Impacts of acute hypoxia on the short-snouted seahorse metabolism and behaviour.

Seahorses are one of the most unique and enigmatic animals, recognized as flagship species for several conservation issues. Unfortunately, seahorses' populations have been declining and their unique lifestyle may constrain the ability of these animals to evolve in the future climate scenarios. They inhabit shallow coastal waters that display daily or seasonal environmental fluctuations. Yet, few studies have scrutinized the impacts of climate changes on these iconic species. Within this context, the objective of this work was to test the effects of an extreme hypoxia exposure (~27 % dissolved oxygen for approximately 7 h) on the metabolism, behaviour and food intake of the temperate seahorse Hippocampus hippocampus. Regarding metabolism, hypoxia exposure led to a significant reduction in metabolic rates and an increase in ventilation rates. Seahorses showed signs of movement lethargy under oxygen depletion. The results show that a small but extreme exposure to hypoxia is tolerable by seahorses despite inducing metabolic and behavioural changes, that may jeopardize the future development and survival of these iconic organisms.

Climate-Change Impacts on Cephalopods: A Meta-Analysis.

Aside from being one of the most fascinating groups of marine organisms, cephalopods play a major role in marine food webs, both as predators and as prey, while representing key living economic assets, namely for artisanal and subsistence fisheries worldwide. Recent research suggests that cephalopods are benefitting from ongoing environmental changes and the overfishing of certain fish stocks (i.e., of their predators and/or competitors), putting forward the hypothesis that this group may be one of the few "winners" of climate change. While many meta-analyses have demonstrated negative and overwhelming consequences of ocean warming (OW), acidification (OA), and their combination for a variety of marine taxa, such a comprehensive analysis is lacking for cephalopod molluscs. In this context, the existing literature was surveyed for peer-reviewed articles featuring the sustained (≥24 h) and controlled exposure of cephalopod species (Cephalopoda class) to these factors, applying a comparative framework of mixed-model meta-analyses (784 control-treatment comparisons, from 47 suitable articles). Impacts on a wide set of biological categories at the individual level (e.g., survival, metabolism, behavior, cell stress, growth) were evaluated and contrasted across different ecological attributes (i.e., taxonomic lineages, climates, and ontogenetic stages). Contrary to what is commonly assumed, OW arises as a clear threat to cephalopods, while OA exhibited more restricted impacts. In fact, OW impacts were ubiquitous across different stages of ontogeny, taxonomical lineages (i.e., octopuses, squids, and cuttlefish). These results challenge the assumption that cephalopods benefit from novel ocean conditions, revealing an overarching negative impact of OW in this group. Importantly, we also identify lingering literature gaps, showing that most studies to date focus on OW and early life stages of mainly temperate species. Our results raise the need to consolidate experimental efforts in a wider variety of taxa, climate regions, life stages, and other key environmental stressors, such as deoxygenation and hypoxia, to better understand how cephalopods will cope with future climate change.

Impacts of Deoxygenation and Hypoxia on Shark Embryos Anti-Predator Behavior and Oxidative Stress.

Climate change is leading to the loss of oxygen content in the oceans and endangering the survival of many marine species. Due to sea surface temperature warming and changing circulation, the ocean has become more stratified and is consequently losing its oxygen content. Oviparous elasmobranchs are particularly vulnerable as they lay their eggs in coastal and shallow areas, where they experience significant oscillations in oxygen levels. Here, we investigated the effects of deoxygenation (93% air saturation) and hypoxia (26% air saturation) during a short-term period (six days) on the anti-predator avoidance behavior and physiology (oxidative stress) of small-spotted catshark (Scyliorhinus canicula) embryos. Their survival rate decreased to 88% and 56% under deoxygenation and hypoxia, respectively. The tail beat rates were significantly enhanced in the embryos under hypoxia compared to those exposed to deoxygenation and control conditions, and the freeze response duration showed a significant opposite trend. Yet, at the physiological level, through the analyses of key biomarkers (SOD, CAT, GPx, and GST activities as well as HSP70, Ubiquitin, and MDA levels), we found no evidence of increased oxidative stress and cell damage under hypoxia. Thus, the present findings show that the projected end-of-the-century deoxygenation levels elicit neglectable biological effects on shark embryos. On the other hand, hypoxia causes a high embryo mortality rate. Additionally, hypoxia makes embryos more vulnerable to predators, because the increased tail beat frequency will enhance the release of chemical and physical cues that can be detected by predators. The shortening of the shark freeze response under hypoxia also makes the embryos more prone to predation.

