Impact of elevated pCO2 on the ecophysiology of Mytilus edulis

Increasing atmospheric CO2 concentrations equilibrate with the surface water of the oceans and thereby increase seawater pCO2 and decrease [CO32-] and pH. This process of ocean acidification is expected to cause a drastic change of marine ecosystem composition and a decrease in calcification ability of various benthic invertebrates. The studied area, Kiel Fjord, is characterized by high pCO2 variability due to upwelling of O2 depleted and CO2 enriched bottom water. Within less than 50 years, eutrophication of the Baltic Sea has drastically increased the mean pCO2 in the fjord. The observed increase and also the rate of this acidification process is much higher than it is expected for the global ocean as a consequence of increasing atmospheric CO2 concentrations. In contrast to other areas subjected to elevated pCO2, calcifying invertebrates inhabit Kiel fjord and the benthic community is dominated by the blue mussel Mytilus edulis. Mussel larvae settle in the period of the year when highest pCO2 (800-2300 µatm) are encountered, which is, at the same time, the main growth period due to highest phytoplankton densities. In laboratory experiments, calcification rates of M. edulis are maintained at elevated pCO2 levels which are expected to occur by the year 2300. Only at high pCO2 above 3000 µatm, calcification is significantly reduced. One possible reason for this tolerance is the fact that even under control conditions, the extracellular body fluids (haemolymph and extrapallial fluid, EPF) of M. edulis are characterized by low pH and [CO32-] and high pCO2. Therefore, the EPF which is in direct contact with the shell is undersaturated with calcium carbonate also at current, low seawater pCO2. Under elevated pCO2, mussels do not buffer the extracellular acidosis by means of bicarbonate accumulation. Thus haemolymph pH and [CO32-] are reduced even further. Calcification might not be affected by the extracellular acidosis, as an amorphous calcium carbonate (ACC) precursor is most probably formed in intracellular vesicles. Since mussels are able to efficiently regulate the intracellular pH, reduced extracellular pH might therefore have only little impact on the initial calcification process. On the other hand, the production of the organic shell components, e.g. the periostracum, consumes high amounts of energy. Especially in young thin shelled life stages with a higher organic shell content most of the energy allocated to growth is required for shell production. Under elevated pCO2, mussels initially (two months acclimation) up - regulate their metabolic rates which may indicate higher energy demand for ion regulatory processes. Long-term acclimated animals (12 months acclimation) probably switch to an energetically less expensive compensation and do not exhibit elevated aerobic metabolism. However, long-term acclimated mussels are characterized by lower filtration rates. As consequence, after both intermediate and long-term exposure, the scope for growth is reduced in high pCO2 acclimated animals. Additionally, after intermediate and also long-term acclimation to elevated pCO2, protein metabolism is increased, as indicated by an elevation of ammonia excretion rates. This mode of energy generation is less efficient than oxidation of lipid or carbohydrate and may contribute to lower energy availability for growth and calcification. Similar to other aquatic animals, ammonia excretion in mussels seems to be facilitated by NH3 diffusion through Rhesus (Rh) and ammonium transporter (Amt) protein channels and subsequent acid-trapping by separate proton excretion. In order to test the importance of energy supply and elevated pCO2 on mussel calcification, juvenile M. edulis were exposed to a crossed experimental design for seven weeks. Higher food supply enables mussels to calcify also under highly elevated pCO2. In general food supply is the most important factor which determines the growth rates of mussels whereas pCO2 has only a minor effect. In a simultaneous field study, mussels were transplanted to the energy rich high pCO2 inner fjord and to the outer parts of the fjord at lower pCO2 and particulated organic carbon concentrations. Similar to the laboratory experiment, mussels exhibit much higher growth rates in the high pCO2 inner fjord with its higher particulate organic carbon concentrations. This reveals the importance of energy availability impacting CO2 tolerance of M. edulis. Mussels seem to be relatively tolerant to elevated pCO2 both in laboratory experiments and under current high pCO2 conditions in Kiel Fjord. The high energy availability present in the eutrophicated habitat may support the tolerance to elevated pCO2. In the future, increasing atmospheric CO2 concentrations will drastically elevate pCO2 level in this habitat. The benthic life stages seem to be able to cope with the expected levels but plantonic larvae might be vulnerable. However, M. edulis exhibit a high adaptation potential to the rate of acidification in the recent past and might be able to adapt also to higher levels in future. In order to predict the success of M. edulis in future, also effects of elevated temperature and the response of their main predators to these conditions needs to be considered

