shrimp haemolymph before (0 h) and after exposure to hypoxia (1 & 2 h). Dissolved oxygen levels were maintained above 85 % saturation in the control and before hypoxia, and 15 % saturation in the hypoxic stress treatment. Results showed 44 metabolites in shrimp haemolymph that were significantly different between before and after hypoxia exposure. These metabolites were energy-related metabolites (e.g., intermediates of citric acid cycle, lactic acid, alanine), fatty acids and amino acids. Pathway analysis revealed 17 pathways that were significantly affected by hypoxia. The changes in metabolites and pathways indicate a shift from aerobic to anaerobic metabolism, disturbance in amino acid metabolism, osmoregulation, oxidative damage and Warburg effect-like response caused by hypoxic stress. Among the altered metabolites, lactic acid was most different between before and after hypoxia exposure and had the highest accurate value for biomarker identification. Future investigations may validate this molecule as a stress biomarker in aquaculture. This study contributes to a better understanding of hypoxia in shrimp and crustaceans at the metabolic level and provides a base for future metabolomics investigations on hypoxia.

Transcriptomic responses to air exposure stress in coelomocytes of the sea cucumber, Apostichopus japonicus

During rearing in hatcheries and transportation to restocking sites, sea cucumbers are often exposed to air for several hours, which may depress their non-specific immunity and lead to mass mortality. We performed transcriptome analysis of Apostichopus japonicus coelomocytes after air exposure to identify stress-related genes and pathways. After exposure to air for 1 h, individuals were re-submerged in aerated seawater and coelomocytes were collected at 0, 1, 4, and 16 h (B, H1, H4, and H16, respectively). We identified 6148 differentially expressed genes, of which 3216 were upregulated and 2932 were downregulated. Many genes involved in the immune response, antioxidant defense, and apoptosis were highly induced in response to air exposure. Enrichment analysis of Gene Ontology terms showed that the most abundant terms in the biological process category were oxidation-reduction process, protein folding and phosphorylation, and receptor-mediated endocytosis for the comparison of H1 vs. B, H4 vs. H1, and H16 vs. H4, respectively. Kyoto Eecyclopedia of Genes and Genomes enrichment analysis showed that six pathways related to the metabolism of proteins, fats, and carbohydrates were shared among the three comparisons. These results indicated that sea cucumbers regulate the expression of genes related to the antioxidant system and energy metabolism to resist the negative effects of air exposure stress. These findings may be applied to optimize juvenile sea cucumber production, and facilitate molecular marker-assisted selective breeding of an anoxia-resistant strain.

Identification and Quantitation of Taste-Active Compounds in Dried Scallops by Combined Application of the Sensomics and a Quantitative NMR Approach

Application of the sensomics concept on dried scallops, a Japanese specialty produced from the adductor muscle of scallops, revealed after activity-guided fractionation with subsequent (comparative) taste dilution analyses besides nucleotides, amino acids, organic acids, and inorganic ions, the presence of taste-modulating quaternary ammonium compounds and opines in highly taste-active fractions. In order to recreate the taste of dried scallops, two independent quantitation approaches were applied and compared. The first approach used multiple targeted UHPLC-MS/MS and high-performance ion chromatography methods. Besides already established quantitation methods for basic taste compounds, a new HILIC-UHPLC-MS/MS_MRM method for the quantitation of chromatographically challenging opines, using synthesized stable isotope-labeled standards, was developed. Furthermore, a qHNMR approach was applied, enabling a direct identification and quantitation of organic taste compounds in a food extract without prior fractionation using a reference ¹H NMR database. Both methods yielded similar quantitative results of taste-active compounds in dried scallop extracts and subsequent taste recombination experiments based on these data were able to recreate the taste of dried scallops.

