Effects of Ocean Acidification and Warming on the mitochondrial physiology of Atlantic cod

The Atlantic cod (Gadus morhua) is an economically important marine fish species exploited by both fishery and aquaculture, especially in the North Atlantic and Arctic oceans. Ongoing climate changes are happening faster in the high latitude oceans with a higher increase of temperature and a steeper decrease in water pH due to anthropogenic CO2 than in the temperate regions threatening the existence of the Atlantic cod in the areas of its maximum exploitation. In this study, we investigated the mitochondrial physiology of two life-stages of cod under the sea water temperatures and pCO2 conditions forecasted for the year 2100 in the North Atlantic (+ 5 °C, 1000 μatm CO2). In embryos, the metabolism during development showed to be sensitive to rising temperatures with a general increase in respiratory activity until 9 °C (5 °C over the natural range) and a drop in activity at 12 °C mainly caused by a dramatic decrease in Complex I activity, which was not compensated by Complex II. In the adults, already well known for their metabolic plasticity, mitochondria from liver and heart are not affected by either increasing temperature or pCO2. However, in heart mitochondria of animals that were reared under warm hypercapnia (10 °C + 1000 μatm CO2), we found OXPHOS to exploit already 100% of the ETS capacity. This suggests that a further increase in temperature or pCO2 might lead to a mismatch in the ATP demand/production and consequently decrease heart performances. The different mitochondrial plasticities of the two life-stages reflect the sensitivity range at population level and thus can provide a more realistic reading frame of the potential survival of the North Atlantic cod population under climate change

Mitochondrial acclimation potential to ocean acidification and warming of Polar cod (Boreogadus saida) and Atlantic cod (Gadus morhua)

Background: Ocean acidification and warming are happening fast in the Arctic but little is known about the effects of ocean acidification and warming on the physiological performance and survival of Arctic fish. Results: In this study we investigated the metabolic background of performance through analyses of cardiac mitochondrial function in response to control and elevated water temperatures and PCO2 of two gadoid fish species, Polar cod (Boreogadus saida), an endemic Arctic species, and Atlantic cod (Gadus morhua), which is a temperate to cold eurytherm and currently expanding into Arctic waters in the wake of ocean warming. We studied their responses to the above-mentioned drivers and their acclimation potential through analysing the cardiac mitochondrial function in permeabilised cardiac muscle fibres after 4 months of incubation at different temperatures (Polar cod: 0, 3, 6, 8 °C and Atlantic cod: 3, 8, 12, 16 °C), combined with exposure to present (400μatm) and year 2100 (1170μatm) levels of CO2. OXPHOS, proton leak and ATP production efficiency in Polar cod were similar in the groups acclimated at 400μatm and 1170μatm of CO2, while incubation at 8 °C evoked increased proton leak resulting in decreased ATP production efficiency and decreased Complex IV capacity. In contrast, OXPHOS of Atlantic cod increased with temperature without compromising the ATP production efficiency, whereas the combination of high temperature and high PCO2 depressed OXPHOS and ATP production efficiency. Conclusions: Polar cod mitochondrial efficiency decreased at 8 °C while Atlantic cod mitochondria were more resilient to elevated temperature; however, this resilience was constrained by high PCO2. In line with its lower habitat temperature and higher degree of stenothermy, Polar cod has a lower acclimation potential to warming than Atlantic cod

The impact of ocean warming and acidification on the behaviour of two co-occurring Gadid species, Boreogadus saida and Gadus morhua from Svalbard

Ocean acidification induces strong behavioural alterations in marine fish as a conse- quence of acid−base regulatory processes in response to increasing environmental CO2 partial pressure. While these changes have been investigated in tropical and temperate fish species, nothing is known about behavioural effects on polar species. In particular, fishes of the Arctic Ocean will experience much greater acidification and warming than temperate or tropical species. Also, possible interactions of ocean warming and acidification are still understudied. Here we analysed the combined effects of warming and acidification on behavioural patterns of 2 fish species co-occurring around Svalbard, viz. polar cod Boreogadus saida and Atlantic cod Gadus morhua. We found a significant temperature effect on the spontaneous activity of B. saida, but not of G. morhua. Environmental CO2 did not significantly influence activity of either species. In con- trast, behavioural laterality of B. saida was affected by CO2 but not by temperature. Behavioural laterality of G. morhua was not affected by temperature or CO2; however, in this species, a possi- ble temperature dependency of CO2 effects on relative laterality may have been missed due to sample size restrictions. This study indicates that fish in polar ecosystems may undergo some, albeit less intense, behavioural disturbances under ocean acidification and in combination with ocean warming than observed in tropical species. It further accentuates species-specific differ- ences in vulnerability

Impact of Ocean Acidification and Warming on the bioenergetics of developing eggs of Atlantic herring Clupea harengus

