Description: Progressive ocean acidification due to anthropogenic CO2 emissions will alter marine ecosytem processes. Calcifying organisms might be particularly vulnerable to these alterations in the speciation of the marine carbonate system. While previous research efforts have mainly focused on external dissolution of shells in seawater under saturated with respect to calcium carbonate, the internal shell interface might be more vulnerable to acidification. In the case of the blue mussel Mytilus edulis, high body fluid pCO2 causes low pH and low carbonate concentrations in the extrapallial fluid, which is in direct contact with the inner shell surface. In order to test whether elevated seawater pCO2 impacts calcification and inner shell surface integrity we exposed Baltic M. edulis to four different seawater pCO2 (39, 142, 240, 405 Pa) and two food algae (310–350 cells mL−1 vs. 1600–2000 cells mL−1) concentrations for a period of seven weeks during winter (5°C). We found that low food algae concentrations and high pCO2 values each significantly decreased shell length growth. Internal shell surface corrosion of nacreous ( = aragonite) layers was documented via stereomicroscopy and SEM at the two highest pCO2 treatments in the high food group, while it was found in all treatments in the low food group. Both factors, food and pCO2, significantly influenced the magnitude of inner shell surface dissolution. Our findings illustrate for the first time that integrity of inner shell surfaces is tightly coupled to the animals' energy budget under conditions of CO2 stress. It is likely that under food limited conditions, energy is allocated to more vital processes (e.g. somatic mass maintenance) instead of shell conservation. It is evident from our results that mussels exert significant biological control over the structural integrity of their inner shell surfaces.

Description: Anthropogenic CO2 emission will lead to an increase in seawater pCO(2) of up to 80-100 Pa (800-1000 mu atm) within this century and to an acidification of the oceans. Green sea urchins (Strongylocentrotus droebachiensis) occurring in Kattegat experience seasonal hypercapnic and hypoxic conditions already today. Thus, anthropogenic CO2 emissions will add up to existing values and will lead to even higher pCO(2) values 〉200 Pa (〉2000 mu atm). To estimate the green sea urchins' potential to acclimate to acidified seawater, we calculated an energy budget and determined the extracellular acid base status of adult S. droebachiensis exposed to moderately (102-145 Pa, 1007-1431 mu atm) and highly (284-385 Pa, 2800-3800 mu atm) elevated seawater pCO(2) for 10 and 45 days. A 45-day exposure to elevated pCO(2) resulted in a shift in energy budgets, leading to reduced somatic and reproductive growth. Metabolic rates were not significantly affected, but ammonium excretion increased in response to elevated pCO(2). This led to decreased O:N ratios. These findings suggest that protein metabolism is possibly enhanced under elevated pCO(2) in order to support ion homeostasis by increasing net acid extrusion. The perivisceral coelomic fluid acid-base status revealed that S. droebachiensis is able to fully (intermediate pCO(2)) or partially (high pCO(2)) compensate extracellular pH (pH(e)) changes by accumulation of bicarbonate (maximum increases 2.5 mM), albeit at a slower rate than typically observed in other taxa (10-day duration for full pH(e) compensation). At intermediate pCO(2), sea urchins were able to maintain fully compensated pH(e) for 45 days. Sea urchins from the higher pCO(2) treatment could be divided into two groups following medium-term acclimation: one group of experimental animals (29%) contained remnants of food in their digestive system and maintained partially compensated pH(e) (+2.3 mM HCO3-), while the other group (71%) exhibited an empty digestive system and a severe metabolic acidosis (-0.5 pH units, -2.4 mM HCO3-). There was no difference in mortality between the three pCO(2) treatments. The results of this study suggest that S. droebachiensis occurring in the Kattegat might be pre-adapted to hypercapnia due to natural variability in pCO(2) in its habitat. We show for the first time that some echinoderm species can actively compensate extracellular pH. Seawater pCO(2) values of 〉200 Pa, which will occur in the Kattegat within this century during seasonal hypoxic events, can possibly only be endured for a short time period of a few weeks. Increases in anthropogenic CO2 emissions and leakages from potential sub-seabed CO2 storage (CCS) sites thus impose a threat to the ecologically and economically important species S. droebachiensis.

Description: CO2 emissions are leading to an acidification of the oceans. Predicting marine community vulnerability towards acidification is difficult, as adaptation processes cannot be accounted for in most experimental studies. Naturally CO2 enriched sites thus can serve as valuable proxies for future changes in community structure. Here we describe a natural analogue site in the Western Baltic Sea. Seawater pCO2 in Kiel Fjord is elevated for large parts of the year due to upwelling of CO2 rich waters. Peak pCO2 values of 〉230 Pa (〉2300 μatm) and pHNBS values of 〈7.5 are encountered during summer and autumn, average pCO2 values are ~70 Pa (~700 μatm). In contrast to previously described naturally CO2 enriched sites that have suggested a progressive displacement of calcifying auto- and heterotrophic species, the macrobenthic community in Kiel Fjord is dominated by calcifying invertebrates. We show that blue mussels from Kiel Fjord can maintain control rates of somatic and shell growth at a pCO2 of 142 Pa (1400 μatm, pHNBS = 7.7). Juvenile mussel recruitment peaks during the summer months, when high water pCO2 values of ~100 Pa (~1000 μatm) prevail. Our findings indicate that calcifying keystone species may be able to cope with surface ocean pHNBS values projected for the end of this century when food supply is sufficient. However, owing to non-linear synergistic effects of future acidification and upwelling of corrosive water, peak seawater pCO2 in Kiel Fjord and many other productive estuarine habitats could increase to values 〉400 Pa (〉4000 μatm). These changes will most likely affect calcification and recruitment, and increase external shell dissolution.