The influence of increased pCO2 on the calcification of Mytilus edulis

One of the most important and abundant calcifying organisms in several marine ecosystems is the blue mussel, Mytilus edulis. It has a wide geographic distribution (Gosling 1992 Developm. Aquacult. Fish. Sci. 25, 1-20) and tolerates a broad range of environmental conditions (Seed and Suchanek 1992 Developm. Aquacult. Fish. Sci. 25, 87-170). Blue mussel beds are also common features in the Kiel Fjord (Baltic Sea), a habitat dominated by low salinity (10-20 PSU), low alkalinity (1900-2150 μmol kg-1), low pH (minimum values < 7.5) and high pCO2 (maximum value of 2340 ppm). The resulting calcium carbonate saturation state (min. values: Ωarag = 0.34 and Ωcalc = 0.58) is significantly lower than in the open ocean (Thomsen et al. submitted). Therefore, pCO2 in Kiel Fjord during summer is already higher than what is predicted for the future (e.g., Caldeira and Wickett 2003 Nature 425, 365). Additionally, Meier (2006 Clim. Dyn. 27, 39-68) projected an increase of temperature (2.6 to 5.0 °C) in the next 100 years for the Baltic Sea. To contribute to the understanding of the ability of calcifying organisms to live under ocean acidification conditions and of biomineralization mechanisms, M. edulis from this naturally CO2-enriched habitat were cultured in a flow-through system. Experiments were conducted using CO2 concentrations ranging from 380 ppm to 4000 ppm and temperatures ranging from 5° to 25°C. At the end of the experiments, hemolymph and extrapallial fluid (EPF) were taken and analyzed for pH, pCO2, bicarbonate and elemental ratios. Fluids showed decreased pH and increased CO2 with increasing water pCO2. Elemental ratios (Mg/Ca and Sr/Ca) in the fluids did not show pCO2 or temperature-related systematic changes. Furthermore, boron isotopes ([Delta]11B), used in isotope geochemistry as a pH proxy, were investigated by LA-MC-ICP-MS in shell portions precipitated during the experimental treatment. We observed high [Delta]11B variability between different individuals, but also within single shells. Average [Delta]11B values showed a weak positive correlation with pH. When comparing our results to published studies, boron isotopes appeared to represent internal pH conditions (EPF) instead of ambient water pH (Kasemann et al. 2009 Chem. Geol. 260, 138-147; Reynaud et al. 2004 Coral Reefs 23, 539-546; Sanyal et al. 2000 Geochim. Cosmochim. Acta 64, 1551-1555)

Moderate seawater acidification does not elicit long-term metabolic depression in the blue mussel Mytilus edulis

Marine organisms are exposed to increasingly acidic oceans, as a result of equilibration of surface ocean water with rising atmospheric CO2 concentrations. In this study, we examined the physiological response of Mytilus edulis from the Baltic Sea, grown for 2 months at 4 seawater pCO2 levels (39, 113, 243 and 405 Pa/385, 1,120, 2,400 and 4,000 latm). Shell and somatic growth, calcification, oxygen consumption and NHþ4 excretion rates were measured in order to test the hypothesis whether exposure to elevated seawater pCO2 is causally related to metabolic depression. During the experimental period, mussel shell mass and shell-free dry mass (SFDM) increased at least by a factor of two and three, respectively. However, shell length and shell mass growth decreased linearly with increasing pCO2 by 6–20 and 10–34%, while SFDM growth was not significantly affected by hypercapnia. We observed a parabolic change in routine metabolic rates with increasing pCO2 and the highest rates (?60%) at 243 Pa. NHþ4 excretion rose linearly with increasing pCO2. Decreased O:N ratios at the highest seawater pCO2 indicate enhanced protein metabolism which may contribute to intracellular pH regulation. We suggest that reduced shell growth under severe acidification is not caused by (global) metabolic depression but is potentially due to synergistic effects of increased cellular energy demand and nitrogen loss

Auswirkungen erhöhter pCO2 Werte auf die Ökophysiologie der Miesmuschel Mytilus edulis

