NMR Profiling of Metabolites in Larval and Juvenile Blue Mussels (Mytilus edulis) under Ambient and Low Salinity Conditions

Uncovering the roles of sirtuin activity and food availability during the onset of the heat shock response in the California mussel (Mytilus californianus): Implications for antioxidative stress responses

Sirtuins are a class of NAD+-dependent deacylases, with known regulatory roles in energy metabolism and cellular stress responses in vertebrates. Previous work using marine mussels have suggested a similar role in invertebrates, providing a potential mechanism linking food availability and thermal sensitivity in Mytilids. Sirtuin inhibitors affect mussels' recovery from environmental stressors, including acute heat shock and well-fed mussels exposed to sirtuin inhibitors and/or acute heat shock respond differently than poorly fed mussels, at the protein and whole-organism levels. While this implies a relationship between sirtuins, food availability, and temperature, the direct effects of sirtuin inhibitors (nicotinamide and suramin) on sirtuin activity or their putative effectors have not been explicitly tested. In this study, adult Mytilus californianus were acclimated to a low or high food availability and exposed to one of the following treatments: control, acute heat shock, sirtuin inhibitors, or acute heat shock and sirtuin inhibitors. Mussels increased sirtuin activity during early recovery (5 h) from sirtuin inhibition and acute heat shock, but only if acclimated to a high food availability. Redox balance was also impacted in mussels acclimated to high food availability and exposed to sirtuin inhibitors, signifying interactions between ration, acute heat shock, and sirtuin inhibitors. Additionally, we found a correlation between sirtuin and superoxide dismutase activities, suggesting a potential regulatory role of oxidative stress by sirtuins. Following prolonged recovery (17 h), we found increased sirtuin activity in mussels acclimated to low food availability, indicating that endogenous sirtuin activity may be related to food availability in mussels.

Metabolomics and lipidomics reveal the effects of the toxic dinoflagellate Alexandrium catenella on immune cells of the blue mussel, Mytilus edulis

The increasing occurrence of harmful algal blooms, mostly of the dinoflagellate Alexandrium catenella in Canada, profoundly disrupts mussel aquaculture. These filter-feeding shellfish feed on A. catenella and accumulate paralytic shellfish toxins, such as saxitoxin, in tissues, making them unsafe for human consumption. Algal toxins also have detrimental effects upon several physiological functions in mussels, but particularly on the activity of hemocytes - the mussel immune cells. The objective of this work was to determine the effects of experimental exposure to A. catenella upon hemocyte metabolism and activity in the blue mussel, Mytilus edulis. To do so, mussels were exposed to cultures of the toxic dinoflagellate A. catenella for 120 h. The resulting mussel saxitoxin load had measurable effects upon survival of hemocytes and induced a stress response measured as increased ROS production. The neutral lipid fraction of mussel hemocytes decreased two-fold, suggesting a differential use of lipids. Metabolomic 1H nuclear magnetic resonance (NMR) analysis showed that A. catenella modified the energy metabolism of hemocytes as well as hemocyte osmolyte composition. The modified energy metabolism was reenforced by contrasting plasma metabolomes between control and exposed mussels, suggesting that the blue mussel may reduce feed assimilation when exposed to A. catenella.

The influence of temperature rise on the metabolic response of Ruditapes philippinarum clams to 17-α-ethinylestradiol

Untargeted Nuclear Magnetic Resonance metabolomics was employed to study the effects of warming conditions (17-21 °C) and exposure to 17-α-ethinylestradiol (EE2) on the polar metabolome of Ruditapes philippinarum clams, to identify metabolic markers for monitoring/prediction of deviant environmental conditions. Warming alone triggered changes in alanine/aspartate/glutamate, aromatic amino acids, taurine/hypotaurine and homarine/trigonelline pathways, as well as in energy metabolism, suggesting osmoregulatory adaptations and glycolytic/tricarboxylic acid (TCA) cycle activation, possibly accompanied to some extent by gluconeogenesis to preserve glycogen reserves. At 17 °C, the lowest EE2 concentration (5 ng/L) specifically engaged branched-chain and aromatic amino acids to activate the glycolysis/TCA cycle. Notably, a partial metabolic recovery was observed at 25 ng/L, whereas higher EE2 concentrations (125 and 625 ng/L) again induced significant metabolic disturbances. These included enhanced glycogen biosynthesis and increased lipid reserves, sustained by low-level glutathione-based antioxidative mechanisms that seemed active. At 21 °C, response to EE2 was notably weak at low/intermediate concentrations, becoming particularly significant at the highest EE2 concentration (625 ng/L), suggesting higher protection capacity of Ruditapes philippinarum clams under warming conditions. At 625 ng/L, disturbances in alanine/aspartate/glutamate and taurine/hypotaurine metabolisms were observed, with no evidence of enhanced carbohydrate/protein catabolism. This low energy function profile was accompanied by marked antioxidative mechanisms and choline compounds modulation for cell membrane protection/repair. These results help monitor clams´ response to temperature rise and EE2 exposure, paving the way for future effective guidance and prediction of environmental damaging effects.