Description: Marine hypoxia has become one of the major concerns of the world, as oceanic dead zones continue expanding horizontally and vertically, a phenomenon primarily caused by global warming and anthropogenic eutrophication. As consequence, drastic changes in community structures, predator-prey relationships (i.e. uncoupling) and/or habitat compression are expected followed by severe impacts on food-webs, ecosystems and fisheries. Moreover, habitat compression is aggravated by the synergistic effects of climate change, as elevated temperature and PCO2 will narrow the habitat from above. The jumbo squid, Dosidicus gigas, undergoes diel vertical migrations into oxygen minimum zones (OMZs) off the Eastern Tropical Pacific, where he plays an important ecological role both as predator and prey. In fact, this species can easily remove more than 4 million tons of food per year from the pelagic food web and is an important component in the diets of birds, fishes, and mammals. Besides its ecological role, the jumbo squid also plays an important economically role being target of the world’s largest cephalopod fishing industry with around 14% of world’s total squid catch and landings estimated at 818,000 tons in 2006. However, the main problem that arises with hypoxia is a reduced gradient that drives O2 uptake via diffusion pathways. At some point, the critical O2 partial pressure (Pcrit), the reduced diffusion gradient cannot support the metabolic demand fully aerobically, and has to be supplemented by anaerobic pathways and/or compensated by a reduction in metabolic rate. Commonly, aquatic animals respond to hypoxia by first attempting to maintain O2 delivery, as aerobic metabolism is much more efficient, followed by conserving energy expenditure and reducing energy turn over and finally by enhancing energetic efficiency of those metabolic processes that remain and derive energy from anaerobic sources. A further problem that vertical migrators of OMZs have to face is the elevated production of radical oxygen species (ROS) during the reoxygenation phase while ascending, as non-neutralized ROS formation can damage biological macromolecules (i.e. lipids, proteins and DNA) resulting in severe functional alterations in cells and tissues. To determine the cost and benefits of such diel vertical migrations, I investigated biochemical and physiological mechanisms in juvenile D. gigas off the Gulf of California with a focus on ventilation, locomotion, metabolism and antioxidant defense. The respiratory regulation in D. gigas was unpredictably high and is mirrored in maximized oxygen extraction efficiencies (EO2) at early (EH, 〈 160 min, 1 kPa O2) and late hypoxia (LH, 〉 180 min, 1 kPa O2). EO2 at EH was maximum 82% and achieved via (1) deep-breathing mechanism with more powerful contractions and an enlarged inflation period, and (2) reduction in the relaxed mantle diameter to favor diffusion. At LH, EO2 was still 40%, despite all other ventilatory mechanisms were drastically reduced, probably by using the collar-flap system (uncoupling of locomotory and ventilatory mechanisms) and a further reduction in the relaxed mantle diameter. Moreover, the drastic change in locomotion between EH and LH (onset of lethargy) was accompanied by a switch in the energy source of anaerobic pathways. At EH, anaerobic energy equivalents (AEE) primarily arrived via rapid energy reserve depletion (ATP, phospho-L-arginine), and, under LH, was mainly obtained via fermentative pathways (mainly octopine). As octopine formation simultaneously creates protons, intracellular acidosis and acid-base disturbances under progressing hypoxia are expected, which might negatively impact squid’s energy household and expenditures from locomotion towards more important cellular processes (i.e. ion regulation). Energy reserve depletion might even trigger lethargic behavior to conserve energy and extend hypoxia residence time. At EH, in contrast, deep-breathing behavior enabled D. gigas to pass the same amount of water through the mantle cavity per period of time and thereby could maintain a stable ventilatory volume per min, which explains its high level of activity observed under such extreme conditions. Moreover, D. gigas suppressed its metabolism (45-60%) at severe hypoxia (below Pcrit), as the reduction in O2 consumption rate (70-80%) could not be compensated by an upregulation in anaerobic energy production (70%). Cephalopods primarily feed on proteins and their glycogen storage potential is low (〈 0.4% of body weight). Therefore anaerobic protein degradation came into focus as strategy in hypoxia tolerant species. Yet, total protein concentration in muscle tissue of D. gigas did not vary significantly under severe hypoxia, but the reduced protein expression of heat shock protein 90 (Hsp90) and α- actinin indicates that, at least under progressing hypoxia, jumbo squids might degrade specific muscle proteins anaerobically. Moreover, the lower α-actinin expression at LH might be related to a decreased protection via the Hsp90 chaperon machinery resulting in increased ubiquitination and subsequent degradation. Therefore, the ubiquitin-proteasome system seems to play an important role in hypoxia tolerance, but further investigations are necessary to discover its full potential and pathways. Antioxidant enzyme activities in D. gigas were generally low and in the range of other squid species, but malondialdehyde concentrations (indicative of cellular damage) did not significantly change between normoxic and hypoxic conditions, demonstrating an efficient antioxidant defense system. Moreover, superoxide dismutase and catalase activities were enhanced under normoxia that seem to constitute an integrated stress response at shallower depths by buffering increased ROS formation, and, in addition, might even be a strategy to cope with the reoxygenation/recovery process. Moreover, heat shock protein 70 concentration was significantly increased under severe hypoxia (1 kPa O2), which may constitute a preparation for the reoxygenation phase during squid’s upward migration. Accordingly, the present thesis demonstrates that D. gigas evolved a variety of adaptive mechanisms and strategies to cope with hypoxia and the imposed challenges of diel vertical migrations. D. gigas might even actively descent into OMZs to suppress metabolism and escape from high metabolic demands at surface waters. Especially the high O2 uptake capacity and respiratory regulation were surprising taking into account cephalopods physiological and anatomical restraints. Therefore, D. gigas seems well-adapted to hypoxic conditions and might even out-compete less hypoxia tolerant species under hypoxia expansion, but the synergistic impacts of climate change, in turn, might endanger its survival.

