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

Resilience against the impacts of climate change in an ecologically and economically significant native oyster

Climate change is acidifying and warming our oceans, at an unprecedented rate posing a challenge for marine invertebrates vital across the globe for ecological services and food security. Here we show it is possible for resilience to climate change in an ecologically and economically significant oyster without detrimental effects to the energy budget. We exposed 24 pair-mated genetically distinct families of the Sydney rock oyster, Saccostrea glomerata to ocean acidification and warming for 4w and measured their resilience. Resilience was identified as the capacity to defend their acid-base balance without a loss of energy available for Scope for Growth (SFG). Of the 24 families, 13 were better able to defend their acid-base balance while eight had no loss of energy availability with a positive SFG. This study has found oyster families with reslience against climate change without a loss of SFG, is an essential mitigation strategy, in a critical mollusc.

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.

Where have all the beads gone? Fate of microplastics in a closed exposure system and their effects on clearance rates in Mytilus spp

The fate of microplastic particles (MP) in exposure experiments is mostly unclear. We measured the recovery of polystyrene (PS) microbeads, which were applied in various concentrations from 0.07 to 47.47 beads/ml, from the different compartments of an experimental system with mussels (Mytilus spp.). At the end of the experiment, we detected a significant loss of MP indicating that the mussels were exposed to less particles than intended. If such a discrepancy remains un-recognized by the experimenter, observed effects are related to an inaccurate particle concentration. Additionally, we observed reduced clearance rates of the mussels in the presence of MP and the effect size increased with increasing particle concentration. This effect was more pronounced in mussels that had recently spawned than in mussels that still had mature gonads. This is a hint that effects of MP may depend on the reproductive status of an organism.

The influence of stochastic temperature fluctuations in shaping the physiological performance of the California mussel, Mytilus californianus

Climate change is forecasted to increase temperature variability and stochasticity. Most of our understanding of thermal physiology of intertidal organisms has come from laboratory experiments that acclimate organisms to submerged conditions and steady-state increases in temperatures. For organisms experiencing the ebb and flow of tides with unpredictable low tide aerial temperatures, the reliability of reported tolerances and thus predicted responses to climate change requires incorporation of environmental complexity into empirical studies. Using the mussel Mytilus californianus, our study examined how stochasticity of the thermal regime influences physiological performance. Mussels were acclimated to either submerged conditions or a tidal cycle that included either predictable, unpredictable or no thermal stress during daytime low tide. Physiological performance was measured through anaerobic metabolism, energy stores and cellular stress mechanisms just before low tide, and cardiac responses during a thermal ramp. Both air exposure and stochasticity of temperature change were important in determining thermal performance. Glycogen content was highest in the mussels from the unpredictable treatment, but there was no difference in the expression of heat shock proteins between thermal treatments, suggesting that mussels prioritise energy reserves to deal with unpredictable low tide conditions. Mussels exposed to fluctuating thermal regimes had lower gill anaerobic metabolism, which could reflect increased metabolic capacity. Our results suggest that while thermal magnitude plays an important role in shaping physiological performance, other key elements of the intertidal environment complexity such as stochasticity, thermal variability, and thermal history are also important considerations for determining how species will respond to climate warming.

Temperature but not ocean acidification affects energy metabolism and enzyme activities in the blue mussel, Mytilus edulis

In mosaic marine habitats, such as intertidal zones, ocean acidification (OA) is exacerbated by high variability of pH, temperature, and biological CO2 production. The nonlinear interactions among these drivers can be context-specific and their effect on organisms in these habitats remains largely unknown, warranting further investigation.We were particularly interested in Mytilus edulis (the blue mussel) from intertidal zones of the Gulf of Maine (GOM), USA, for this study. GOM is a hot spot of global climate change (average sea surface temperature (SST) increasing by >0.2°C/year) with >60% decline in mussel population over the past 40 years.Here, we utilize bioenergetic underpinnings to identify limits of stress tolerance in M. edulis from GOM exposed to warming and OA. We have measured whole-organism oxygen consumption rates and metabolic biomarkers in mussels exposed to control and elevated temperatures (10 vs. 15°C, respectively) and current and moderately elevated P CO2 levels (~400 vs. 800 µatm, respectively).Our study demonstrates that adult M. edulis from GOM are metabolically resilient to the moderate OA scenario but responsive to warming as seen in changes in metabolic rate, energy reserves (total lipids), metabolite profiles (glucose and osmolyte dimethyl amine), and enzyme activities (carbonic anhydrase and calcium ATPase).Our results are in agreement with recent literature that OA scenarios for the next 100-300 years do not affect this species, possibly as a consequence of maintaining its in vivo acid-base balance.

