Since the beginning of the Industrial Revolution, carbon dioxide (CO2) has been emitted into the atmosphere at rates unprecedented to Earth’s history. Nearly 30% of the anthropogenic CO2 in the atmosphere has been absorbed in surface waters of the ocean, pushing carbonate chemistry towards increased bicarbonate ions and hydrogen protons and decreased carbonate ions. Consequently, seawater pH has decreased from pre-Industrial Revolution levels of 8.2 to current levels of 8.1, and it is expected to continue to drop to 7.8 by the year 2100 if carbon emissions continue as predicted. The combination of these effects is referred to as ocean acidification. It is at the forefront of marine research as it poses a serious threat to several marine organisms and ecosystems. Ocean acidification has the most notable direct effect on calcifying organisms with calcium carbonate skeletons and shells, because fewer carbonate ions in the water column result in reduced calcification. Coral reefs are especially vulnerable to ocean acidification since reefs are composed of complex carbonate structures. Coral reefs have a high biodiversity; thus, not only will the corals themselves be affected by ocean acidification, but so will many of the animals that dwell in them. The primary objective of this thesis was to examine the effects of ocean acidification on demersal zooplankton that reside in coral reefs. Ocean acidification research on zooplankton has primarily been single- species experiments on calcifying species or generalist copepod species. Scaling-up to experiments examining ocean acidification effects on entire zooplankton communities is logistically difficult, thus the ability to predict community changes in zooplankton due to ocean acidification has been rather limited. However, a few locations around the world have submarine volcanic CO2 seeps that can be used as natural laboratories to study ecosystem effects of ocean acidification. Two CO2 seeps located in coral reefs in Papua New Guinea were used as windows into the future to examine the effects of ocean acidification on entire zooplankton communities while they live naturally in their environment. Over three expeditions to two CO2 seeps, nocturnal plankton were sampled with horizontal net tows and emergence traps. Additional experiments were also conducted, and collectively this work is summarized in chapters 2-5 as outlined below. Chapter 2 reports on the observed changes in zooplankton abundance and community composition between control and high-CO2 sites. Consistent results between seep sites and expeditions showed that zooplankton abundances were reduced three-fold under high-CO2 conditions. The abundance loss was partially attributed to habitat change within the coral reef, from more structurally complex corals in the control sites to a replacement of massive bouldering corals in the high-CO2 sites. The loss of structural complexity in the reef meant there were fewer hiding spaces for the zooplankton to seek refuge in during the day. All zooplankton taxa were reduced under high-CO2 conditions but to varying levels, suggesting that each taxon reacts differently to ocean acidification. Since each taxonomic group within the zooplankton communities was reduced to varying levels under ocean acidification, the copepod genus with the largest reduction in abundance was investigated in more detail. Labidocera spp. are pontellid copepods that are generally considered surface-dwellers and are not known to inhabit coral reefs. Therefore, as a preface to the ocean acidification study, the new discovery of these copepods living in coral reefs is first described (Chapter 3). Not only were they found to be residential to the reef, but Labidocera spp. living at the control reefs preferred to reside in coral rubble, macroalgae, and turf algae. Labidocera spp. were one of the most sensitive copepods to high-CO2 conditions and were reduced by nearly 70% in abundance, prompting a more detailed investigation about the effect of ocean acidification on their physiology and habitat preference (Chapter 4). Physiological parameters, e.g. size, feeding, and oocyte development, were unaffected by ocean acidification. Unlike the zooplankton community as a whole, the main cause for the abundance loss of Labidocera spp. was not a shift in the habitat because their preferred substrata were of equal percent coverage across high-CO2 and control sites. Instead, Labidocera spp. were no longer associated with any substrata type. Multiple direct and indirect effects of ocean acidification will act on each zooplankton taxa separately, and their collective response will contribute to the community response. The effects of ocean acidification on zooplankton communities were then scaled up to potential impacts on entire ecosystems. Zooplankton are the primary food source for corals, fish, and other zooplanktivores. The impacts of ocean acidification on zooplankton communities will have cascade effects on the food chain via the pathway of zooplanktivorous organisms. A case study on the stony coral Galaxea fascicularis explored the effects of ocean acidification on the ability of corals, which had lived their entire lives under high-CO2 conditions, to feed on zooplankton (Chapter 5). Under anthropogenic changes, whether it is from bleaching, high turbidity, or ocean acidification, some corals rely more on heterotrophy and consume more zooplankton. Contrary to expectation, this study showed that when given equal quantities of food particles these corals consumed less zooplankton under ocean acidification. Corals rely on heterotrophy for essential nutrients, like nitrogen and phosphorus, which they cannot otherwise obtain from autotrophy and their symbiotic zooxanthellae. In conclusion, my thesis shows that not only is there fewer zooplankton available to consume, but the existing zooplankton is consumed with lower capture rates under high CO2 conditions. Coral reefs in future oceans will likely have reduced zooplankton abundances as an indirect effect of ocean acidification, partially caused by a change in habitat from branching corals to more massive bouldering corals. Zooplankton abundances were reduced yet the community composition was unaffected by ocean acidification. All zooplankton taxa were reduced yet present under high-CO2 conditions suggesting that the zooplankton are at least able to survive under ocean acidification. Fewer zooplankton will be available to zooplanktivores, but the fatty acid content and nutritional value of the zooplankton as a food source is expected to be similar to current food. Together this is expected to negatively impact the entire coral reef ecosystem, with some coral species unable to consume zooplankton at normal rates. In an ecosystem already highly vulnerable to ocean acidification, coral reefs may be even more threatened if the very basis of their food webs is reduced.