Metabolic variation and oxidative stress response of blue mussels (Mytilus sp.) perturbed by norfloxacin exposure

Antibiotics are currently widely applied in agricultural cultivation, animal husbandry, and medical treatment, but the effects and ecological risks of antibiotics need to be further investigated. Norfloxacin is one of the most widely applied fluoroquinolone antibiotics and is commonly detected in aquatic ecosystems. In this study, the activities of catalase (CAT) and glutathione S-transferase (GST) in blue mussels (Mytilus sp.) exposed to norfloxacin (from 25 to 200 mg/L) for 2 d of acute exposure and 7 d of subacute exposure were measured. ¹H nuclear magnetic resonance (¹H-NMR)-based metabolomics was applied to identify the metabolites and to investigate the physiological metabolism of blue mussels (Mytilus sp.) under different concentrations of norfloxacin. The activity of the CAT enzyme was induced in acute exposure, while the activity of GST was inhibited in subacute exposure when the concentration of norfloxacin reached 200 mg/L. Orthogonal partial least squares discriminant analysis (OPLS-DA) revealed that the increased concentrations of norfloxacin might cause greater metabolic differences between the treatment and control groups and cause greater metabolic variation within the same treatment group. The contents of taurine in the 150 mg/L acute exposure group were 5.17 times higher than those in the control group. The pathway analysis indicated that exposure to high concentrations of norfloxacin disturbed different pathways involved in energy metabolism, amino acid metabolism, neuroregulation, and the regulation of osmotic pressure. These results may provide a molecular and metabolic view of the effects of norfloxacin and the regulatory mechanism of blue mussels when exposed to extremely high doses of antibiotics.

Influence of Ecological Factors on the Metabolomic Composition of Fish Lenses

Multiple stressors related to changes in environmental conditions (such as water temperature, salinity, and natural and anthropogenic pollution) may cause biological responses of aquatic organisms that lead to significant variations in the biochemical reactions in their tissues and thereby change the concentrations of metabolites. We used a quantitative NMR-based metabolomic analysis of the fish lens for the evaluation of the influence of environmental factors on metabolic processes in aquatic animals. For this purpose, three species of freshwater fish-Perca fluviatilis, Rutilus rutilus lacustris, and Gymnocephalus cernua-were caught at approximately the same time at three locations in Siberia (Russia) that differed in levels of dissolved oxygen (LDO) and water purity, and the concentrations of 57 major metabolites in the fish lenses were determined. We found that the metabolomic profiles of the fish lenses strongly depended on the location. The obtained data demonstrated that two typical stressors for aquatic animals-a reduced LDO and anthropogenic water pollution-caused a largely similar metabolic response in the fish lenses that led to an increase in the concentrations of several amino acids and a decrease in sarcosine and phosphoethanolamine. At the same time, the composition of the major lens osmolytes depended mostly on the oxygen level, while variations in AMP (decrease) and NAD (increase) corresponded to the water pollution. We suggest that the eye lens is a very convenient tissue for studying the impact of ecological factors on the metabolic state of aquatic animals, fish in particular.