Combined effects of climate change and BDE-209 dietary exposure on the behavioural response of the white seabream, Diplodus sargus.

Decabromodiphenyl-ether (BDE-209) is a persistent organic pollutant ubiquitously found in marine environments worldwide. Even though this emerging chemical contaminant is described as highly toxic, bioaccumulative and biomagnifiable, limited studies have addressed the ecotoxicological implications associated with its exposure in non-target marine organisms, particularly from a behavioural standpoint. Alongside, seawater acidification and warming have been intensifying their impacts on marine ecosystems over the years, compromising species welfare and survival. BDE-209 exposure as well as seawater acidification and warming are known to affect fish behaviour, but information regarding their interactive effects is not available. In this study, long-term effects of BDE-209 contamination, seawater acidification and warming were studied on different behavioural traits of Diplodus sargus juveniles. Our results showed that D. sargus exhibited a marked sensitivity in all the behaviour responses after dietary exposure to BDE-209. Fish exposed to BDE-209 alone revealed lower awareness of a risky situation, increased activity, less time spent within the shoal, and reversed lateralization when compared to fish from the Control treatment. However, when acidification and/or warming were added to the equation, behavioural patterns were overall altered. Fish exposed to acidification alone exhibited increased anxiety, being less active, spending more time within the shoal, while presenting a reversed lateralization. Finally, fish exposed to warming alone were more anxious and spent more time within the shoal compared to those of the Control treatment. These novel findings not only confirm the neurotoxicological attributes of brominated flame retardants (like BDE-209), but also highlight the relevance of accounting for the effects of abiotic variables (e.g. pH and seawater temperature) when investigating the impacts of environmental contaminants on marine life.

Corrigendum: The significance of cephalopod beaks as a research tool: An update.

[This corrects the article DOI: 10.3389/fphys.2022.1038064.].

Elemental and spectral chemometric analyses of Octopus vulgaris beaks as reliable markers of capture location.

The high demand and economic relevance of cephalopods make them prone to food fraud, including related to harvest location. Therefore, there is a growing need to develop tools to unequivocally confirm their capture location. Cephalopod beaks are nonedible, making this material ideal for traceability studies as it can also be removed without a loss of commodity economic value. Within this context, common octopus (Octopus vulgaris) specimens were captured in five fishing areas along the Portuguese coast. Untargeted multi-elemental total X-ray fluorescence analysis of the octopus beaks revealed a high abundance of Ca, Cl, K, Na, S, and P, concomitant with the keratin and calcium phosphate nature of the material. We tested a suite of discrimination models on both elemental and spectral data, where the elements contributing most to discriminate capture location were typically associated with diet (As), human-related pressures (Zn, Se, and Mn), or geological features (P, S, Mn, and Zn). Among the six different chemometrics approaches used to classify individuals to their capture location according to their beaks' element concentration, classification trees attained a classification accuracy of 76.7%, whilst reducing the number of explanatory variables for sample classification and highlighting variable importance for group discrimination. However, using X-ray spectral features of the octopus beaks further improved classification accuracy, with the highest classification of 87.3% found with partial least-squares discriminant analysis. Ultimately, element and spectral analyses of nonedible structures such as octopus beaks can provide an important, complementary, and easily accessible means to support seafood provenance and traceability, whilst integrating anthropogenic and/or geological gradients.

Can marine heatwaves affect the fatty acid composition and energy budget of the tropical fish Zebrasoma scopas?

Marine heatwaves (MHWs) are extreme weather events featuring abnormally high seawater temperature, and expected to increase in frequency, duration and severity over this century. The impacts of these phenomena on physiological performance of coral reef species require understanding. This study aimed to evaluate the effects of a simulated MHW (category IV; ΔT = +2 °C, 11 days) (after exposure and 10-day recovery period) on fatty acid (FA) composition (as a biochemical indicator) and energy budget (i.e., growth, G, excretion (faecal, F and nitrogenous losses, U), respiration, R and food consumption, C) of a juvenile tropical surgeonfish species (Zebrasoma scopas). Significant and different changes were found under MHW scenario for some of the most abundant FA and respective groups (i.e., an increase in the contents of 14:0, 18:1n-9, ΣMonounsaturated (ΣMUFA) and 18:2n-6; and a decrease in the levels of 16:0, ΣSaturated (ΣSFA), 18:1n-7, 22:5n-3 and ΣPolyunsaturated (ΣPUFA)). The contents of 16:0 and ΣSFA were also significantly lower after MHW exposure compared to control (CTRL). Additionally, lower feed efficiency (FE), relative growth rate (RGR) and specific growth rate in terms of wet weight (SGRw), as well as higher energy loss for respiration were observed under MHW exposure conditions in comparison with CTRL and MHW recovery period. The energy proportion channelled for faeces dominated the mode of energy allocation, followed by growth in both treatments (after exposure). After MHW recovery, this trend was reversed, and a higher percentage was spent for growth and a lower fraction for faeces than in the MHW exposure period. Overall, FA composition, growth rates and energy loss for respiration of Z. Scopas were the physiological parameters most influenced (mainly in a negative way) by an 11-day MHW event. The observed effects in this tropical species can be exacerbated with increasing intensity and frequency of these extreme events.