Atlantic herring (Clupea harengus) is a benthic spawner, therefore its eggs are prone to encounter different water conditions during embryonic development, with bottom waters often depleted of oxygen and enriched in CO2. Some Atlantic herring spawning grounds are predicted to be highly affected by ongoing Ocean Acidification and Warming with water temperature increasing by up to +3°C and CO2 levels reaching ca. 1000 μatm (RCP 8.5). Although many studies investigated the effects of high levels of CO2 on the embryonic development of Atlantic herring, little is known about the combination of temperature and ecologically relevant levels of CO2. In this study, we investigated the effects of Ocean Acidification and Warming on embryonic metabolic and developmental performance such as mitochondrial function, respiration, hatching success (HS) and growth in Atlantic herring from the Oslo Fjord, one of the spawning grounds predicted to be greatly affected by climate change. Fertilized eggs were incubated under combinations of two PCO2 conditions (400 μatm and 1100 μatm) and three temperatures (6, 10 and 14°C), which correspond to current and end-of-the-century conditions. We analysed HS, oxygen consumption (MO2) and mitochondrial function of embryos as well as larval length at hatch. The capacity of the electron transport system (ETS) increased with temperature, reaching a plateau at 14°C, where the contribution of Complex I to the ETS declined in favour of Complex II. This relative shift was coupled with a dramatic increase in MO2 at 14°C. HS was high under ambient spawning conditions (6–10°C), but decreased at 14°C and hatched larvae at this temperature were smaller. Elevated PCO2 increased larval malformations, indicating sub-lethal effects. These results indicate that energetic limitations due to thermally affected mitochondria and higher energy demand for maintenance occur at the expense of embryonic development and growth

Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession : First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations

Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes-summarized by the term ocean acidification (OA)-could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (similar to 380 mu atm pCO(2)), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (mu 760 mu atm pCO(2)). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a "long-term mesocosm" approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.Peer reviewe

Zellulärer Metabolismus verschiedener Lebensstadien mariner Knochenfische unter Ozeanversauerung und -erwärmung.

The anthropogenic emissions of greenhouse gases are causing an increase in atmospheric and oceanic temperatures. In the oceans, seawater temperature rises in parallel to the decrease in pH caused by the reaction of rising atmospheric CO2 with water. The combined phenomenon is known as Ocean Acidification and Warming (OAW). Rising temperatures and decreased water pH may induce adjustments in the energy budget of fish, requiring more energy for protein turnover and for ion and acid-base balance. Since most of these processes depend on the mitochondrial provision of ATP, this PhD project investigated if rising temperature and PCO2 affect the mitochondrial functioning with consequences for the energy budget and acclimation potential of the animals to climate changes. Mitochondria are specialised cellular organelles and their degree of specialisation may vary according to species and life-stage. To cover a broad range of this variability, this thesis analysed firstly the mitochondria of juvenile polar cod (Boreogadus saida) as polar fish and of juvenile Atlantic cod (Gadus morhua) from the Northeast Arctic population (NEAC) as temperate fish, which presently co- occur in the waters around Svalbard. Secondly, the mitochondria of juvenile NEAC were compared with the mitochondria of embryos of the same species (Øresund population) to assess the differences between life-stages. Lastly, mitochondria of Atlantic cod embryos were analysed together with the ones of Atlantic herring embryos (Clupea harengus) because of the different spawning behaviour of the two species (pelagic for Atlantic cod, benthic for herring). Polar cod and NEAC were acclimated for four months at combinations of temperature (polar cod: 0, 3, 6, 8°C; NEAC: 3, 8, 12, 16°C) and PCO2 (400 and 1170 μatm) at the end of which their cardiac mitochondrial respiration was tested. In addition, the lipid class composition in pooled cellular membranes and the capacity of a number of mitochondrial enzymes were analysed. Embryos of Atlantic cod and herring were incubated from fertilization to hatch at present and projected temperatures (Atlantic cod: 0, 3, 6, 9, 12°C; herring: 6, 10, 14°C) and PCO2 (400 and 1100 μatm). When the embryos reached the "50% eye pigmentation" developmental stage, whole-body mitochondrial functioning was assessed. Moreover, the hatching success, length at hatch and larval malformation rates were recorded. The mitochondrial parameters measured in all species were OXPHOS, proton leak, citrate synthase (CS) capacity and the capacity of the single components of the Electron Transport System (ETS) i.e., Complex I (CI), Complex II (CII) and Complex IV (CCO). Juvenile polar cod presented some stenothermal traits like the lack of adjustments of the membrane lipids, stable values of OXPHOS and ETS despite increasing temperatures and low values of CCO/ETS. The relation between OXPHOS and proton leak suggested an optimum temperature for ATP production in the 3-6°C range, while the proton leak increased dramatically at 8°C, which was not paired by OXPHOS, hence decreasing the ATP production and therefore the available energy in the cardiac cells. Since the heart plays a fundamental role in acclimation to temperature, the lower energy yield may be related to the higher mortality occurring at this temperature. Yet, polar cod mitochondria were not affected by elevated PCO2 besides the increase in CS activity, probably as compensatory response to overcome its inhibition. NEAC, on the other hand, displayed more eurythermal features like modifying the lipid components of the cellular membranes, high CCO/ETS and increasing OXPHOS and ETS with rising temperatures. Although proton leak also increased with temperature, the stable ATP production efficiency indicates the ability to control proton leak and ensure the required energy to the cellular processes in a broader range of temperatures. However, the cardiac mitochondria of NEAC were negatively impacted by incubation under elevated PCO2, especially in combination with the highest tested temperature (16°C). Individuals from that group presented lower OXPHOS, lower ETS and lower capacity of the ETS enzymes CI and CCO whereas the TCA cycle-related enzymes CS and CII were stimulated. Possibly, elevated PCO2 inhibited CS and CII which were up-regulated in order to compensate for the lower activity. If the compensation was just partial, the decrease in activity of the TCA cycle may have led to a decrease of the ETS activity and therefore of the OXPHOS capacity with negative consequences on the energy yield of the heart cells. In contrast to their juvenile conspecifics, Atlantic cod embryos possessed mitochondria with a narrower thermal window which were not sensitive to elevated PCO2. In fact, OXPHOS, ETS and CI increased with temperature until 9°C, where they reached a plateau, and CII presented the same capacity at control and high levels of CO2. However, the combination of high temperature (12°C) and elevated PCO2 exerted a negative effect at higher organizational levels, decreasing hatching success and hatchlings' length. Moreover, elevated PCO2 increased the larval malformation rates at all incubation temperatures. Similar trends were found in the embryonic mitochondria of Atlantic herring. In this species OXPHOS and CI increased with temperature until 10°C and then reached a plateau, while elevated PCO2 did not affect the mitochondrial functioning. While elevated PCO2, especially in combination with high temperatures, decreased the survival of Atlantic cod embryos, herring hatching success and hatchlings' size was only related to temperature, suggesting higher CO2-tolerance in this benthic spawner. In conclusion, with regard to the mitochondrial functioning, NEAC appeared more eurytherm and plastic in the range of temperatures projected for the waters around Svalbard at the end of the century. Despite their higher CO2- sensitivity they may outperform polar cod, displacing them or forcing them to retreat in the fjord bottom waters. The plasticity of juvenile NEAC was lowest at the embryonic level, where the thermal window was narrower and more susceptible to elevated PCO2, suggesting that embryos may be a bottle-neck for the population acclimation process. Moreover, tolerance to high CO2 may be related to the spawning behaviour, with benthic species being more tolerant than pelagic ones