Effect of hypo- and hypersaline stress conditions on physiological, metabolic, and immune responses in the oyster Crassostrea corteziensis (Bivalvia: Ostreidae)

Salinity in the oceans is changing due to climate change and global warming. Intense rainfalls and freshwater runoff decrease salinity along the coastal areas. In contrast, intense drought seasons and river damming have certainly increased salinity in lagoons and estuaries. Few studies have focused on aspects of the biology and culture of oyster Crassostrea corteziensis, but until now, physiological and immunological responses in this species have not been assessed under acute hypo- and hypersaline stress conditions. Oysters obtained from a local farm were acclimated for three weeks in laboratory conditions. To avoid closure of oyster valves during salinity induced-stress conditions, a notch was done on each organism shell not only to facilitate oyster tissue exposure to rearing water but also for sampling hemolymph. Oysters (N = 180) were abruptly exposed to three salinity treatments: (HO) hypo-, (C) control, and (HP) hypersaline stress conditions (10, 35, and 50 PSU, respectively). Four oysters per treatment were sampled at 1, 2, 3, 6, 12, 24, and 48 h after exposure. Hemolymph osmolality, water content and total protein concentration in tissues, metabolic and immune responses were assessed for each organism. Oyster survival was not different among treatments and was maintained above 96% at the end of the experimental trial. Hemolymph osmolality reached the value of rearing water at 6 and 48 h of exposure to HP and HO stress conditions, where oysters exposed to salinity increase showed less resilience than those to decrease. Higher glucose levels in plasma and lower ones of hemocyanin were assessed in the oysters exposed to HP compared to HO conditions, suggesting more stressful conditions or susceptibility of oysters during salinity increase. Total hemocyte (THC), hyalinocyte (HC), and granulocyte (GC) counts decreased in oysters exposed to HP condition, while total and differential hemocyte counts were similar among oysters exposed to HO and control conditions. Despite hemocyte phagocytosis was not different among treatments, viability decreased in those exposed to HP condition. Contrastingly, superoxide anion (SOA) production (oxidative capacity) increased in oysters exposed to both induced salinity-stress conditions, which suggest susceptibility increase in oysters, particularly during salinity increase. The results show that HP condition is particularly stressful for C. corteziensis. In turn, this condition could increase both their vulnerability to other environmental stressors, such as temperature and/or acidification or susceptibility to opportunistic pathogenic microorganisms that cause the most common oyster diseases.

Metabolite profiling of the post-ovulatory oocytes of the common carp, Cyprinus carpio: A 1H NMR-based metabolomics approach

A metabolomics study was conducted to investigate the molecular bases of oocyte over-ripening in common carp, Cyprinus carpio from a metabolic point of view. The ovulation was induced in fish brooders by intramuscular injection of pituitary extract and oocytes were collected four times post-ovulation with 30 min intervals. A set of 32 metabolites were identified on the NMR spectra of the oocytes, which mainly included energy-linked metabolites, amino acids, methylated metabolites and citric acid cycle (TCA) intermediates. PCA and PLS-DA models clearly separated the post ovulations times, indicating the effects of post-ovulation time on oocyte metabolome content. Based on the loading plot outputs, 15 metabolites including tryptophan, cysteine, AMP, tyrosine, valine, creatine phosphate (PCr), ATP, leucine, inosine, malate, acetate, TMAO, glucose, fumarate and lysine had more effects on the separation of post ovulation times. According to the results of metabolite profiling, the concentrations of glutamine, alanine, tryptophan, lysine and cysteine mostly significantly (P < 0.01) increased at 90 and 120 min post-ovulation. The concentrations of PCr, ATP, inosine and guanosine were relatively stable until 60 min post-ovulation, while significantly (P < 0.01) decreased at 90 and 120 min post ovulation. The TCA metabolites succinate, malate and fumarate significantly (P < 0.01) elevated at 90 and 120 min post-ovulation. AMP concentrations remained relatively unchanged until 30 min and then progressively decreased with time post ovulation (P < 0.01). The concentrations of lactate showed significant elevations at 90 and 120 min post ovulation (P < 0.01). In conclusion, the energetic potentials of the oocytes reduced with time post ovulation. There were apparent elevations in the concentrations of free amino acids, which may be associated with the onset of proteolytic activities in the post ovulatory oocytes. In addition, we found some changes in the apoptotic-related metabolites, which may support the results of previous studies regarding the oxidative stress and following apoptosis in post ovulatory oocytes of fish.