Gymnodinium catenatum Paralytic Shellfish Toxin Production and Photobiological Responses under Marine Heat Waves.

Marine heatwaves (MHWs) have doubled in frequency since the 1980s and are projected to be exacerbated during this century. MHWs have been shown to trigger harmful algal blooms (HABs), with severe consequences to marine life and human populations. Within this context, this study aims to understand, for the first time, how MHWs impact key biological and toxicological parameters of the paralytic shellfish toxin (PST) producer Gymnodinium catenatum, a dinoflagellate inhabiting temperate and tropical coastal waters. Two MHW were simulated-category I (i.e., peak: 19.9 °C) and category IV (i.e., peak: 24.1 °C)-relative to the estimated baseline in the western coast of Portugal (18.5 °C). No significant changes in abundance, size, and photosynthetic efficiency were observed among treatments. On the other hand, chain-formation was significantly reduced under category IV MHW, as was PSP toxicity and production of some PST compounds. Overall, this suggests that G. catenatum may have a high tolerance to MHWs. Nevertheless, some sublethal effects may have occurred since chain-formation was affected, suggesting that these growth conditions may be sub-optimal for this population. Our study suggests that the increase in frequency, intensity, and duration of MHWs may lead to reduced severity of G. catenatum blooms.

Lack of detrimental effects of ocean acidification and warming on proximate composition, fitness and energy budget of juvenile Senegalese sole (Solea senegalensis).

Rising levels of atmospheric carbon dioxide (CO2) are driving ocean warming and acidification, which may negatively affect the nutritional quality and physiological performance of commercially important fish species. Thus, this study aimed to evaluate the effects of ocean acidification (OA; ΔpH = -0.3 units equivalent to ΔpCO2 ~ +600 µatm) and warming (OW; ΔT = +4 °C) (and combined, OAW) on the proximate composition, fitness and energy budget of juvenile Senegalese sole (Solea senegalensis). After an exposure period of 75 days, growth (G), metabolism (R) and excretion (faecal, F and nitrogenous losses, U) were assessed to calculate the energy intake (C). Biometric and viscera weight data were also registered to determine animal fitness. Overall, the proximate composition and gross energy were not significantly affected by acidification and warming (alone or in combination). Weight gain, maximum and standard metabolic rates (MMR and SMR, respectively), aerobic scope (AS) and C were significantly higher in fish subjected to OA, OW and OAW than in CTR conditions. Furthermore, the highest relative growth rates (RGR), specific growth rates in terms of wet weight (SGRw) and protein (SGRp), as well as feed efficiencies (FE) occurred in fish submitted to OW and OAW. On the other hand, fish exposed to CTR conditions had significantly higher feed conversion ratio (FCR) and ammonia excretion rate (AER) than those exposed to simulated stressors. Regarding energy distribution, the highest fraction was generally allocated to growth (48-63 %), followed by excretion through faeces (36-51 %), respiration (approximately 1 %) and ammonia excretion (0.1-0.2 %) in all treatments. Therefore, ocean warming and acidification can trigger physiological responses in juvenile Senegalese sole, particularly in their energy budget, which can affect the energy flow and allocation of its population. However, and in general, this species seems highly resilient to these predicted ocean climate change stressors.

Impacts of Low Oxygen on Marine Life: Neglected, but a Crucial Priority for Research.