Evaluation of immune stimulatory products for whiteleg shrimp (Penaeus vannamei) by a metabolomics approach

The use of probiotics, prebiotics and dietary fiber has become a common practice in shrimp aquaculture as alternatives to antibiotic treatment. However, not much is known about the metabolic mechanisms underlying the effects of probiotics and immunostimulant used in shrimp aquaculture. In this study, a gas chromatography-mass spectrometry (GC-MS) based metabolomics approach was used to characterize metabolite profiles of haemolymph and gills of whiteleg shrimp (Penaeus vannamei) exposed to four treatments (cellulose fiber, probiotics with Vibrio alginolyticus, a combination of cellulose fiber and V. alginolyticus and a control treatment). The cellulose fiber was administrated as a feed additive (100 mg⋅Kg^-1 feed), while the probiotics was applied in the water (10⁵ UFC⋅mL^-1 culture water). The results showed significant differences in haemolymph metabolite profiles of immune stimulated treatments compared to the control and among treatments. The combination of cellulose fiber and probiotics resulted in greater differences in metabolic profiles, suggesting a better immune stimulation with this approach. The changes in haemolymph metabolome of treated shrimp reflected several biochemical pathway modifications, including changes in amino acid and fatty acid metabolism, disturbances in energy metabolism and antimicrobial activity and stress responses. For gill tissues, significant differences were only found in lactic acid between the probiotic group and the control. Among the altered metabolites, the increases of itaconic acid in haemolymph, and lactic acid in both haemolymph and gill tissues of immune-stimulated suggest the potential use of these metabolites as biomarkers for health assessment in aquaculture.

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.

The effect of ocean acidification on the enzyme activity of Apostichopus japonicus

The influence of ocean acidification (OA) is particularly significant on calcifying organisms. The sea cucumber Apostichopus japonicus is an important cultured calcifying organism in the northern China seas. Little was known about the effects of OA on this economically important species. In this study, individuals from embryo to juveniles stage of A. japonicus, cultured in different levels of acidified seawater, were measured their enzymes activities, including five metabolic enzymes and three immune enzymes. The activity of acid phosphatase (ACP) and alkaline phosphatase (ALP) was significantly lower in the severely acid group (pH 7.1), while the content of lactate dehydrogenase (LDH) was significantly higher. Superoxide dismutase (SOD) and catalase (CAT) were significantly lower in the severely acid group. The multivariate statistical results showed that the significant difference of enzyme assemblage existed among three experimental groups. This study indicated that OA could reduce the biomineralization capacity, influence the anaerobic metabolism and severely affect the immune process of A. japonicas. More researches are needed in the future to reveal the mechanisms of enzyme regulation and expression of A. japonicas underlying mixture environmental stress.

Metabolomics investigation of summer mortality in New Zealand Greenshell™ mussels (Perna canaliculus)

Increasing water temperatures due to climate change have resulted in more frequent high mortality events of New Zealand Greenshell™ mussels (Perna canaliculus Gmelin 1791). These events have significant impacts within mussel farms which support a major shellfish industry for New Zealand. The present study investigates metabolic responses of farmed mussels during a summer mortality event in order to identify health impacts and elucidate mechanistic effects of external stressors on mussels. A gas chromatography-mass spectrometry (GC-MS)-based metabolomics approach was used to identify metabolic perturbations and flow cytometry assays were used to assess viability, oxidative stress and apoptosis of haemocytes from healthy and unhealthy mussels during a summer mortality event. The results showed significantly higher mortality and apoptosis of haemocytes in unhealthy mussels compared to healthy mussels. Reactive oxygen species (ROS) production, which is an indicator of oxidative stress was very high in both mussel groups, but no differences were observed between the two mussel groups. Metabolomics revealed alterations of many metabolites in both haemolymph and hepatopancreas (digestive gland) of unhealthy mussels compared to healthy mussels, reflecting perturbations in several molecular pathways, including energy metabolism, amino acid metabolism, protein degradation/tissue damage and oxidative stress. An increased level of itaconic acid which is an antimicrobial metabolite and biomarker of pathogen infection was observed in haemolymph, but not in hepatopancreas samples. This investigation provides the first detailed metabolic characterization of mussel immune responses to a summer mortality event and illustrates the benefits of using an integrated metabolomics and flow cytometry workflow for mussel health assessment and biomarker identification for summer mortality early detection.