Time-course changes in the regulation of ions and amino acids in the hard clam Meretrix lusoria upon lower salinity challenge

In this study, we examined ion and amino acid regulation in the gill and mantle of the hard clam Meretrix lusoria. We found that the osmolality and Na⁺ and Cl^- concentrations of hard clam hemolymph were significantly reduced after transferring clams from the salinity of their natural habitat [20‰ saltwater (SW)] to a lower salinity environment (10‰ SW). Specific activities of Na⁺ , K⁺ -ATPase (NKA), which provides the driving force for the secondary ion transport associated with cell osmoregulation in gills and mantles, were unaffected during the acclimation to lower salinity. In contrast, there was a significant decline in the contents of free amino acids (FAAs) in the gills and mantles of hard clams during lower salinity acclimation. Taurine was established to be the dominant FAA, the content of which is considerably higher than that of other FAAs in the hard clam. Following acclimation to the lower salinity environment, mRNA expression of the taurine transporter (TAUT), which plays a pivotal role in regulating intracellular taurine contents, was significantly upregulated in the gill and downregulated in the mantle of hard clams at different time points. However, the relative abundance of TAUT protein in the gill and mantle was significantly increased after transfer from 20‰ SW to 10‰ SW, which may reflect feedback regulation in response to reduced taurine contents in the gill and mantle of hard clams. Collectively, the findings of this study provide important insights on the dynamic processes of ion and amino acid regulation in the peripheral tissues of bivalves.

Nanopore long-read RNAseq reveals regulatory mechanisms of thermally variable reef environments promoting heat tolerance of scleractinian coral Pocillopora damicornis

Some scleractinian corals exhibit high thermal adaptability to climate changes, although the mechanism of their adaptation is unclear. This study investigated the adaptability of scleractinian coral Pocillopora damicornis to thermally variable reef environments by applying a nanopore-based RNA sequencing method to characterize different transcription responses that promote heat tolerance of P. damicornis. We identified 1414 novel genes and optimized 6256 mis-annotated loci. Based on full-length transcriptome data, we identified complex alternative polyadenylation and alternative splicing events, which can improve our understanding of the genome annotation and gene structures of P. damicornis. Furthermore, we constructed differentially expressed lncRNA-mRNA co-expression networks, which may play a crucial role in the P. damicornis thermal adaptive response. KEGG function enrichment analysis revealed that P. damicornis from the high-temperature pool had a lower metabolic rate than that from the low-temperature pool. We hypothesize that metabolic readjustment, in the form of a lower metabolic rate, positively correlated with increased heat tolerance in P. damicornis in thermally variable reef environments. Our study provides novel insights into lncRNAs that promote thermally tolerance of scleractinian corals in the thermally variable reef environment, suggesting potential mechanisms for their adaptation to global warming in the future.

Polyps of the Jellyfish Aurelia aurita Are Unaffected by Chronic Exposure to a Combination of Pesticides

Pesticides are a major contaminant in coastal waters and can cause adverse effects in marine invertebrates such as jellyfish. Most studies have investigated short-term responses of organisms to unrealistically high concentrations of pesticides; however, chronic exposure to persistent low concentrations, which are more likely to occur in the environment, are rarely analyzed. We tested the response of polyps of the moon jellyfish Aurelia aurita to environmental concentrations of the herbicide atrazine and the insecticide chlorpyrifos, individually and in combination, over 9 wk. We hypothesized that exposure to individual pesticides would reduce rates of asexual reproduction and alter polyps' metabolite profiles, and that the results would be more severe when polyps were exposed to the combined pesticides. Polyps survived and reproduced (through budding) in all treatments, and no differences among treatments were observed. Proton nuclear magnetic resonance spectroscopy revealed no difference in profiles of polar metabolites of polyps exposed to the individual or combined pesticides. Our results suggest that A. aurita polyps are unaffected by chronic exposure to atrazine and chlorpyrifos at concentrations recommended as being protective by current Australian water quality guidelines. Environ Toxicol Chem 2020;39:1685-1692. © 2020 SETAC.