AbstractGlobal ocean O2 content has varied significantly across the eons, both shaping and being shaped by the evolutionary history of life on planet Earth. Indeed, past O2 fluctuations have been associated with major extinctions and the reorganization of marine biota. Moreover, its most recent iteration-now anthropogenically driven-represents one of the most prominent challenges for both marine ecosystems and human societies, with ocean deoxygenation being regarded as one of the main drivers of global biodiversity loss. Yet ocean deoxygenation has received far less attention than concurrent environmental variables of marine climate change, namely, ocean warming and acidification, particularly in the field of experimental marine ecology. Together with the lack of consistent criteria defining gradual and acute changes in O2 content, a general lack of multifactorial studies featuring all three drivers and their interactions prevents an adequate interpretation of the potential effects of extreme and gradual deoxygenation. We present a comprehensive overview of the interplay between O2 and marine life across space and time and discuss the current knowledge gaps and future steps for deoxygenation research. This work may also contribute to the ongoing call for an integrative perspective on the combined effects of these three drivers of change for marine organisms and ecosystems worldwide.

Integrative Approaches to Understanding Organismal Responses to Aquatic Deoxygenation.

AbstractOxygen bioavailability is declining in aquatic systems worldwide as a result of climate change and other anthropogenic stressors. For aquatic organisms, the consequences are poorly known but are likely to reflect both direct effects of declining oxygen bioavailability and interactions between oxygen and other stressors, including two-warming and acidification-that have received substantial attention in recent decades and that typically accompany oxygen changes. Drawing on the collected papers in this symposium volume ("An Oxygen Perspective on Climate Change"), we outline the causes and consequences of declining oxygen bioavailability. First, we discuss the scope of natural and predicted anthropogenic changes in aquatic oxygen levels. Although modern organisms are the result of long evolutionary histories during which they were exposed to natural oxygen regimes, anthropogenic change is now exposing them to more extreme conditions and novel combinations of low oxygen with other stressors. Second, we identify behavioral and physiological mechanisms that underlie the interactive effects of oxygen with other stressors, and we assess the range of potential organismal responses to oxygen limitation that occur across levels of biological organization and over multiple timescales. We argue that metabolism and energetics provide a powerful and unifying framework for understanding organism-oxygen interactions. Third, we conclude by outlining a set of approaches for maximizing the effectiveness of future work, including focusing on long-term experiments using biologically realistic variation in experimental factors and taking truly cross-disciplinary and integrative approaches to understanding and predicting future effects.

The significance of cephalopod beaks as a research tool: An update.

The use of cephalopod beaks in ecological and population dynamics studies has allowed major advances of our knowledge on the role of cephalopods in marine ecosystems in the last 60 years. Since the 1960's, with the pioneering research by Malcolm Clarke and colleagues, cephalopod beaks (also named jaws or mandibles) have been described to species level and their measurements have been shown to be related to cephalopod body size and mass, which permitted important information to be obtained on numerous biological and ecological aspects of cephalopods in marine ecosystems. In the last decade, a range of new techniques has been applied to cephalopod beaks, permitting new kinds of insight into cephalopod biology and ecology. The workshop on cephalopod beaks of the Cephalopod International Advisory Council Conference (Sesimbra, Portugal) in 2022 aimed to review the most recent scientific developments in this field and to identify future challenges, particularly in relation to taxonomy, age, growth, chemical composition (i.e., DNA, proteomics, stable isotopes, trace elements) and physical (i.e., structural) analyses. In terms of taxonomy, new techniques (e.g., 3D geometric morphometrics) for identifying cephalopods from their beaks are being developed with promising results, although the need for experts and reference collections of cephalopod beaks will continue. The use of beak microstructure for age and growth studies has been validated. Stable isotope analyses on beaks have proven to be an excellent technique to get valuable information on the ecology of cephalopods (namely habitat and trophic position). Trace element analyses is also possible using beaks, where concentrations are significantly lower than in other tissues (e.g., muscle, digestive gland, gills). Extracting DNA from beaks was only possible in one study so far. Protein analyses can also be made using cephalopod beaks. Future challenges in research using cephalopod beaks are also discussed.

Camouflage and Exploratory Avoidance of Newborn Cuttlefish under Warming and Acidification.

Ocean warming and acidification have been shown to elicit deleterious effects on cephalopod mollusks, especially during early ontogeny, albeit effects on behavior remain largely unexplored. This study aimed to evaluate, for the first time, the effect of end-of-the-century projected levels of ocean warming (W; + 3 °C) and acidification (A; 980 µatm pCO2) on Sepia officinalis hatchlings' exploratory behavior and ability to camouflage in different substrate complexities (sand and black and white gravel). Cuttlefish were recorded in open field tests, from which mobility and exploratory avoidance behavior data were obtained. Latency to camouflage was registered remotely, and pixel intensity of body planes and background gravel were extracted from photographs. Hatching success was lowered under A and W combined (AW; 72.7%) compared to control conditions (C; 98.8%). Motion-related behaviors were not affected by the treatments. AW delayed camouflage response in the gravel substrate compared to W alone. Moreover, cuttlefish exhibited a higher contrast and consequently a stronger disruptive pattern under W, with no changes in background matching. These findings suggest that, although climate change may elicit relevant physiological challenges to cuttlefish, camouflage and mobility of these mollusks are not undermined under the ocean of tomorrow.