Nontraditional systems in aging research: an update

Research on the evolutionary and mechanistic aspects of aging and longevity has a reductionist nature, as the majority of knowledge originates from experiments on a relatively small number of systems and species. Good examples are the studies on the cellular, molecular, and genetic attributes of aging (senescence) that are primarily based on a narrow group of somatic cells, especially fibroblasts. Research on aging and/or longevity at the organismal level is dominated, in turn, by experiments on Drosophila melanogaster, worms (Caenorhabditis elegans), yeast (Saccharomyces cerevisiae), and higher organisms such as mice and humans. Other systems of aging, though numerous, constitute the minority. In this review, we collected and discussed a plethora of up-to-date findings about studies of aging, longevity, and sometimes even immortality in several valuable but less frequently used systems, including bacteria (Caulobacter crescentus, Escherichia coli), invertebrates (Turritopsis dohrnii, Hydra sp., Arctica islandica), fishes (Nothobranchius sp., Greenland shark), reptiles (giant tortoise), mammals (blind mole rats, naked mole rats, bats, elephants, killer whale), and even 3D organoids, to prove that they offer biogerontologists as much as the more conventional tools. At the same time, the diversified knowledge gained owing to research on those species may help to reconsider aging from a broader perspective, which should translate into a better understanding of this tremendously complex and clearly system-specific phenomenon.

Nitric oxide mediates metabolic functions in the bivalve Arctica islandica under hypoxia

The free radical nitric oxide (NO) is a powerful metabolic regulator in vertebrates and invertebrates. At cellular concentrations in the nanomolar range, and simultaneously reduced internal oxygen partial pressures (pO₂), NO completely inhibits cytochrome-c-oxidase (CytOx) activity and hence mitochondrial- and whole-tissue respiration. The infaunal clam Arctica islandica regulates pO₂ of hemolymph and mantle cavity water to mean values of <5 kPa, even in a completely oxygen-saturated environment of 21 kPa. These low internal pO₂ values support a longer NO lifespan and NO accumulation in the body fluids and can thus trigger a depression of metabolic rate in the clams. Measurable amounts of NO formation were detected in hemocyte cells (~110 pmol NO 100⁻¹ hemocytes h^-1 at 6 kPa), which was not prevented in the presence of the NO synthase inhibitor L-NAME, and in the gill filaments of A. islandica. Adding a NO donor to intact gills and tissue homogenate significantly inhibited gill respiration and CytOx activity below 10 kPa. Meanwhile, the addition of the NO-oxidation product nitrite did not affect metabolic rates. The high nitrite levels found in the hemolymph of experimental mussels under anoxia do not indicate cellular NO production, but could be an indication of nitrate reduction by facultative anaerobic bacteria associated with tissue and/or hemolymph biofilms. Our results suggest that NO plays an important role in the initiation of metabolic depression during self-induced burrowing and shell closure of A. islandica. Furthermore, NO appears to reduce mitochondrial oxygen radical formation during surfacing and cellular reoxygenation after prolonged periods of hypoxia and anoxia.

Revamping the evolutionary theories of aging

Radical lifespan disparities exist in the animal kingdom. While the ocean quahog can survive for half a millennium, the mayfly survives for less than 48 h. The evolutionary theories of aging seek to explain why such stark longevity differences exist and why a deleterious process like aging evolved. The classical mutation accumulation, antagonistic pleiotropy, and disposable soma theories predict that increased extrinsic mortality should select for the evolution of shorter lifespans and vice versa. Most experimental and comparative field studies conform to this prediction. Indeed, animals with extreme longevity (e.g., Greenland shark, bowhead whale, giant tortoise, vestimentiferan tubeworms) typically experience minimal predation. However, data from guppies, nematodes, and computational models show that increased extrinsic mortality can sometimes lead to longer evolved lifespans. The existence of theoretically immortal animals that experience extrinsic mortality - like planarian flatworms, panther worms, and hydra - further challenges classical assumptions. Octopuses pose another puzzle by exhibiting short lifespans and an uncanny intelligence, the latter of which is often associated with longevity and reduced extrinsic mortality. The evolutionary response to extrinsic mortality is likely dependent on multiple interacting factors in the organism, population, and ecology, including food availability, population density, reproductive cost, age-mortality interactions, and the mortality source.