Ocean acidification and warming have been shown to affect a wide range of marine organisms and impact assemblages and ecosystems. Many of the species experiencing negative biological effects provide valuable ecosystem services, yet it is unclear how these biological effects will affect ecosystem services provision. This thesis aimed to appraise the consequences of ocean acidification and warming on important shellfish species, from physiology to provision of ecosystem services, using a multidisciplinary approach. The responses to ocean acidification and warming of two ecologically and commercially important species of oysters – the native European Flat oyster Ostrea edulis, and the non-native Pacific oyster Magallana gigas – were assessed in laboratory mesocosms following long-term exposures to a range of scenarios predicted for 2050 and 2100. Oysters provide numerous ecosystem services, including improvement of water quality, reef formation, and food provision, but are at risks from ocean acidification and other stressors due to negative impacts occurring at multiple life-stages and threatening reef maintenance and functioning (Chapter 1). The physiology of adult oysters appeared susceptible to ocean acidification and warming, with evident sub-lethal effects (Chapter 2). Magallana gigas experienced a greater degree of stress than O. edulis, displaying increased Standard Metabolic Rate, reduced Clearance Rate, and poorer Condition Indices. Reductions in Clearance Rates of M. gigas are especially concerning and may have important ecological impacts by limiting their ability to improve water quality in the future. The physiological changes experienced by individual oysters held important implications for the functioning of the reefs through changes in predation resistance. Again, M. gigas appeared to undergo more pronounced changes than O. edulis, displaying increased muscle strength but weakened shell strength. These changes are expected to alter its susceptibility to predators and influence community level interactions. Both O. edulis and M. gigas also underwent important changes to their biochemical composition with trends for impoverished nutritional quality, which holds direct implications on the provision of sea food. In particular, M. gigas contained lower lipid, carbohydrate, and protein levels, but higher contaminant concentration (copper); this change holds concerns for both future food security and future food safety. It was apparent that the physiological stress experienced (Chapter 2), led to significant energy reallocation from somatic growth to metabolism by depleting energetic reserves (Chapter 4), at the detriment of its nutritional quality. No negative effects on the eating quality of M. gigas (appearance, aroma, texture, taste, and overall acceptability) were recorded following a short-term exposure to ocean acidification and warming (Chapter 5), which was considered positive for the aquaculture sector. In order to secure future food provision and economic revenue, the UK aquaculture industry might need to reconsider its management strategy in the future, and encourage the production and consumption of O. edulis, in addition to the already popular M. gigas. It is clear that the impacts of ocean acidification and warming on oysters are multifaceted and occurring at multiple scales and levels of organisation. The risks to oysters and oyster reefs appear species-specific; in the UK, introduced M. gigas may be more vulnerable than native O. edulis. To secure benefits and minimise costs related to the management of introduced species, these findings could be integrated into the current management and conservation measures in place for these species and the reefs they can form.

Hypoxia and ocean acidification (OA) are amongst the major environmental threats to marine ecosystems worldwide. Biofilms, the signpost to guide larval settlement of many benthic invertebrates, are known to be responsive to environmental changes and thus can become the crucial factor for the response of benthic invertebrate communities. This study aimed at investigating the individual and interactive effects of hypoxia and OA on biofilms and the subsequent effects on larval settlement. Biofilms collected from two sites (clean, hypoxic) were treated with a factorial design of low dissolved oxygen and/or low pH conditions in microcosms and the bacterial cell density and viability (by LIVE/DEAD® cell viability assays) were analyzed. Larval settlement preference was tested with the marine invertebrate, Crepidula onyx. The total bacterial cell densities of biofilms of the hypoxia and hypoxia and OA combination treatment were lower than that of the control biofilms for both sites. There was generally no significant difference in cell viability among control and different treatments for both sites. While the larval settlement rate on hypoxia and hypoxia and OA combination treated biofilms was significantly lower. In conclusion, this study revealed that hypoxia and OA are likely to affect larval settlement by alteration of biofilms, and this may lead to alterations in future coastal communities.

Ocean acidification may cause biodiversity loss, alter ecosystems and impact food security, yet uncertainty over ecological responses to ocean acidification remains considerable. Most work on the impact of ocean acidification on foraminifera has been short-term laboratory experiments on single species. To expand this, benthic foraminiferal assemblages were examined across shallow water CO2 gradients in the Gulf of California, off the islands of Ischia and Vulcano in Italy and off Papua New Guinea. Living assemblages from the Gulf of California did not appear to show a response across a pH range of 7.55 – 7.88, although the species assemblage was impoverished in all locations and the dead assemblage was less diverse at the lowest pH sites where there was evidence of post mortem dissolution. At Vulcano, the small macroalga, Padina pavonica, did not protect calcareous foraminifera from the adverse effects of ocean acidification. Calcareous taxa disappeared from the assemblage and were replaced by agglutinated foraminifera as mean pH reduced from 8.19 to 7.71. Settlement of benthic foraminifera onto artificial collectors off Vulcano was adversely affected in the acidified water, with few species as pCO2 increased and evidence of post-mortem dissolution. The foraminiferal tests, collected off Papua New Guinea, had lower δ11B as mean pH decreased from 7.99 – 7.82 for small (250 – 500 µm) Amphistegina lessonii, but not for A. lessonii or Calcarina spengleri >500 µm. In the larger foraminifera, photosynthetic activity by symbionts may begin to dominate the boron isotopic signature. Overall, the responses of foraminiferal assemblages to ocean acidification are complex, but there was an overall reduction in species diversity in infaunal, epifaunal and epiphytic assemblages as pCO2 increased. This raises serious concerns for the survival of shallow water calcareous benthic foraminifera as the oceans continue to acidify, with implications for benthic ecosystems and inorganic carbon cycling.