Description: Since the last two centuries anthropogenic C02 emissions arising from the combustion of fossil fuels have altered seawater chemistry far more rapidly than previously experienced in earth history. The rate and extent of this change are expected to affect marine organisms and entire ecosystems. Excess C0 2 diffuses from the atmosphere into ocean surface waters, resulting in elevated seawater C02 partial pressure, as weil as reduced [CO3 2-] and pH. These changes in carbonate system speciation have been demonstrated to especially impact calcifying organisms. The present study focuses on the effects of ocean acidification on adult specimens of the green sea urchin Strongylocentrotus droebachiensis, a keystone predator and grazer in ecosystems of the northern hemisphere. Laboratory experiments revealed that the potential of S. droebachiensis to cope with COrdriven ocean acidification is surprisingly high. The green sea urchin was able to fully compensate its extracellular pH by active accumulation of HC03- ions (3-4 mM) under exposure to 140 Pa. Thereby, accumulation of HC03- was facilitated by active ion regulation processes and not due to passive shell dissolution. Between the pC02 treatments 140 and 400 Pa, an ion exchange capacity limit was detected, beyond this the extracellular pH could no langer be achieved and declined by about 0.2 units. Simultaneously, in high pC02 treatments (400 Pa), extracellular bicarbonate concentration was maintained, resulting in a partial compensation of S. droebachiensis extracellular body fluid. Under long-term exposure ( 400 Pa), an upregulation of respiratory chain cytochrome oxidase in the podia of the green sea urchin could point at mitochondrial proliferation and a general elevation in aerobic metabolism, as active compensation of extracellular pH increases total energy demand. The suggested metabolic upregulation could potentially ameliorate some of the effects of increased acidity, but at similar feeding intake rates it might come at a substantial cost (e.g. decreased reproduction, slower growth) if sustained in the long-term. However, the unexpected high capacity to compensate near-future ocean acidification could be linked to an adaptation to the environmental stressor hypoxia, occurring periodically in the Western Baltic. Previous work on the biological consequences of COrdriven ocean acidification has suggested that calcification and metabolic processes in many invertebrates (e.g. molluscs, crustaceans and echinoderms) are compromised. This raises questions concerning the potentially broad range of sensitivities to changes in acid-base status amongst invertebrates, as well as concerning the underlying mechanistic origins. Further studies are needed to evaluate potential impacts on noncalcifiers, as well as the synergistic impacts of ocean acidification and global warming. Studies should also focus on the adaptive capability of marine organisms, knowledge that will be crucial to forecast how marine organisms and ecosystems will respond to proceeding world ocean acidification and warming.

Description: The impact of a realistic warming scenario on the metabolic physiology of early cephalopod (squid Loligo vulgaris and cuttlefish Sepia officinalis) life stages was investigated. During exposure to the warming conditions (19 °C for the western coast of Portugal in 2100), the increase in oxygen consumption rates throughout embryogenesis was much steeper in squid (28-fold increase) than in cuttlefish (11-fold increase). The elevated catabolic activity–accelerated oxygen depletion within egg capsules, which exacerbated metabolic suppression toward the end of embryogenesis. Squid late-stage embryos appear to be more impacted by warming via metabolic suppression than cuttlefish embryos. At all temperature scenarios, the transition from encapsulated embryos to planktonic paralarvae implied metabolic increments higher than 100 %. Contrary to the nektobenthic strategy of cuttlefish newborns, the planktonic squid paralarvae rely predominantly on pulsed jet locomotion that dramatically increases their energy requirements. In the future, hatchlings will require more food per unit body size and, thus, feeding intake success will be crucial, especially for squid with high metabolic rates and low levels of metabolic reserves.