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.

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

Ocean acidification (OA), which is caused by increasing levels of dissolved CO2 in the ocean, is a major threat to marine ecosystems. Multiple lines of scientific evidence show that marine bivalves, including scallops, are vulnerable to OA due to their poor capacities to regulate extracellular ions and acid-based status. However, the physiological mechanisms of scallops responding to OA are not well understood. In this study, we evaluated the effects of 45 days of exposure to OA (pH 7.5) on the energy metabolism and antioxidant capability of Yesso scallops. Some biochemical markers related to energy metabolism (e.g., content of glycogen and ATP, activity of ATPase, lactate dehydrogenase, glutamate oxaloacetate transaminase, and glutamate-pyruvate transaminase), antioxidant capacity (e.g., reactive oxygen species level, activity of superoxide dismutase, and catalase) and cellular damage (e.g., lipid peroxidation level) were measured. Our results demonstrate that the effects of the reduced pH (7.5) on scallops are varied in different tissues. The energy reserves are mainly accumulated in the adductor muscle and hepatopancreas. Yesso scallops exhibit energy modulation by increasing lactate dehydrogenase activities to stimulate anaerobic metabolism. The highly active Na+/K+-ATPase and massive ATP consumption in the mantle and gill indicate that a large amount of energy was allocated for the ion regulation process to maintain the acid-base balance in the reduced-pH environment. Moreover, the increase in the reactive oxygen species level and the superoxide dismutase and catalase activities in the gill and adductor muscle, indicate that oxidative stress was induced after long-term exposure to the reduced-pH environment. Our findings indicate that the effects of OA are tissue-specific, and physiological homeostasis could be modulated through different mechanisms for Yesso scallops.

Hypoxia adaptation in termites: hypoxic conditions enhance survival and reproductive activity in royals

Termite royals (queen and king) exhibit extraordinary longevity without sacrificing reproductive performance, unlike most animals, in whom lifespan is generally negatively associated with reproduction. Therefore, the regulatory mechanisms underlying longevity have attracted much attention. Although the ageing process is influenced by environmental factors in many insects during their life cycle, it remains unclear whether any factors have an effect on the extended survival and high reproductive capacity of termite royals. Here, we show that hypoxia, possibly an important environmental factor in the nests, enhances survival and reproductive activity in incipient royals of the subterranean termite Reticulitermes speratus compared with those in control conditions. Quantitative real-time PCR analysis revealed that the expression levels of the vitellogenin gene in queens are maintained to a greater extent under hypoxic conditions than under control conditions. The expression levels of the antioxidant enzyme genes RsCAT1 and RsPHGPX are also significantly promoted by hypoxia in queens and kings respectively. These results suggest that hypoxic exposure can contribute in part to achieving high reproductive output by altering gene expression after founding of colonies in the royals. Our study provides novel insights into the effect of a nest environment on the reproductive characteristics in termite royals.

Assessment of muscular energy metabolism and heat shock response of the green abalone Haliotis fulgens (Gastropoda: Philipi) at extreme temperatures combined with acute hypoxia and hypercapnia

The interaction between ocean warming, hypoxia and hypercapnia, suggested by climate projections, may push an organism earlier to the limits of its thermal tolerance window. In a previous study on juveniles of green abalone (Haliotis fulgens), combined exposure to hypoxia and hypercapnia during heat stress induced a lowered critical thermal maximum (CT_max), indicated by constrained oxygen consumption, muscular spams and loss of attachment. Thus, the present study investigated the cell physiology in foot muscle of H. fulgens juveniles exposed to acute warming (18 °C to 32 °C at +3 °C day^-1) under hypoxia (50% air saturation) and hypercapnia (~1000 μatm PCO₂), alone and in combination, to decipher the mechanisms leading to functional loss in this tissue. Under exposure to either hypoxia or hypercapnia, citrate synthase (CS) activity decreased with initial warming, in line with thermal compensation, but returned to control levels at 32 °C. The anaerobic enzymes lactate and tauropine dehydrogenase increased only under hypoxia at 32 °C. Under the combined treatment, CS overcame thermal compensation and remained stable overall, indicating active mitochondrial regulation under these conditions. Limited accumulation of anaerobic metabolites indicates unchanged mode of energy production. In all treatments, upregulation of Hsp70 mRNA was observed already at 30 °C. However, lack of evidence for Hsp70 protein accumulation provides only limited support to thermal denaturation of proteins. We conclude that under combined hypoxia and hypercapnia, metabolic depression allowed the H. fulgens musculature to retain an aerobic mode of metabolism in response to warming but may have contributed to functional loss.