Elevated atmospheric concentrations of carbon dioxide (CO₂), partly driven by anthropogenic activity, are decreasing the pH of the oceans. This thesis aimed to assess the biological response of foraminifera to ocean acidification. Foraminifera are single-celled organisms that form the dominant component of many marine communities. A series of laboratory experiments were carried out on benthic intertidal foraminifera from the Eden and Ythan estuaries, NE Scotland, to assess the impacts of ocean acidification. The responses of two dominant intertidal species of foraminifera (Haynesina germanica and Elphidium williamsoni) to ocean acidification were initially investigated in a short-term (6 week) experiment. Multiple species and multiple stressors (seasonal temperature regime and elevated CO₂) were then incorporated in a long-term (18 month) mesocosm study to investigate the physiological consequences (e.g. survival, growth) of ocean acidification. Survival of both Haynesina germanica and Elphidium williamsoni was significantly reduced under low pH conditions. Live specimens of both these calcareous species were however recorded at low pH, in reduced numbers. Following long-term exposure to ocean acidification, foraminiferal populations were still dominated by calcareous forms. Agglutinated foraminifera were recorded throughout the long-term incubations but their numbers were not high enough in the initial sediment collections to allow them to contribute significantly to the populations. Overall, survival of all foraminifera was greatly reduced in elevated CO₂ treatments. Temperature effects were observed on foraminiferal survival and diversity with the largest CO₂ effects recorded under the seasonally varying temperature regime. Foraminiferal test damage for all live species was also highest under elevated CO₂ conditions. Test dissolution was particularly evident in Haynesina germanica with important morphological features, such as functional ornamentation, becoming reduced or completely absent under elevated CO₂ conditions. A reduction in functionally important ornamentation could lead to a reduction in feeding efficiency with consequent impacts on this organism's survival and fitness. In addition, changes in the relative abundance and activities of these important species could affect biological interactions (e.g. food web function) and habitat quality.

As climate change intensifies, the capacity of organisms to adapt to changing environments becomes increasingly relevant. Heat-induced coral bleaching –the breakdown of the symbiotic association between coral hosts and photosynthetic algae of the family Symbiodiniaceae– is rapidly degrading reefs worldwide. Hence, there is a growing interest to study symbioses that can persist in extreme conditions. The Red Sea is such a place, known as one of the hottest seas where healthy coral reef systems thrive. Here (Chapter 1), we tested the potential of symbiont manipulation as means to improve the thermal resilience of the cnidarian holobiont, particularly using heat tolerant symbiont species from the Red Sea. We used clonal lineages of the model system Aiptasia (host and symbiont), originating from different thermal environments to assess how interchanging either partner affected their short- and long-term performance under heat stress. Our findings revealed that symbioses are not only intra-specific but have also adapted to native, local environments, thus potentially limiting the acclimation capacity of symbiotic cnidarians to climate change. As such, infection with more heat resistant species, even if native, might not necessarily improve thermotolerance of the holobiont. We further investigated (Chapter 2) how environment-dependent specificity, in this case elevated temperature, affects the establishment of novel symbioses. That is, if Aiptasia hosts are, despite exhibiting a high degree of partner fidelity, capable of acquiring more thermotolerant symbionts under stress conditions. Thus, we examined the infection dynamics of multi-species symbioses under different thermal environments and assessed their performance to subsequent heat stress. We showed that temperature, more than host identity, plays a critical role in symbiont uptake and overall performance when heatchallenged. Additionally, we found that pre-exposure to high temperature plays a fundamental role in improving the response to thermal stress, yet, this can be heavily influenced by other factors like feeding. Like climate change, ocean acidification is a serious threat to corals. Yet, most research has focused on the host and little is known for the algal partner. Thus, here we studied (Chapter 3) the global transcriptomic response of an endosymbiotic dinoflagellate to long-term seawater acidification stress. Our results revealed that despite observing an enrichment of processes related to photosynthesis and carbon fixation, which might seem beneficial to the symbiont, low pH has a detrimental effect on its photo-physiology. Taken together, this dissertation provides valuable insights into the responses of symbiotic cnidarians to future climate and ocean changes.