Metabolic shifts in the Antarctic fish Notothenia rossii in response to rising temperature and PCO2

Introduction: Ongoing ocean warming and acidification increasingly affect marine ecosystems, in particular around the Antarctic Peninsula. Yet little is known about the capability of Antarctic notothenioid fish to cope with rising temperature in acidifying seawater. While the whole animal level is expected to be more sensitive towards hypercapnia and temperature, the basis of thermal tolerance is set at the cellular level, with a putative key role for mitochondria. This study therefore investigates the physiological responses of the Antarctic Notothenia rossii after long-term acclimation to increased temperatures (7°C) and elevated PCO2 (0.2 kPa CO2) at different levels of physiological organisation. Results: For an integrated picture, we analysed the acclimation capacities of N. rossii by measuring routine metabolic rate (RMR), mitochondrial capacities (state III respiration) as well as intra- and extracellular acid–base status during acute thermal challenges and after long-term acclimation to changing temperature and hypercapnia. RMR was partially compensated during warm- acclimation (decreased below the rate observed after acute warming), while elevated PCO2 had no effect on cold or warm acclimated RMR. Mitochondrial state III respiration was unaffected by temperature acclimation but depressed in cold and warm hypercapnia-acclimated fish. In both cold- and warm-exposed N. rossii, hypercapnia acclimation resulted in a shift of extracellular pH (pHe) towards more alkaline values. A similar overcompensation was visible in muscle intracellular pH (pHi). pHi in liver displayed a slight acidosis after warm normo- or hypercapnia acclimation, nevertheless, long-term exposure to higher PCO2 was compensated for by intracellular bicarbonate accumulation. Conclusion: The partial warm compensation in whole animal metabolic rate indicates beginning limitations in tissue oxygen supply after warm-acclimation of N. rossii. Compensatory mechanisms of the reduced mitochondrial capacities under chronic hypercapnia may include a new metabolic equilibrium to meet the elevated energy demand for acid–base regulation. New set points of acid–base regulation under hypercapnia, visible at the systemic and intracellular level, indicate that N. rossii can at least in part acclimate to ocean warming and acidification. It remains open whether the reduced capacities of mitochondrial energy metabolism are adaptive or would impair population fitness over longer timescales under chronically elevated temperature and PCO2