Inducible variation in anaerobic energy metabolism reflects hypoxia tolerance across the intertidal and subtidal distribution of the Pacific oyster (Crassostrea gigas)

Pacific oyster (Crassostrea gigas) distribute a steep gradient of environmental stress between intertidal and subtidal habits and provide insight into population-scale patterns and underlying processes of variation in physiological tolerance. In this study, 1-year-old-F₁ oysters, collected from subtidal and intertidal habitats, were obtained after common garden experiment. Genetic differentiation and physiological responses under air exposure were examined to determine whether they had evolved into local adapted subpopulations. Mortality rate, anaerobic glycolysis metabolism, and energy status indicated that oyster had initiated metabolism depression and anaerobic glycolysis metabolism in both intertidal and subtidal oysters under air exposure. However, the subtidal oysters displayed the larger energy metabolism depressions and the earlier anaerobic glycolysis responses. This may indicate that subtidal oysters were more sensitives to hypoxia stress, which may lead the higher mortality rate under long term of air exposure. Based on a common garden experimental design, we propose that this diversification may have a genetic background. Overall, the clear differences between intertidal and subtidal oysters under air exposure have provided an important reference for their aquaculture and transportation used in commercial production.

Inducing the Alternative Oxidase Forms Part of the Molecular Strategy of Anoxic Survival in Freshwater Bivalves

Hypoxia in freshwater ecosystems is spreading as a consequence of global change, including pollution and eutrophication. In the Patagonian Andes, a decline in precipitation causes reduced lake water volumes and stagnant conditions that limit oxygen transport and exacerbate hypoxia below the upper mixed layer. We analyzed the molecular and biochemical response of the North Patagonian bivalve Diplodon chilensis after 10 days of experimental anoxia (<0.2 mg O₂/L), hypoxia (2 mg O₂/L), and normoxia (9 mg O₂/L). Specifically, we investigated the expression of an alternative oxidase (AOX) pathway assumed to shortcut the regular mitochondrial electron transport system (ETS) during metabolic rate depression (MRD) in hypoxia-tolerant invertebrates. Whereas, the AOX system was strongly upregulated during anoxia in gills, ETS activities and energy mobilization decreased [less transcription of glycogen phosphorylase (GlyP) and succinate dehydrogenase (SDH) in gills and mantle]. Accumulation of succinate and induction of malate dehydrogenase (MDH) activity could indicate activation of anaerobic mitochondrial pathways to support anoxic survival in D. chilensis. Oxidative stress [protein carbonylation, glutathione peroxidase (GPx) expression] and apoptotic intensity (caspase 3/7 activity) decreased, whereas an unfolded protein response (HSP90) was induced under anoxia. This is the first clear evidence of the concerted regulation of the AOX and ETS genes in a hypoxia-tolerant freshwater bivalve and yet another example that exposure to hypoxia and anoxia is not necessarily accompanied by oxidative stress in hypoxia-tolerant mollusks.