Ocean acidification is predicted to lead to global oceanic decreases in pH of up to 0.3 units within the next 100 years. However, those levels are already being reached currently in coastal regions due to natural CO2 variability. Squid are a vital component of the pelagic ecosystem, holding a unique niche as a highly active predatory invertebrate and major prey stock for upper trophic levels. This study examined the effects of a range of ocean acidification regimes on the early life history of a coastal squid species, the Atlantic longfin squid, Doryteuthis pealeii. Eggs were raised in a flow-through ocean acidification system at CO2 levels ranging from ambient (400ppm) to 2200ppm. Time to hatching, hatching efficiency, and hatchling mantle lengths, yolk sac sizes, and statoliths were all examined to elucidate stress effects. Delays in hatching time of at least a day were seen at exposures above 1300ppm in all trials under controlled conditions. Mantle lengths were significantly reduced at exposures above 1300 ppm. Yolk sac sizes varied between CO2 treatments, but no distinct pattern emerged. Statoliths were increasingly porous and malformed as CO2 exposures increased, and were significantly reduced in surface area at exposures above 1300ppm. Doryteuthis pealeii appears to be able to withstand acidosis stress without major effects up to 1300ppm, but is strongly impacted past that threshold. Since yolk consumption did not vary among treatments, it appears that during its early life stages, D. pealeii reallocates its available energy budget away from somatic growth and system development in order to mitigate the stress of acidosis.

The uptake of CO2 by the ocean is causing major changes in its chemistry. These changes are likely to have detrimental effects on many organisms with a severe impact on calcifying species. With future OA, marine organisms will be submitted to hypercapnia (increased pCO2) and acidosis (decreased pH). Skeleton production and maintenance could be impacted due to the increased energetic cost to calcify in less favourable conditions and direct corrosive effect of undersaturated seawater resulting in dissolution of calcium-carbonate unprotected structures. Postmetamorphic echinoderms (juveniles and adults) endoskeleton is made of high magnesium-calcite, one of the most soluble form of CaCO3. Because of their low metabolism and their heavily calcified skeleton, echinoderms were designated as species particularly at risk under OA. However, the effects of OA on calcification and on skeleton maintenance vary among closely related taxa. Hypotheses explaining these contrasted tolerance to OA were stated: (1) regulation of the acid-base balance, which occurs in some echinoderms taxa and not others, plays a major role; (2) populations living in highly fluctuating habitats are adapted or selected, which may confer them a better resistance to acidified conditions. The goal of this thesis was to evaluate these hypotheses using a comparative approach in asteroids (two species) and regular euechinoids (five species). The chosen species differ by their ability to regulate their acid-base physiology and by the amplitude of fluctuations in their habitats. The impacts of OA on corrosion and mechanical properties of their skeletal elements as well as, in selected species, the expression of biomineralization-related genes were investigated. All samples were obtained from individuals exposed to acidified conditions during long-term aquarium experiments or in situ exposures (CO2 vents).Bending and compression mechanical tests analysed by Weibull statistics and expression of biomineralization-related genes appeared particularly unitive endpoints. On the contrary, occurrence of corrosion, i.e. observation in scanning electron microscopy, did not match with mechanical effects, and nanoindentation never revealed differences according to treatment. The results showed that species which were not able to regulate their acid-base physiology also presented the most affected skeleton integrity when submitted to OA. This was particularly true for the temperate sea star Asterias rubens and the Mediterranean sea urchin Arbacia lixula whose skeleton was significantly impacted. In the latter, this went together with a down-regulation of biomineralization-related genes. Temperate and tropical sea urchins that regulated their acid-base physiology (Paracentrotus lividus, Echinometra sp. B and sp. C) presented no or very limited impact of OA on their skeleton and biomineralization-related gene expression (P. lividus). The Antarctic species (the sea star Odontaster validus and the sea urchin Sterechinus neumayeri) showed no sign of acid-base physiology regulation but also no impact of acidified conditions on their skeleton. This could be linked to their particularly low metabolism or to food availability. Living in fluctuating habitats did not appear to confer a particular resistance of the skeleton in front of OA. In particular, the sea star A. rubens from Kiel Fjord, living in highly fluctuating sea water conditions, was impacted by acidified conditions they transiently encounter every year. The same was true for the temperate sea urchin A. lixula. So, it appears that sea stars and sea urchins living in fluctuating habitats might already be at the limit of their tolerance window if they do not regulate their acid-base physiology. In conclusion, it appears that the acid-base regulation may be the key biological trait to address the impact of OA on the skeleton of adult echinoderms. Studies coupling mechanical testing and analysing biomineralization-related gene expression should be extended to more taxa (within and outside echinoderms) to ascertain the relationship between OA sensitivity and