Projecting Future Climate Change-Mediated Impacts in Three Paralytic Shellfish Toxins-Producing Dinoflagellate Species.

Toxin-producing microalgae present a significant environmental risk for ecosystems and human societies when they reach concentrations that affect other aquatic organisms or human health. Harmful algal blooms (HAB) have been linked to mass wildlife die-offs and human food poisoning episodes, and climate change has the potential to alter the frequency, magnitude, and geographical extent of such events. Thus, a framework of species distribution models (SDMs), employing MaxEnt modeling, was used to project changes in habitat suitability and distribution of three key paralytic shellfish toxin (PST)-producing dinoflagellate species (i.e., Alexandrium catenella, A. minutum, and Gymnodinium catenatum), up to 2050 and 2100, across four representative concentration pathway scenarios (RCP-2.6, 4.5, 6.0, and 8.5; CMIP5). Despite slightly different responses at the regional level, the global habitat suitability has decreased for all the species, leading to an overall contraction in their tropical and sub-tropical ranges, while considerable expansions are projected in higher latitudes, particularly in the Northern Hemisphere, suggesting poleward distributional shifts. Such trends were exacerbated with increasing RCP severity. Yet, further research is required, with a greater assemblage of environmental predictors and improved occurrence datasets, to gain a more holistic understanding of the potential impacts of climate change on PST-producing species.

Does Ocean Sunfish Mola spp. (Tetraodontiformes: Molidae) Represent a Risk for Tetrodotoxin Poisoning in the Portuguese Coast?

Tetrodotoxin (TTX) is a potent neurotoxin naturally occurring in terrestrial and marine organisms such as pufferfish. Due to the risk of TTX poisoning, fish of Tetraodontidae family and other puffer-related species must not be placed in the EU markets. This restriction applies to fish of the family Molidae even though no data on toxins' occurrence is available. In this study, the presence of TTX and its analogues was investigated in the main edible tissue (the white muscle) and the main xenobiotics storage organ (the liver) of ocean sunfish Mola spp. (n = 13) from the South Portuguese coast. HILIC-MS/MS analyses did not reveal TTX in the analyzed samples, suggesting an inexistent or very limited risk of TTX poisoning.

Effects of Hypoxia on Coral Photobiology and Oxidative Stress.

Global ocean oxygen (O2) content is decreasing as climate change drives declines in oxygen solubility, strengthened stratification of seawater masses, increased biological oxygen consumption and coastal eutrophication. Studies on the biological effects of nocturnal decreased oxygen concentrations (hypoxia) on coral reefs are very scarce. Coral reefs are fundamental for supporting one quarter of all marine species and essential for around 275 million people worldwide. This study investigates acute physiological and photobiological responses of a scleractinian coral (Acropora spp.) to overnight hypoxic conditions (<2 mg/L of O2). Bleaching was not detected, and visual and physical aspects of corals remained unchanged under hypoxic conditions. Most photobiological-related parameters also did not show significant changes between treatments. In addition to this, no significant differences between treatments were observed in the pigment composition. However, hypoxic conditions induced a significant decrease in coral de-epoxidation state of the xanthophyll cycle pigments and increase in DNA damage. Although the present findings suggest that Acropora spp. is resilient to some extent to short-term daily oxygen oscillations, long-term exposure to hypoxia, as predicted to occur with climate change, may still have deleterious effects on corals.

Impacts of ocean warming and acidification on the energy budget of three commercially important fish species.