Energetic response of Atlantic surfclam Spisula solidissima to ocean acidification

In this study, we assessed the Atlantic surfclam (Spisula solidissima) energy budget under different ocean acidification conditions (OA). During 12 weeks, 126 individuals were maintained at three different ρCO₂ concentrations. Every two weeks, individuals were sampled for physiological measurements and scope for growth (SFG). In the high ρCO₂ treatment, clearance rate decreased and excretion rate increased relative to the low ρCO₂ treatment, resulting in reduced SFG. Moreover, oxygen:nitrogen (O:N) excretion ratio dropped, suggesting that a switch in metabolic strategy occurred. The medium ρCO₂ treatment had no significant effects upon SFG; however, metabolic loss increased, suggesting a rise in energy expenditure. In addition, a significant increase in food selection efficiency was observed in the medium treatment, which could be a compensatory reaction to the metabolic over-costs. Results showed that surfclams are particularly sensitive to OA; however, the different compensatory mechanisms observed indicate that they are capable of some temporary resilience.

Impacts of elevated pCO2 on Mediterranean mussel (Mytilus galloprovincialis): Metal bioaccumulation, physiological and cellular parameters

Ocean acidification alters physiology, acid-base balance and metabolic activity in marine animals. Near future elevated pCO₂ conditions could be expected to influence the bioaccumulation of metals, feeding rate and immune parameters in marine mussels. To better understand such impairments, a series of laboratory-controlled experiment was conducted by using a model marine mussel, Mytilus galloprovincialis. The mussels were exposed to three pH conditions according to the projected CO₂ emissions in the near future (one ambient: 8.10 and two reduced: 7.80 and 7.50). At first, the bioconcentration of Ag and Cd was studied in both juvenile (2.5 cm) and adult (5.1 cm) mussels by using a highly sensitive radiotracer method (^110mAg and ¹⁰⁹Cd). The uptake and depuration kinetics were followed 21 and 30 days, respectively. The biokinetic experiments demonstrated that the effect of ocean acidification on bioconcentration was metal-specific and size-specific. The uptake, depuration and tissue distribution of ^110mAg were not affected by elevated pCO₂ in both juvenile and adult mussels, whereas ¹⁰⁹Cd uptake significantly increased with decreasing pH in juveniles but not in adults. Regardless of pH, ^110mAg accumulated more efficiently in juvenile mussels than adult mussels. After executing the biokinetic experiment, the perturbation was sustained by using the same mussels and the same experimental set-up, which enabled us to determine filtration rate, haemocyte viability, lysosomal membrane stability, circulating cell-free nucleic acids (ccf-NAs) and protein (ccf-protein) levels. The filtration rate and haemocyte viability gradually decreased by increasing pCO₂ level, whereas the lysosomal membrane stability, ccf-NAs, and ccf-protein levels remained unchanged in the mussels exposed to elevated pCO₂ for eighty-two days. This study suggests that acidified seawater partially shift metal bioaccumulation, physiological and cellular parameters in the mussel Mytilus galloprovincialis.

Single and combined effects of the "Deadly trio" hypoxia, hypercapnia and warming on the cellular metabolism of the great scallop Pecten maximus

In the ocean the main climate drivers affecting marine organisms are warming, hypercapnia, and hypoxia. We investigated the acute effects of warming (W), warming plus hypercapnia (WHc, ~1800 μatm CO₂), warming plus hypoxia (WHo, ~12.1 kPa O₂), and a combined exposure of all three drivers (Deadly Trio, DT) on king scallops (Pecten maximus). All exposures started at 14 °C and temperature was increased by 2 °C once every 48 h until the lethal temperature was reached (28 °C). Gill samples were taken at 14 °C, 18 °C, 22 °C, and 26 °C and analyzed for their metabolic response by ¹H-nuclear magnetic resonance (NMR) spectroscopy. Scallops were most tolerant to WHc and most susceptible to oxygen reduction (WHo and DT). In particular under DT, scallops' mitochondrial energy metabolism was affected. Changes became apparent at 22 °C and 26 °C involving significant accumulation of glycogenic amino acids (e.g. glycine and valine) and anaerobic end-products (e.g. acetic acid and succinate). In line with these observations the LT₅₀ was lower under the exposure to DT (22.5 °C) than to W alone (~ 25 °C) indicating a narrowing of the thermal niche due to an imbalance between oxygen demand and supply.