absence of acid-base regulation. / Depuis la Révolution Industrielle, la concentration en dioxyde de carbone (CO2) atmosphérique augmente continuellement. Les océans absorbent en partie ce CO2, ce qui induit une diminution de la concentration en ions carbonate ainsi qu’une augmentation de la concentration en protons, processus connus sous le nom d’acidification des océans (AO). Ces changements sont susceptibles d'avoir des effets néfastes sur une variété d'organismes marins qui seront soumis à l’hypercapnie (augmentation de la pCO2) et à l’acidose (diminution du pH). De plus, la calcification fait l’objet d’une attention particulière étant donné la remontée des horizons de saturation en carbonate de calcium dans les océans. La production ainsi que le maintien du squelette pourraient être limités en raison de l'augmentation du coût énergétique de la calcification dans des conditions moins favorables. Cependant, les effets de l’AO sur la calcification et sur le maintien du squelette varient selon les taxons voire au sein d’un même taxon. Des hypothèses ont donc été émises quant aux mécanismes sous-tendant ces différences. Une meilleure résistance face à l’AO pourrait aller de pair avec (1) une capacité à réguler le pH des fluides extracellulaires, (2) une préadaptation due à l’occupation d’habitats fortement fluctuants.Les échinodermes post-métamorphiques (juvéniles et adultes), espèces marines clés, possèdent une biologie générale assez semblable et sont constitués d’un endosquelette composé de calcite hautement magnésienne, une des formes les plus solubles de carbonate de calcium. En raison de leur faible métabolisme et de leur squelette fortement calcifié, les échinodermes ont été désignés comme des espèces particulièrement vulnérables sous l’effet de l’AO. Toutefois, de récentes études montrent que le squelette de certaines espèces d’échinodermes au stade adulte n’est pas affecté lorsque les organismes sont soumis à de bas pH d’eau de mer. L'objectif principal de la présente thèse était d’évaluer les différentes hypothèses par une approche comparative chez les astéroïdes (deux espèces) et les euéchinoïdes réguliers (cinq espèces). Les espèces choisies se différencient par leur capacité à réguler leur physiologie acide-base et par l’amplitude des fluctuations de leur habitat. Les effets de l’AO sur la corrosion et les propriétés mécaniques de leurs éléments squelettiques, ainsi que, chez certaines espèces, l’expression de gènes liés à la biominéralisation ont été étudiés. Tous les échantillons ont été obtenus à partir d’individus exposés à des conditions acidifiées lors d’expériences à long terme ou d’expositions in situ (évents à CO2).Les résultats ont montré que les espèces qui n'étaient pas en mesure de réguler leur physiologie acide-base (l’étoile de mer Asterias rubens et l’oursin Arbacia lixula) présentaient également des squelettes plus affectés lorsqu'elles étaient soumises à l'AO. Les oursins tempérés et tropicaux qui régulent leur physiologie acide-base (Parancetrotus lividus, Echinometra spp.) n’ont présenté aucun impact ou un impact très limité de l’AO sur leur squelette et l’expression des gènes liés à la biominéralisation (P. lividus). Les espèces antarctiques (l’oursin Sterechinus neumayeri et l'étoile de mer Odontaster validus) n’ont montré aucun signe de régulation de la physiologie acide-base mais également aucun impact sur leur squelette dû à une diminution du pH de l'eau de mer. Cela pourrait être lié à leur métabolisme bas ou à la disponibilité de nourriture dans leur environnement. Vivre dans des habitats fluctuants ne semble pas conférer une résistance particulière du squelette face à l’AO. En particulier, l’étoile de mer A. rubens du Fjord de Kiel, qui vit dans des conditions très fluctuantes, a été affectée par des conditions d’acidification qu’elles rencontrent de manière transitoire chaque année. Il en va de même pour l’oursin tempéré A. lixula. Il semble donc que les étoiles de mer et les oursins vivant dans des habitats fluctuants pourraient déjà être à la limite de leur fenêtre de tolérance lorsqu’ils ne régulent pas leur physiologie acide-base. En conclusion, la régulation de la physiologie acide-base est une caractéristiques biologique clé pour adresser les effets de l’AO sur le squelette des échinodermes adultes. Les études couplant les tests mécaniques à l’analyse de l’expression de gènes liés à la biominéralisation devraient être étendues à plus de taxons (au sein et en dehors de échinodermes) afin de déterminer la relation entre la sensibilité de la calcification face à l’AO et l’absence de régulation acide-base. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Mississippi River affects the carbon dynamics in the northern Gulf of Mexico (N-GoM) significantly. Hence, total alkalinity (TA) algorithms developed for major ocean basins produce inaccurate estimations for this region. A TA algorithm was developed, which addresses the local effects of coastal processes and complex spatial influences. In-situ data collected during numerous previous research cruises in the N-GoM were compiled and used to calculate the efficiency of an existing TA algorithm that uses Sea-Surface-Temperature (SST) and Sea-Surface-Salinity (SSS) as explanatory variables. To improve this algorithm, statistical analyses were performed to improve the coefficients and functional form of this algorithm. Then, chlorophyll-a (Chla) was included as an additional explanatory variable. Chla worked as a proxy for addressing the organic carbon pump’s pronounced effects on coastal waters. Finally, a Geographically Weighted Regression (GWR) algorithm was developed to address spatial non-stationarity, which apparently could not be addressed in the previously developed global algorithm.