A mechanistic model based on Dynamic Energy Budget (DEB) theory was developed to predict the combined effects of ocean warming, acidification and decreased food availability on growth and reproduction of three commercially important marine fish species: white seabream (Diplodus sargus), zebra seabream (Diplodus cervinus) and Senegalese sole (Solea senegalensis). Model simulations used a parameter set for each species, estimated by the Add-my-Pet method using data from laboratory experiments complemented with bibliographic sources. An acidification stress factor was added as a modifier of the somatic maintenance costs and estimated for each species to quantify the effect of a decrease in pH from 8.0 to 7.4 (white seabream) or 7.7 (zebra seabream and Senegalese sole). The model was used to project total length of individuals along their usual lifespan and number of eggs produced by an adult individual within one year, under different climate change scenarios for the end of the 21st century. For the Intergovernmental Panel on Climate Change SSP5-8.5, ocean warming led to higher growth rates during the first years of development, as well as an increase of 32-34% in egg production, for the three species. Ocean acidification contributed to reduced growth for white seabream and Senegalese sole and a small increase for zebra seabream, as well as a decrease in egg production of 48-52% and 14-33% for white seabream and Senegalese sole, respectively, and an increase of 4-5% for zebra seabream. The combined effect of ocean warming and acidification is strongly dependent on the decrease of food availability, which leads to significant reduction in growth and egg production. This is the first study to assess the combined effects of ocean warming and acidification using DEB models on fish, therefore, further research is needed for a better understanding of these climate change-related effects among different taxonomic groups and species.

Access to Cleaning Services Alters Fish Physiology Under Parasite Infection and Ocean Acidification.

Cleaning symbioses are key mutualistic interactions where cleaners remove ectoparasites and tissues from client fishes. Such interactions elicit beneficial effects on clients' ecophysiology, with cascading effects on fish diversity and abundance. Ocean acidification (OA), resulting from increasing CO2 concentrations, can affect the behavior of cleaner fishes making them less motivated to inspect their clients. This is especially important as gnathiid fish ectoparasites are tolerant to ocean acidification. Here, we investigated how access to cleaning services, performed by the cleaner wrasse Labroides dimidiatus, affect individual client's (damselfish, Pomacentrus amboinensis) aerobic metabolism in response to both experimental parasite infection and OA. Access to cleaning services was modulated using a long-term removal experiment where cleaner wrasses were consistently removed from patch reefs around Lizard Island (Australia) for 17 years or left undisturbed. Only damselfish with access to cleaning stations had a negative metabolic response to parasite infection (maximum metabolic rate-MO2Max; and both factorial and absolute aerobic scope). Moreover, after an acclimation period of 10 days to high CO2 (∼1,000 µatm CO2), the fish showed a decrease in factorial aerobic scope, being the lowest in fish without the access to cleaners. We propose that stronger positive selection for parasite tolerance might be present in reef fishes without the access to cleaners, but this might come at a cost, as readiness to deal with parasites can impact their response to other stressors, such as OA.

Ocean Acidification Alleviates Dwarf Eelgrass (Zostera noltii) Lipid Landscape Remodeling under Warming Stress.

Coastal seagrass meadows provide a variety of essential ecological and economic services, including nursery grounds, sediment stabilization, nutrient cycling, coastal protection, and blue carbon sequestration. However, these ecosystems are highly threatened by ongoing climatic change. This study was aimed to understand how the dwarf eelgrass Zostera noltii leaf lipid landscapes are altered under predicted ocean warming (+4 °C) and hypercapnic (ΔpH 0.4) conditions. Warming and hypercapnic conditions were found to induce a severe reduction in the leaf total fatty acid, though the combined treatment substantially alleviated this depletion. The lipid discrimination revealed a significant increase in the relative monogalactosyldiacylglycerol (MGDG) content in both hypercapnic and warming conditions, allied to plastidial membrane stabilization mechanisms. Hypercapnia also promoted enhanced phosphatidylglycerol (PG) leaf contents, a mechanism often associated with thylakoid reinvigoration. In addition to changing the proportion of storage, galacto- and phospholipids, the tested treatments also impacted the FA composition of all lipid classes, with warming exposure leading to decreases in polyunsaturated fatty acids (PUFAs); however, the combination of both stress conditions alleviated this effect. The observed galactolipid and phospholipid PUFA decreases are compatible with a homeoviscous adaptation, allowing for the maintenance of membrane stability by counteracting excessive membrane fluidity. Neutral lipid contents were substantially increased under warming conditions, especially in C18 fatty acids (C18), impairing their use as substrates for fatty acylated derivatives essential for maintaining the osmotic balance of cells. An analysis of the phospholipid and galactolipid fatty acid profiles as a whole revealed a higher degree of discrimination, highlighting the higher impact of warming and the proposed stress alleviation effect induced by increased water-dissolved CO2 availability. Still, it is essential to remember that the pace at which the ocean is warming can overcome the ameliorative capacity induced by higher CO2 availability, leaving seagrasses under severe heat stress beyond their lipid remodeling capacity.