The humble blue mussel (Mytilus edulis), a filter feeder clinging to rocks and docks, plays a vital role in coastal ecosystems and breathes life into the mussel farming industry. However, these bivalves face a growing threat: the increasing temperature of the ocean. As the world warms, marine heatwaves cause deadly spikes, and mussels struggle to survive.
A study published by scientists from the Center for Biological Research of the Northwest, the Université du Québec à Rimouski, and the Université de Bretagne Occidentale delves into the intricate mechanisms mussels employ to combat thermal stress, revealing a fascinating tale of resilience and adaptation.
Effects of marine heatwaves
Sudden increases in water temperature have caused massive mortalities of mussels in natural populations and mariculture systems, leaving behind empty shells. But how does this heat affect them? The culprit lies in the delicate balance of proteins within their cells. High temperatures disturb this balance, causing damage.
Mussels’ defense mechanisms
Mussels face immense thermal stress in their natural habitat, enduring tidal fluctuations that expose them to drastic temperature changes. But when these fluctuations become extreme, as during marine heatwaves, their survival is at stake. Massive die-offs have been documented, highlighting the urgency of understanding their survival mechanisms.
To counteract this, mussels deploy so-called heat shock proteins (HSP). Imagine HSP as repair teams, rushing to repair and stabilize proteins disorganized by heat. This vital defense mechanism helps mussels withstand stressful temperatures.
But the battle against high temperatures is multifaceted. Increased temperatures also speed up metabolic engines, generating harmful reactive oxygen species (ROS). These free radicals act as “cell vandals,” causing internal damage. To combat this, mussels produce antioxidant enzymes, which mitigate the effect of ROS. They eliminate and neutralize these harmful molecules, protecting the cell from oxidative stress.
Prostaglandins and chronic stress
Adding another layer of complexity is the involvement of prostaglandins (PG). These tiny molecules act as chemical messengers and regulate various functions, including the stress response. Interestingly, PG can either aid or hinder the fight against heat, depending on the situation. Some PG trigger apoptosis, controlled cell death, to eliminate severely damaged cells. Others stimulate the production of HSP and the activity of antioxidant enzymes, enhancing defenses.
However, prolonged thermal stress presents a different challenge. Continuously elevated PG levels, initially beneficial, can become counterproductive and even deadly. Therefore, mussels need to adjust their PG production under chronic stress.
To test this, scientists compared mussels subjected to acute thermal stress (a single peak of intense heat) and chronic stress (repeated cycles of heating and cooling). They analyzed the levels of PG produced through enzymatic and non-enzymatic pathways, along with the distribution of ARA in different cellular compartments. Additionally, they examined gill structure to understand how these changes affected mussel health.
Interestingly, PG profiles differed significantly between the two groups.
- PGE2, PGE3, and 13-HDHA: These PGs, synthesized from fatty acids like arachidonic acid (ARA), increased in the gills of mussels facing acute stress. This increase likely drove apoptosis of damaged cells and enhanced defenses against oxidative stress.
- Hemocyte infiltration: A higher number of immune cells were observed in the gills, suggesting an inflammatory response to counter the immediate threat.
- 3,14-dihydro-15-keto-PGE2: This non-enzymatic PG showed no significant differences between acute and chronic stress, implying its function could be independent of stress duration.
- PGF3α: This PG, associated with anti-inflammatory processes, was lower in mussels under chronic stress, suggesting negative regulation of inflammation as a possible adaptation.
- 8-isoprostane: This marker of oxidative stress was significantly higher in chronic stress, indicating continuous cell damage despite decreased inflammation.
- Gill filament width: Chronic stress leads to thinner gill filaments, which could affect oxygen absorption and respiration.
Secrets of mussel resilience
The study’s findings suggest that mussels adjust their PG production and cellular responses based on the nature of thermal stress. Acute stress triggers a rapid and aggressive defense with high levels of protective PGs and inflammation. Chronic stress, on the other hand, seems to involve a more balanced approach, potentially conserving resources while mitigating continuous damage.
Understanding how PGs and non-enzymatic PG-like compounds modulate the response to thermal stress is crucial. The study results may help us develop strategies to enhance mussel resilience in a warming world, perhaps through selective breeding of individuals with stronger stress responses or through dietary interventions that influence PG production.
“These findings suggest that acute and chronic stressors have distinct effects on lipid composition, prostaglandin levels, and gill filament morphology, highlighting potential adaptations or responses to different stress conditions,” the scientists conclude.
They also note that the study provides information on PG modulation in response to thermal stress in the blue mussel Mytilus edulis. “While there was a strong increase in the production of the proinflammatory prostaglandin PGE2 during acute thermal stress that was related to the activation of cytoprotective mechanisms, such as inflammation and the immune response, the levels of these decreased during chronic stress,” they concluded.
The study was funded by the Ressources Aquatiques Québec Research Network (Fonds de Recherche du Québec-Nature et Technologies).
Centro de Investigaciones Biológicas del Noroeste
Instituto Politécnico Nacional 195
Playa Palo de Santa Rita Sur
La Paz, Baja California Sur, Mexico
Duran-Encinas, Y., Tremblay, R., Genard, B., Rivera, C., Lora-Vilchis, M. C., Kraffe, E., & Palacios, E. (2023). Modulation of thermal stress response by prostaglandins in gills of the blue mussel Mytilus edulis. Aquaculture, 740478.