Antioxidant defenses and metabolic responses of blue mussels (Mytilus edulis) exposed to various concentrations of erythromycin

Erythromycin, one of the most widely used macrolide antibiotics, has been detected in various aquatic environments, so erythromycin ecotoxicity should deserve more attention. In this study, blue mussels (Mytilus edulis) were exposed to erythromycin to explore its potential physiological toxicity. After 2d acute and 7d sub-acute exposure to erythromycin, blue mussel glutathione S-transferase (GST) and catalase (CAT) activities were determined with microplate methods and metabolic responses were analyzed using ¹H nuclear magnetic resonance (¹H NMR). The results revealed that GST was approximately 1.6 times higher in exposed mussels at 200 mg/L and higher concentrations. CAT was about 1.9 times higher in exposed mussels at 200 mg/L, indicating that erythromycin exposure led that blue mussels enhanced antioxidant responses. Low doses of erythromycin exposure had a relatively small impact on the metabolism, while high doses of erythromycin exposure (200 and 400 mg/L) disturbed metabolic balance. With the increase of erythromycin concentrations, the individual metabolic differences within the same treatment groups also increased. The significant increase in alanine, glutamate, taurine, glycine and betaine were observed after acute and subacute exposure. Betaine played an important role in protecting antioxidant enzyme activities through adjusting osmotic pressure. The metabolomic results also showed the modes of erythromycin acted on the energy metabolism, osmoregulation, nerve activities and amino acid metabolism. This study highlighted how metabolomics can provide a comprehensive picture of metabolic responses, although significant antioxidant and metabolic responses were observed at high exposure concentrations.

Marine Metabolomics: a Method for Nontargeted Measurement of Metabolites in Seawater by Gas Chromatography-Mass Spectrometry

Nontargeted approaches using metabolomics to analyze metabolites that occur in the oceans is less developed than those for terrestrial and limnic ecosystems. One of the challenges in marine metabolomics is that salt limits metabolite analysis in seawater to methods requiring salt removal. Building on previous sample preparation methods for metabolomics, we developed SeaMet, which overcomes the limitations of salt on metabolite detection. Considering that the oceans contain the largest dissolved organic matter pool on Earth, describing the marine metabolome using nontargeted approaches is critical for understanding the drivers behind element cycles, biotic interactions, ecosystem function, and atmospheric CO₂ storage. Our method complements both targeted marine metabolomic investigations as well as other “omics” (e.g., genomics, transcriptomics, and proteomics) approaches by providing an avenue for studying the chemical interaction between marine microbes and their habitats.

Microbial communities exchange molecules with their environment, which plays a major role in regulating global biogeochemical cycles and climate. While extracellular metabolites are commonly measured in terrestrial and limnic ecosystems, the presence of salt in marine habitats limits the nontargeted analyses of the ocean exometabolome using mass spectrometry (MS). Current methods require salt removal prior to sample measurements, which can alter the molecular composition of the metabolome and limit the types of compounds detected by MS. To overcome these limitations, we developed a gas chromatography MS (GC-MS) method that avoids sample altering during salt removal and that detects metabolites down to nanomolar concentrations from less than 1 ml of seawater. We applied our method (SeaMet) to explore marine metabolomes in vitro and in vivo. First, we measured the production and consumption of metabolites during the culture of a heterotrophic bacterium, Marinobacter adhaerens. Our approach revealed successional uptake of amino acids, while sugars were not consumed. These results show that exocellular metabolomics provides insights into nutrient uptake and energy conservation in marine microorganisms. We also applied SeaMet to explore the in situ metabolome of coral reef and mangrove sediment porewaters. Despite the fact that these ecosystems occur in nutrient-poor waters, we uncovered high concentrations of sugars and fatty acids, compounds predicted to play a key role for the abundant and diverse microbial communities in coral reef and mangrove sediments. Our data demonstrate that SeaMet advances marine metabolomics by enabling a nontargeted and quantitative analysis of marine metabolites, thus providing new insights into nutrient cycles in the oceans.

IMPORTANCE Nontargeted approaches using metabolomics to analyze metabolites that occur in the oceans is less developed than those for terrestrial and limnic ecosystems. One of the challenges in marine metabolomics is that salt limits metabolite analysis in seawater to methods requiring salt removal. Building on previous sample preparation methods for metabolomics, we developed SeaMet, which overcomes the limitations of salt on metabolite detection. Considering that the oceans contain the largest dissolved organic matter pool on Earth, describing the marine metabolome using nontargeted approaches is critical for understanding the drivers behind element cycles, biotic interactions, ecosystem function, and atmospheric CO₂ storage. Our method complements both targeted marine metabolomic investigations as well as other “omics” (e.g., genomics, transcriptomics, and proteomics) approaches by providing an avenue for studying the chemical interaction between marine microbes and their habitats.