Biosynthesis of an Opine Metallophore by Pseudomonas aeruginosa

Bacterial pathogenesis frequently requires metal acquisition by specialized, small-molecule metallophores. We hypothesized that the Gram-negative Pseudomonas aeruginosa encodes the enzymes nicotianamine synthase (NAS) and opine dehydrogenase (ODH), biosynthesizing a new class of opine metallophore, previously characterized only in the unrelated Gram-positive organism Staphylococcus aureus. The identity of this metallophore, herein named pseudopaline, was determined through measurements of binding affinity, the in vitro reconstitution of the biosynthetic pathway to screen potential substrates, and the confirmation of product formation by mass spectrometry. Pseudopaline and the S. aureus metallophore staphylopine exhibit opposite stereochemistry for the histidine moiety, indicating unique recognition by NAS. Additionally, we demonstrate SaODH catalysis in the presence of pyruvate, as previously shown, but also oxaloacetate, suggesting the potential for the production of a variant form of staphylopine, while PaODH specifically recognizes α-ketoglutarate. Both the staphylopine and pseudopaline operons have been implicated in the pathogenesis of key infectious disease states and warrant further study.

What modulates animal longevity? Fast and slow aging in bivalves as a model for the study of lifespan

Delineating the physiological and biochemical causes of aging process in the animal kingdom is a highly active area of research not only because of potential benefits for human health but also because aging process is related to life history strategies (growth and reproduction) and to responses of organisms to environmental conditions and stress. In this synthesis, we advocate studying bivalve species as models for revealing the determinants of species divergences in maximal longevity. This taxonomic group includes the longest living metazoan on earth (Arctica islandica), which insures the widest range of maximum life span when shorter living species are also included in the comparative model. This model can also be useful for uncovering factors modulating the pace of aging in given species by taking advantages of the wide disparity of lifespan among different populations of the same species. For example, maximal lifespan in different populations of A islandica range from approximately 36 years to over 500 years. In the last 15 years, research has revealed that either regulation or tolerance to oxidative stress is tightly correlated to longevity in this group which support further investigations on this taxon to unveil putative mechanistic links between Reactive Oxygen Species and aging process.

Intra-population variability of ocean acidification impacts on the physiology of Baltic blue mussels (Mytilus edulis): integrating tissue and organism response

Increased maintenance costs at cellular, and consequently organism level, are thought to be involved in shaping the sensitivity of marine calcifiers to ocean acidification (OA). Yet, knowledge of the capacity of marine calcifiers to undergo metabolic adaptation is sparse. In Kiel Fjord, blue mussels thrive despite periodically high seawater PCO₂, making this population interesting for studying metabolic adaptation under OA. Consequently, we conducted a multi-generation experiment and compared physiological responses of F1 mussels from 'tolerant' and 'sensitive' families exposed to OA for 1 year. Family classifications were based on larval survival; tolerant families settled at all PCO₂ levels (700, 1120, 2400 µatm) while sensitive families did not settle at the highest PCO₂ (≥99.8% mortality). We found similar filtration rates between family types at the control and intermediate PCO₂ level. However, at 2400 µatm, filtration and metabolic scope of gill tissue decreased in tolerant families, indicating functional limitations at the tissue level. Routine metabolic rates (RMR) and summed tissue respiration (gill and outer mantle tissue) of tolerant families were increased at intermediate PCO₂, indicating elevated cellular homeostatic costs in various tissues. By contrast, OA did not affect tissue and routine metabolism of sensitive families. However, tolerant mussels were characterised by lower RMR at control PCO₂ than sensitive families, which had variable RMR. This might provide the energetic scope to cover increased energetic demands under OA, highlighting the importance of analysing intra-population variability. The mechanisms shaping such difference in RMR and scope, and thus species' adaptation potential, remain to be identified.

Environmental control and control of the environment: the basis of longevity in bivalves

Longevity and ageing are two sides of a coin, leaving the question open as to which one is the cause and which one the effect. At the individual level, the physiological rate of ageing determines the length of life (= individual longevity, as long as death results from old age and not from disease or other impacts). Individual longevity depends on the direct influence of environmental conditions with respect to nutrition, and the possibility for and timing of reproduction, as well as on the energetic costs animals invest in behavioural and physiological stress defence. All these environmental effectors influence hormonal and cellular signalling pathways that modify the individual physiological condition, the reproductive strategy, and the rate of ageing. At the species level, longevity (= maximum lifespan) is the result of an evolutionary process and, thus, largely determined by the species' behavioural and physiological adaptations to its ecological niche. Specifically, reproductive and breeding strategies have to be optimized in relation to local environmental conditions in different habitats. As a result of adaptive and evolutionary processes, species longevity is genetically underpinned, not necessarily by a few ageing genes, but by an evolutionary process that has hierarchically shaped and optimized species genomes to function in a specific niche or environmental system. Importantly, investigations and reviews attempting to unravel the mechanistic basis of the ageing process need to differentiate clearly between the evolutionary process shaping longevity at the species level and the regulatory mechanisms that alter the individual rate of ageing.