Transgenerational biochemical effects of seawater acidification on the Manila clam (Ruditapes philippinarum)

Ocean acidification can negatively impact marine bivalves. Pivotal to projecting their fate is the ability to acclimate and adapt to shifts in seawater chemistry. Transgenerational plasticity enables marine bivalves to acclimate, yet the underlying mechanisms at different levels of biological organization remain poorly understood. Here, we performed a transgenerational experiment to understand biochemical responses of the Manila clam, Ruditapes philippinarum, following exposure to moderately reduced seawater pH (from 8.1 to 7.7). Activities of tissue calcification-relevant enzymes, such as carbonic anhydrase (CA), acid phosphatase (ACP) and alkaline phosphatase (ALP), energy-metabolizing enzymes, such as Na⁺/K⁺-ATPase (NKA) and Ca²⁺/Mg²⁺-ATPase (CMA), as well as tissue energy reserves (glycogen, lipid and protein) were assayed. With decreasing seawater pH, adult R. philippinarum exhibited significantly increased CA activity, and especially the clams with a history of transgenerational exposure displaying significantly higher CA activity than those spawned from parents exposed to ambient seawater pH. Yet, ACP and ALP activities remained unaffected. Transgenerational exposure to reduced seawater pH led to significant increases of NKA activity, while no transgenerational response of CMA activity was observed. Tissue glycogen and lipid contents were significantly depleted under acidified conditions regardless of transgenerational exposure. Yet, transgenerational alleviation in the net protein degradation was found. These findings suggest that our current understanding of transgenerational responses is still limited by the achievable time-window possible in the laboratory. While the energetic budget is lower under acidified conditions, there is no evidence of transgenerational recovery in term of energetic budget. Therefore, this work demonstrates that the critical basis of ocean acidification resilience can most likely be explained in energetic terms.

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

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

Ecological and functional consequences of coastal ocean acidification: Perspectives from the Baltic-Skagerrak System

Ocean temperatures are rising; species are shifting poleward, and pH is falling (ocean acidification, OA). We summarise current understanding of OA in the brackish Baltic-Skagerrak System, focussing on the direct, indirect and interactive effects of OA with other anthropogenic drivers on marine biogeochemistry, organisms and ecosystems. Substantial recent advances reveal a pattern of stronger responses (positive or negative) of species than ecosystems, more positive responses at lower trophic levels and strong indirect interactions in food-webs. Common emergent themes were as follows: OA drives planktonic systems toward the microbial loop, reducing energy transfer to zooplankton and fish; and nutrient/food availability ameliorates negative impacts of OA. We identify several key areas for further research, notably the need for OA-relevant biogeochemical and ecosystem models, and understanding the ecological and evolutionary capacity of Baltic-Skagerrak ecosystems to respond to OA and other anthropogenic drivers.

Deciphering carbon sources of mussel shell carbonate under experimental ocean acidification and warming

Ocean acidification and warming is widely reported to affect the ability of marine bivalves to calcify, but little is known about the underlying mechanisms. In particular, the response of their calcifying fluid carbonate chemistry to changing seawater carbonate chemistry remains poorly understood. The present study deciphers sources of the dissolved inorganic carbon (DIC) in the calcifying fluid of the blue mussel (Mytilus edulis) reared at two pH (8.1 and 7.7) and temperature (16 and 22 °C) levels for five weeks. Stable carbon isotopic ratios of seawater DIC, mussel soft tissues and shells were measured to determine the relative contribution of seawater DIC and metabolically generated carbon to the internal calcifying DIC pool. At pH 8.1, the percentage of seawater DIC synthesized into shell carbonate decreases slightly from 83.8% to 80.3% as temperature increases from 16 to 22 °C. Under acidified conditions, estimates of percent seawater DIC incorporation decreases clearly to 65.6% at 16 °C and to 62.3% at 22 °C, respectively. These findings indicate that ongoing ocean acidification and warming may interfere with the calcification physiology of M. edulis through interfering with its ability to efficiently extract seawater DIC to the calcifying front.

The effects of low seawater pH on energy storage and heat shock protein 70 expression in a bivalve Limecola balthica

Though biological consequences of CCS (Carbon Capture and Storage) implementation into the marine environment have received substantial research attention, the impact of potential CO₂ leakage on benthic infauna in the Baltic Sea remained poorly recognized. This study quantified medium-term (56-day laboratory exposure) effects of CO₂-induced seawater acidification (pH 7.7, 7.0 and 6.3) on energetic reserves and heat-shock protein HSP70 expression of adult bivalve Limecola balthica from the southern Baltic. While no clear impact was evident in the most acidic treatment (pH 6.3), moderate seawater hypercapnia (pH 7.0) induced elevated catabolism of high caloric reserves (carbohydrates including glycogen and lipids) in order to provide energy to cover enhanced metabolic requirements for acid-base regulation. Biochemical response did not involve, however, breakdown of proteins, suggesting that they were not utilized as metabolic substrates. As indicated also by subtle variations in the chaperone protein HSP70, the clams demonstrated high CO₂ tolerance, presumably through development of efficient defensive/compensatory mechanisms during their larval and/or ontogenic life stages.