The current increase in the atmospheric CO2 concentration results in two major consequences in the marine environment: an increase of the sea surface temperature (0.7 °C since pre-industrial times) and a decreased seawater pH. This decrease is being measured continuously in different parts of the world and ranges from -0.0017 to -0.04 units per year according to the location considered. Based on CO2 emissions models provided by the IPCC, it was predicted that the average open ocean pH would decrease further by 0.4 units by 2100 and 0.8 by 2300 (corresponding to about a three-fold and six-fold increase of the proton concentration). Also, saturation states of seawater for the different forms of calcium carbonate, such as calcite, magnesium calcite and aragonite which are produced by calcifying marine organisms, are decreasing and consequently the saturation horizons of these minerals are shoaling. Today, some environments are characterized by pH values lower than the average open ocean pH. These are intertidal rock pools, upwelling zones, the deep-sea and CO2 vents. In these environments, pH is either constantly low or fluctuates. Those changes are either due to biological activity, geological CO2 leakage, or water masses movements. Within these environments, it has been hypothesized that organisms could be adapted or acclimatized to low pH values such as those predicted for the near-future. <p><p>Tolerance to ocean acidification in metazoans is linked to their acid-base regulation capacities when facing environmental hypercapnia (i.e. increased CO2 concentration in the surrounding environment). The latter may result in a hypercapnia of the internal fluids and a concomitant acidosis (i.e. reduced pH of the internal fluids due to the dissociation of CO2 in this case). Organisms have two buffer systems allowing the compensation of this acidosis: the CO2-bicarbonate and the non-bicarbonate buffers. Homeostasis of the internal fluids thanks to these systems is essential for the proper functioning of enzymes and processes. As hypometabolic calcifying osmoconformers, three of the characteristics conferring a relative vulnerability to ocean acidification, echinoderms are considered “at risk” for the near-future conditions. Nonetheless, post-metamorphic (juveniles and adults) echinoderms inhabit all environments showing naturally low pH. Furthermore, sea urchins which are highly calcified (compared to sea stars or sea cucumbers) are also found in these environments. This suggests that echinoderms have strategies to adapt or acclimate to low pH environments. Recent studies indicated that while sea urchins are able to regulate their coelomic (extracellular) fluid by accumulation of bicarbonate, sea stars seem to tolerate the acidosis linked to environmental hypercapnia. However, this information was obtained on a reduced number of species and significant interspecific differences were evidenced. Some taxa have not been investigated at all. Furthermore, several aspects of the acid-base physiology were unexplored, like the buffering capacity of the extracellular fluid and the origin of carbon within these fluids.<p><p>Accordingly, the goal of this study was to characterize the acid-base physiology in post-metamorphic echinoderms of different taxa in order to understand their response to ocean acidification.<p><p>The acid-base regulation capacities within the different echinoderm taxa were compared. A method was designed to measure the total alkalinity in small volumes (500 µl) of the main extracellular fluid (the coelomic fluid). This study showed that regular euechinoids have an increased buffer capacity in their coelomic fluid compared to seawater and the other echinoderm groups. In sea urchins, bicarbonate and non-bicarbonate buffers come into play, the former playing the major role. This buffer capacity was increased in fed individuals compared to fasted ones and increased further when seawater pH was lowered.<p><p>The acid-base regulation capacities of sea urchins from different taxa were investigated. Regular euechinoids possess an increased buffer capacity of the coelomic fluid allowing them to maintain a higher pH compared to cidaroids at current seawater pH. This pattern was found for temperate, tropical and Antarctic sea urchins. Data was also obtained for irregular echinoids which also showed a particularly low extracellular pH and a buffer capacity close to seawater like cidaroids. When exposed to reduced seawater pH (8.0, 7.7, and 7.4) for 4-6 weeks, regular euechinoids showed an increasing buffer capacity of the coelomic fluid accompanied by a homeostasis of the pH. On the contrary, cidaroids showed no changes in their acid-base status whatever the seawater pH (8.0 to 7.4). The origin of coelomic fluid carbon, investigated by stable carbon isotope analysis, also differs according to taxa. The δ13CDIC of regular euechinoids evidenced a mixing between CO2 from metabolic origin and that from the surrounding seawater. This is further supported by the correlation between the seawater signal of reduced pH conditions (modified by the addition of industrial gas, changing the δ13C to more negative values) and that of the coelomic fluid. On the other hand, cidaroids exhibit a signal reflecting principally metabolic CO2 (very negative δ13C), and the δ13C did not change under varying pH conditions (i.e. did not adapt to the seawater δ13CDIC signature). For irregular echinoids, the carbon origin is unclear as some species show signals close to that of regular euechinoids whereas others are similar to cidaroids.<p><p>The impact of acid-base regulation was investigated by testing the effect of ocean acidification on the mechanical properties of the skeleton (test plates) in the sea urchin Paracentrotus lividus. Individuals from intertidal pools, CO2 vents and a one year acidification experiment (pH 8.0, 7.9 and 7.7) were compared. Only the intertidal pool individuals showed a difference of the Young’s modulus and fracture forces of their plates. Sea urchins from the tide pool with the largest pH fluctuations showed a lower stiffness and strengthened test. On the contrary, sea urchins from CO2 vents and experimental acidification did not display any differences in the several mechanical properties tested. We suggest that the different food qualities (calcified vs. uncalcified algae) in the different tide pools significantly contributed to the observed difference.