Gene expression and physiological changes of different populations of the long-lived bivalve Arctica islandica under low oxygen conditions

The bivalve Arctica islandica is extremely long lived (>400 years) and can tolerate long periods of hypoxia and anoxia. European populations differ in maximum life spans (MLSP) from 40 years in the Baltic to >400 years around Iceland. Characteristic behavior of A. islandica involves phases of metabolic rate depression (MRD) during which the animals burry into the sediment for several days. During these phases the shell water oxygen concentrations reaches hypoxic to anoxic levels, which possibly support the long life span of some populations. We investigated gene regulation in A. islandica from a long-lived (MLSP 150 years) German Bight population and the short-lived Baltic Sea population, experimentally exposed to different oxygen levels. A new A. islandica transcriptome enabled the identification of genes important during hypoxia/anoxia events and, more generally, gene mining for putative stress response and (anti-) aging genes. Expression changes of a) antioxidant defense: Catalase, Glutathione peroxidase, manganese and copper-zinc Superoxide dismutase; b) oxygen sensing and general stress response: Hypoxia inducible factor alpha, Prolyl hydroxylase and Heat-shock protein 70; and c) anaerobic capacity: Malate dehydrogenase and Octopine dehydrogenase, related transcripts were investigated. Exposed to low oxygen, German Bight individuals suppressed transcription of all investigated genes, whereas Baltic Sea bivalves enhanced gene transcription under anoxic incubation (0 kPa) and, further, decreased these transcription levels again during 6 h of re-oxygenation. Hypoxic and anoxic exposure and subsequent re-oxygenation in Baltic Sea animals did not lead to increased protein oxidation or induction of apoptosis, emphasizing considerable hypoxia/re-oxygenation tolerance in this species. The data suggest that the energy saving effect of MRD may not be an attribute of Baltic Sea A. islandica chronically exposed to high environmental variability (oxygenation, temperature, salinity). Contrary, higher physiological flexibility and stress hardening may predispose these animals to perform a pronounced stress response at the expense of life span.

Physiological responses to self-induced burrowing and metabolic rate depression in the ocean quahog Arctica islandica

Arctica islandica is the longest-lived non-colonial animal found so far, and reaches individual ages of 150 years in the German Bight (GB) and more than 350 years around Iceland (IC). Frequent burrowing and physiological adjustments to low tissue oxygenation in the burrowed state are proposed to lower mitochondrial reactive oxygen species (ROS) formation. We investigated burrowing patterns and shell water partial pressure of oxygen (P(O(2))) in experiments with live A. islandica. Furthermore, succinate accumulation and antioxidant defences were recorded in tissues of bivalves in the normoxic or metabolically downregulated state, as well as ROS formation in isolated gills exposed to normoxia, hypoxia and hypoxia/reoxygenation. IC bivalves burrowed more frequently and deeper in winter than in summer under in situ conditions, and both IC and GB bivalves remained burrowed for between 1 and 6 days in laboratory experiments. Shell water P(O(2)) was <5 kPa when bivalves were maintained in fully oxygenated seawater, and ventilation increased before animals entered the state of metabolic depression. Succinate did not accumulate upon spontaneous shell closure, although shell water P(O(2)) was 0 kPa for over 24 h. A ROS burst was absent in isolated gills during hypoxia/reoxygenation, and antioxidant enzyme activities were not enhanced in metabolically depressed clams compared with normally respiring clams. Postponing the onset of anaerobiosis in the burrowed state and under hypoxic exposure presumably limits the need for elevated recovery respiration upon surfacing and oxidative stress during reoxygenation.