Climate change and eutrophication increase the frequency and duration of heat waves (HWs) along with diurnal oxygen and pH fluctuations (FH) in coastal ecosystems. Mussels (Mytilus edulis) are marine ecosystem engineers commonly exposed to temperature, pH and oxygen fluctuations and an excellent model species for investigating the cumulative effects of these stressors on survival and physiological performance of keystone benthic organisms. Here, we evaluated whether a 3-week exposure to FH would alter mussels’ responses to a HW scenario. 
We examined the physiological responses including respiration (RR), clearance (CR) and nitrogen excretion (ER) rates, and byssus production, as well as biochemical indicators of energy status and oxidative stress in the gills of mussels exposed to well-oxygenated or FH conditions before and after 24h, 1 and 2 weeks of HW (1826°C). 
ER and RR increased transiently during the HW, and FH exposure prevented the return to baseline (ER) or slight decrease (RR) observed after 2w HW in normoxia. A parallel increase of the protein concentrations between 24h and 1w HW indicate that protein turnover is linked to the thermal acclimation. Decrease in the lipid peroxidation levels was found after 1w HW, indicating adjustments of redox balance after prolonged warming, although FH altered the antioxidant upregulation pattern. Mussels CR decreased, but cellular energy status remained stable during HW.
These results highlight that FH acclimation affects the physiological responses of M. edulis to HW and could reduce mussels’ resilience to the intense scenarios predicted for future Baltic Sea coasts.

Does fluctuating hypoxia and hypercapnia interact with the response of the bivalve Mytilus edulis to a heat wave scenario ?

Mussels from the Mytilus edulis species complex are euryhaline osmoconformers that adjust their internal osmolarity to match that of their surroundings. This adaptive strategy poses a significant challenge to the metabolic machinery of the mussels, including mitochondria. An earlier study showed that mussels from different populations achieve optimal mitochondrial respiration at salinities near their prevailing habitat salinity. Given the euryhaline biology of mussels, we hypothesised that osmotic optima for the mitochondrial function will show plasticity to salinity acclimation. To test this hypothesis, mussels from the southern Baltic Sea (Mytilus edulis x trossulus transition zone) were acclimated to either 6 (hypoosmotic) or 15 (control) salinity and the mitochondrial performance curve was generated by measuring respiration of isolated hepatopancreas mitochondria at a range of osmotic concentrations (230–915 mOsm/kg). We found that the optima of the oxidative phosphorylation (OXPHOS, indicative of ATP synthesis capacity) and mitochondrial coupling were shifted towards a lower osmolality under hypoosmotic conditions. Additionally, the OXPHOS respiration was consistently higher, while efflux of reactive oxygen species (ROS) and fractional electron leak were lower in mussels acclimated to salinity 6 compared to those at salinity 15. This response of mitochondria from hypoosmotically exposed mussels resembles that of mitochondria from north Baltic Sea mussels (M. trossulus) that were collected and acclimated at their natural salinity (6) in a previous study. Thus, it appears as though a salinity of 6 offers optimal conditions for mussel populations from the north and south Baltic Sea.

Salinity dependent mitochondrial respiration in mussels of the Mytilus edulis species complex

Mussels from the Mytilus edulis species complex experience a salinity gradient from the North Sea into the Baltic Proper ranging from 32 to 5 practical salinity units. As osmoconformers, mussels adjust their internal osmolarity to match that of their surroundings. This adaptive strategy poses a significant challenge to the metabolic machinery of the mussels, including their mitochondria. Thus, populations from different sites have adapted to reach their highest mitochondrial function at a salinity close to their respective habitat salinity. We hypothesised that this adaptation is accompanied by a change in accumulated metabolites. To test this hypothesis, mussels from three populations along the salinity gradient were assessed: north Baltic Sea (M. trossulus zone), south Baltic Sea (transition zone) and North Sea (M. edulis zone). Metabolites from gill tissue were measured via liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) and their abundances analysed using a principal component analysis (PCA) and pathway enrichment analysis (MetaboAnalyst 6.0). The measured metabolites separated the populations from one another either by osmoregulation or by energy metabolism. Additionally, the pathway enrichment analysis identified three pathways that differed between each population. γ-aminobutyric acid (GABA) was one of the main drivers of the population specific differences, along with amino acids that are directly or indirectly connected to the citrate cycle. The citrate cycle in turn feeds the mitochondrial electron transport system (ETS). Overall, our findings suggest population specific differences in drivers of the citrate cycle as well as differences in the utilisation of sources for the mitochondrial respiration.