<p><p>The acid-base regulation ability of sea cucumbers was assessed in two species from contrasted habitats (mangrove intertidal vs. coral reef species). These organisms underwent acidosis of the coelomic fluid when exposed to reduced seawater pH for a short time (6 to 12 days). The δ13C signal of the coelomic fluid mirrored that of the surrounding seawater in all conditions, indicating that the CO2 accumulated (cause of the acidosis) comes also from the seawater. This is still unexplained to date. However, metabolic processes such as respiration and ammonium excretion rates were not affected. No difference was evidenced between the two species.<p><p>The results obtained in this study compiled with data from the literature indicate that post-metamorphic echinoderms have contrasted acid-base physiology with most regular euechinoids compensating the coelomic fluid pH by accumulation of bicarbonate ions (and possibly ophiuroids also), cidaroids and at least one regular euechinoid (Arbacia lixula) having a naturally low coelomic fluid pH which is not affected by acidification, and sea stars and sea cucumbers which do not compensate their coelomic fluid pH when submitted to acidified seawater. In regular euechinoids, negative effects are linked to resource allocation with growth usually being reduced in favor of acid-base regulation mechanisms. Starfish and sea cucumbers appear as resilient to acidification, with very few functions being negatively impacted. In conclusion, it seems that post-metamorphic echinoderms studied so far will not be particularly at risk when facing ocean acidification levels expected by 2100. Furthermore, tolerance to ocean acidification does not seem linked to the present day ambient pH regime. Nevertheless, more studies need to be carried out on brittle stars and sea cucumbers to confirm preliminary results, as well as crinoids which have not been investigated to date. Long-term exposure experiments to estimate energy budget changes as well as more assessments of evolutionary potential in echinoderms are crucially needed./L’augmentation actuelle de la concentration en CO2 atmosphérique a deux conséquences majeures dans l’environnement marin :une augmentation de la température des eaux de surface (0.7°C depuis l’époque préindustrielle) et une diminution du pH de l’eau de mer. Cette diminution est mesurée continuellement dans différentes régions du monde et varie de -0.0017 à -0.04 unités de pH par an en fonction du site considéré. Basé sur des modèles d’émissions de CO2 du GIEC, il a été prédit que le pH moyen de l’océan diminuerait encore de 0.4 unités d’ici 2100 et 0.8 d’ici 2300 (correspondant à une augmentation de la concentration en protons d’environ 3 fois et 6 fois). De même, les états de saturation de l’eau de mer vis-à-vis des différentes formes de carbonate de calcium, telles que la calcite, la calcite magnésienne et l’aragonite produites par les organismes calcifiants, sont en train de diminuer et par conséquent, les horizons de saturation remontent vers les eaux de surface. Aujourd’hui, certains environnements sont caractérisés par des valeurs de pH plus basses que celle de l’océan. Ceux-ci sont les mares intertidales, les zones d’upwelling, l’océan profond et les évents volcaniques. Dans ces environnements, le pH est soit constamment bas ou fluctue. Ces changements sont soit dû à une activité biologique, une fuite de CO2 géologique, ou au mouvement des masses d’eau. Dans ces environnements, il a été suggéré que les organismes pourraient être adaptés ou acclimatés à des valeurs basses de pH, telles que celles prédites pour le futur proche.<p> <p>La tolérance à l’acidification des océans chez les métazoaires est liée à leur capacité de régulation acide-base lorsqu’ils sont exposés à une hypercapnie environnementale (c’est-à-dire, une augmentation de la concentration en CO2 dans l’environnement entourant l’organisme). Ce phénomène peut résulter en une hypercapnie des liquides internes et une acidose concomitante (c’est-à-dire, un pH des liquides internes réduit dû à la dissociation du CO2 dans ce cas précis). Les organismes ont deux systèmes tampons leur permettant de compenser l’acidose :les tampons CO2-bicarbonate et non-bicarbonate. L’homéostasie des liquides internes grâce à ces systèmes est essentielle pour le fonctionnement correct des enzymes et processus. En tant qu’osmoconformes calcifiant hypométaboliques, trois caractéristiques menant à une certaine vulnérabilité face à l’acidification des océans, les échinodermes sont considérés « à risque » pour les conditions du futur proche. Cependant, les échinodermes post-métamorphiques (juvéniles et adultes) occupent tous les environnements montrant un pH faible naturellement. De plus, les oursins qui sont hautement calcifiés (par rapport aux étoiles de mer ou aux concombres de mer) sont également retrouvés dans ces environnements. Ceci suggère que les échinodermes ont des stratégies d’adaptation ou d’acclimatation à ces environnements à bas pH. Alors que des études récentes montrent que les oursins sont capables de réguler le pH du liquide cœlomique (extracellulaire) par l’accumulation de bicarbonates, les étoiles semblent tolérer l’acidose liée à l’hypercapnie environnementale. Néanmoins, ces informations ont été obtenues sur un petit nombre d’espèces et des différences interspécifiques significatives ont été mises en évidence. Certains taxa n’ont pas été étudié du tout. Par ailleurs, différents aspects de la physiologie acide-base sont inexplorés, tels que la capacité tampon du liquide extracellulaire et l’origine du carbone dans ces liquides.<p><p>Par conséquent, le but de cette étude était de caractériser la physiologie acide-base chez les échinodermes post-métamorphiques de différents taxa afin de comprendre leur réponse à l’acidification des océans.<p><p>Les capacités de régulation acide-base au sein des différents groupes d’échinodermes ont été comparées. Une méthode a été mise au point afin de mesurer l’alcalinité totale dans de petits volumes (500 µl) de liquide extracellulaire (le liquide cœlomique). Cette étude démontra que la capacité tampon du liquide cœlomique des euéchinoïdes réguliers est accrue comparée à celle de l’eau de mer ainsi que celle des autres groupes d’échinodermes. Dans les oursins, les tampons bicarbonate et non-bicarbonate entrent en jeux, le premier étant majoritaire. Cette capacité tampon est augmentée chez les individus nourris par rapport à ceux à jeuns et est augmentée plus encore lorsque le pH de l’eau de mer est diminué.<p><p>Les capacités de régulation acide-base ont été étudiées plus spécifiquement dans les différents groupes d’oursins. Les euéchinoïdes réguliers possèdent une capacité tampon accrue du liquide cœlomique leur permettant de maintenir un pH élevé comparé aux oursins cidaroïdes, au pH de l’eau de mer actuel. Ce patron se retrouve dans les oursins tempérés, tropicaux et antarctiques. Des données ont également été obtenues pour les oursins irréguliers qui ont également un pH extracellulaire particulièrement bas et une capacité tampon proche de celle de l’eau de mer comme les cidaroïdes. Lorsqu’ils sont exposés à un pH de l’eau de mer réduit (7.7 et 7.4 par rapport à 8.0) pour 4 à 6 semaines, les euéchinoïdes réguliers ont montré une augmentation de la capacité tampon du liquide cœlomique accompagnée d’une homéostasie du pH de ce liquide. A l’inverse, les cidaroïdes n’ont montré aucune modification de leur statut acide-base quel que soit le pH (8.0 à 7.4). L’origine du carbone du liquide cœlomique, étudié par analyse des isotopes stables du carbone, diffère également en fonction du groupe. Le δ13CDIC des euéchinoïdes réguliers met en évidence un mélange entre du CO2 d’origine métabolique et celui de l’eau environnante. Ceci est également démontré par la corrélation entre le signal de l’eau de mer dont le pH est réduit (modifié par l’ajout de CO2 industriel, changent le δ13C vers des valeurs plus négatives) et celui du liquide cœlomique. En revanche, les cidaroïdes montrent un signal reflétant principalement celui du CO2 métabolique (δ13C très négatif), et le δ13C n’est pas influencé par des conditions de pH variées (c’est-à-dire, qu’il ne s’adapte pas à la signature du δ13CDIC de l’eau de mer). Pour les oursins irréguliers, l’origine du carbone est incertaine puisque certaines espèces montrent un signal proche de celui des euéchinoïdes réguliers et d’autres similaire à celui des cidaroïdes.<p><p>L’impact de la régulation acide-base a été étudié en testant l’effet de l’acidification des océans sur les propriétés mécaniques du squelette (plaques squelettiques) de l’oursin Paracentrotus lividus. Des individus de mares intertidales, d’évents volcaniques et d’une expérience d’acidification d’un an (pH 8.0, 7.9 et 7.7) ont été comparés. Seuls les individus des mares intertidales montrèrent une différence pour le module de Young et la force des fractures des plaques. Les oursins venant de la mare intertidale montrant les plus grandes variations de pH avaient une rigidité plus faible et un squelette renforcé. A l’inverse, les oursins des évents volcaniques et de l’expérience d’acidification n’ont montrés aucune différence dans les diverses propriétés mécaniques étudiées. Nous suggérons que les variations en termes de qualité de nourriture (algues calcifiées vs. non-calcifiées) dans les différentes mares intertidales ont contribués de manière significative à la différence observée.<p><p>L’habilité des concombres de mer à réguler leur balance acide-base a été évaluée dans deux espèces d’habitats contrastés (espèce intertidale des mangroves vs. subtidale des récifs coralliens). Ces organismes ont subis une acidose du liquide cœlomique lorsqu’ils ont été exposés à un pH réduit de l’eau de mer pour une courte durée (6 à 12 jours). Le signal δ13C du liquide cœlomique reflétait celui de l’eau environnante dans toutes les conditions, indiquant que le CO2 accumulé (cause de l’acidose) venait de l’eau. Ceci est encore inexpliqué à l’heure actuelle. Cependant, les processus métaboliques tels que la respiration ou l’excrétion d’ammonium n’ont pas été affecté. Aucune différence n’a été observée entre les deux espèces.<p><p>Les résultats obtenus dans cette étude compilés avec ceux de la littérature indiquent que les échinodermes post-métamorphiques ont une physiologie acide-base contrastée avec la plupart des euéchinoïdes réguliers qui compensent le pH du liquide cœlomique par l’accumulation d’ions bicarbonates (et peut-être les ophiures aussi), les cidaroïdes et au moins un euéchinoïde régulier (Arbacia lixula) qui ont naturellement un pH du liquide cœlomique bas et qui ne sont pas affectés par l’acidification, et les étoiles de mer et les concombres de mers qui ne compensent pas le pH du liquide cœlomique lorsqu’ils sont soumis à une eau acidifiée. Chez les euéchinoïdes réguliers, des effets négatifs sont liés à un changement de l’allocation des ressources avec souvent un taux de croissance réduit en faveur des mécanismes de régulation acide-base. Les étoiles de mer et les concombres de mer apparaissent plus tolérants à l’acidification, avec peu de fonctions négativement impactées. En conclusion, il semble que les échinodermes post-métamorphiques étudiés jusqu’à présent ne seront pas particulièrement à risque lorsqu’ils seront exposés au niveau d’acidification attendu pour 2100. De plus, la tolérance à l’acidification des océans ne semble pas liée au régime de pH subit actuellement. Cependant, plus d’études doivent être menées sur les ophiures et les concombres de mer afin de confirmer les résultats préliminaires, ainsi que sur les crinoïdes qui n’ont à l’heure actuelle pas encore été étudiés. Des expériences à long terme afin d’estimer le budget énergétique des organismes ainsi que plus d’évaluations du potentiel d’évolution chez les échinodermes sont absolument nécessaires.<p> / Doctorat en Sciences / info:eu-repo/semantics/nonPublished