Metabolite profiling reveals energetic and osmoregulatory differences in populations from three zones of the Mytilus edulis species complex distribution area

Energy metabolism (encompassing energy assimilation, conversion and utilization) plays a central role in all life processes and serves as a link between the organismal physiology, behavior, and ecology. Metabolic rates define the physiological and life-history performance of an organism and have direct implications for Darwinian fitness. Anthropogenic and natural stressors affect energy balance because the stress-induced disruption of homeostasis must be corrected to ensure the organism's survival. Pollutants can negatively affect different aspects of energy metabolism of an organism interfering with energy assimilation and conversion or increasing energy costs for basal maintenance. This can diminish the energy fluxes available for other fitness functions and negatively affect the organism’s growth, reproduction and ability to deal with additional stressors or diseases. In this talk, I will discuss the role of bioenergetics in toxicity of legacy and emerging pollutants (including trace metals, nanoparticles and pharmaceuticals) and the pollutant interactions with other abiotic stressors such as temperature, salinity or oxygen deficiency. I will address the mechanisms underlying the pollutant-induced bioenergetics disturbances and discuss the use of bioenergetics-based approaches in linking the molecular and cellular stress responses to the whole-organism fitness in coastal environments exposed to multiple stressors.

Bioenergetic aspects of pollutant toxicity and their role in determining the tolerance limits to multiple stressors in aquatic organisms

Mitochondria play a central role in ATP provisioning, redox and Ca2+ homeostasis and stress signaling of aerobic organisms. Animal mitochondria are extremely sensitive to fluctuating oxygen (O2) levels such as occur during tissue ischemia and/or environmental hypoxia. In hypoxia-sensitive organisms, hypoxia and especially post-hypoxic reoxygenation cause mitochondrial injury due to the elevated production of reactive oxygen species, Ca2+ overload and damage to the metabolic machinery; yet many hypoxia-tolerant species (including intertidal invertebrates) endure frequent hypoxia-reoxygenation cycles without apparent ill effects. The mechanisms of such exceptional mitochondrial robustness are not yet fully understood. I will discuss the mitochondrial responses to intermittent hypoxia in marine intertidal mollusks emphasizing the potentially adaptive functional and proteomic changes of the mitochondria and integration of the mitochondrial responses with cellular protection and stress response pathways (including antioxidant defense, autophagy and apoptosis) that might contribute to high tolerance to fluctuating O2 levels and support recovery in these organisms. I will also discuss the current knowledge gaps with an outlook to future studies needed to shed light on mitochondrial adaptations and evolution of metabolism in the environments with highly variable oxygen levels such as the intertidal zone.

Mitochondrial mechanisms of tolerance to fluctuating oxygen conditions: What can we learn from intertidal animals?

Phenotypic plasticity allows species to cope with fluctuations in their environment. It is thus seen as the main route by which species may adjust to rapidly changing environmental conditions. However, there are still many unanswered questions regarding the capacity it provides for adjusting to anthropogenic stressors. For instance, why are some species and some traits more plastic than others, why are some stressors more likely to induce a plastic response than others, how do the mechanisms underlying intergenerational plasticity compare to those operating within an individual and why is there interspecific variation in the plasticity rate (i.e. the rate at which the phenotype changes)? Of particular relevance in a changing world is the plastic phenotypic response referred to as acclimation. The capacity of animals to cope with heat waves is likely to depend on their acclimation capacity but also their rate of acclimation. Despite frequent tests of acclimation (particularly in relation to thermal challenges), understudied aspects of acclimation remain. For example, what physiological mechanisms underlie acclimation? How long do animals require to fully acclimate? And what limits acclimation capacity? We are interested in showcasing the dynamics of acclimation and underlying mechanisms that allow animals to acclimate to various environments, as well as the factors which may affect how they acclimate. The session will first cover general phenotypic plasticity, with a more specific focus on the dynamics and mechanisms of thermal acclimation in the second session.

**Session Organisers:** Dr. Zara-Louise Cowan, Dr. Anna H. Andreassen, Jeremy De Bonville, Dr. Tim Burton, Prof. Neil B. Metcalfe and Dr. Jose C. Noguera

**Invited Speakers:** Dr. Madison Earhart, Ivan Gomez-Mestre, Dr. Jennifer Hellmann and Dr. Alex Little

This session has kindly been sponsored by 

**American Physiological Society** 

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**PyroScience**

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A18 - Phenotypic Plasticity and Acclimation Mechanisms in a Changing World

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Animal Poster Session                                                                

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A16:  Adaptation or maladaption of coastal populations to anthropogenic and environmental stressors

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Presentations

Does fluctuating hypoxia and hypercapnia interact with the response of the bivalve Mytilus edulis to a heat wave scenario ?

Salinity dependent mitochondrial respiration in mussels of the Mytilus edulis species complex

Metabolite profiling reveals energetic and osmoregulatory differences in populations from three zones of the Mytilus edulis species complex distribution area

Bioenergetic aspects of pollutant toxicity and their role in determining the tolerance limits to multiple stressors in aquatic organisms

Mitochondrial mechanisms of tolerance to fluctuating oxygen conditions: What can we learn from intertidal animals?

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