Increasing atmospheric CO2 concentrations equilibrate with the surface water of the oceans and thereby increase seawater pCO2 and decrease [CO32-] and pH. This process of ocean acidification is expected to cause a drastic change of marine ecosystem composition and a decrease in calcification ability of various benthic invertebrates. The studied area, Kiel Fjord, is characterized by high pCO2 variability due to upwelling of O2 depleted and CO2 enriched bottom water. Within less than 50 years, eutrophication of the Baltic Sea has drastically increased the mean pCO2 in the fjord. The observed increase and also the rate of this acidification process is much higher than it is expected for the global ocean as a consequence of increasing atmospheric CO2 concentrations. In contrast to other areas subjected to elevated pCO2, calcifying invertebrates inhabit Kiel fjord and the benthic community is dominated by the blue mussel Mytilus edulis. Mussel larvae settle in the period of the year when highest pCO2 (800-2300 µatm) are encountered, which is, at the same time, the main growth period due to highest phytoplankton densities. In laboratory experiments, calcification rates of M. edulis are maintained at elevated pCO2 levels which are expected to occur by the year 2300. Only at high pCO2 above 3000 µatm, calcification is significantly reduced. One possible reason for this tolerance is the fact that even under control conditions, the extracellular body fluids (haemolymph and extrapallial fluid, EPF) of M. edulis are characterized by low pH and [CO32-] and high pCO2. Therefore, the EPF which is in direct contact with the shell is undersaturated with calcium carbonate also at current, low seawater pCO2. Under elevated pCO2, mussels do not buffer the extracellular acidosis by means of bicarbonate accumulation. Thus haemolymph pH and [CO32-] are reduced even further. Calcification might not be affected by the extracellular acidosis, as an amorphous calcium carbonate (ACC) precursor is most probably formed in intracellular vesicles. Since mussels are able to efficiently regulate the intracellular pH, reduced extracellular pH might therefore have only little impact on the initial calcification process. On the other hand, the production of the organic shell components, e.g. the periostracum, consumes high amounts of energy. Especially in young thin shelled life stages with a higher organic shell content most of the energy allocated to growth is required for shell production. Under elevated pCO2, mussels initially (two months acclimation) up - regulate their metabolic rates which may indicate higher energy demand for ion regulatory processes. Long-term acclimated animals (12 months acclimation) probably switch to an energetically less expensive compensation and do not exhibit elevated aerobic metabolism. However, long-term acclimated mussels are characterized by lower filtration rates. As consequence, after both intermediate and long-term exposure, the scope for growth is reduced in high pCO2 acclimated animals. Additionally, after intermediate and also long-term acclimation to elevated pCO2, protein metabolism is increased, as indicated by an elevation of ammonia excretion rates. This mode of energy generation is less efficient than oxidation of lipid or carbohydrate and may contribute to lower energy availability for growth and calcification. Similar to other aquatic animals, ammonia excretion in mussels seems to be facilitated by NH3 diffusion through Rhesus (Rh) and ammonium transporter (Amt) protein channels and subsequent acid-trapping by separate proton excretion. In order to test the importance of energy supply and elevated pCO2 on mussel calcification, juvenile M. edulis were exposed to a crossed experimental design for seven weeks. Higher food supply enables mussels to calcify also under highly elevated pCO2. In general food supply is the most important factor which determines the growth rates of mussels whereas pCO2 has only a minor effect. In a simultaneous field study, mussels were transplanted to the energy rich high pCO2 inner fjord and to the outer parts of the fjord at lower pCO2 and particulated organic carbon concentrations. Similar to the laboratory experiment, mussels exhibit much higher growth rates in the high pCO2 inner fjord with its higher particulate organic carbon concentrations. This reveals the importance of energy availability impacting CO2 tolerance of M. edulis. Mussels seem to be relatively tolerant to elevated pCO2 both in laboratory experiments and under current high pCO2 conditions in Kiel Fjord. The high energy availability present in the eutrophicated habitat may support the tolerance to elevated pCO2. In the future, increasing atmospheric CO2 concentrations will drastically elevate pCO2 level in this habitat. The benthic life stages seem to be able to cope with the expected levels but plantonic larvae might be vulnerable. However, M. edulis exhibit a high adaptation potential to the rate of acidification in the recent past and might be able to adapt also to higher levels in future. In order to predict the success of M. edulis in future, also effects of elevated temperature and the response of their main predators to these conditions needs to be considered.Increasing atmospheric CO2 concentrations equilibrate with the surface water of the oceans and thereby increase seawater pCO2 and decrease [CO32-] and pH. This process of ocean acidification is expected to cause a drastic change of marine ecosystem composition and a decrease in calcification ability of various benthic invertebrates. The studied area, Kiel Fjord, is characterized by high pCO2 variability due to upwelling of O2 depleted and CO2 enriched bottom water. Within less than 50 years, eutrophication of the Baltic Sea has drastically increased the mean pCO2 in the fjord. The observed increase and also the rate of this acidification process is much higher than it is expected for the global ocean as a consequence of increasing atmospheric CO2 concentrations. In contrast to other areas subjected to elevated pCO2, calcifying invertebrates inhabit Kiel fjord and the benthic community is dominated by the blue mussel Mytilus edulis. Mussel larvae settle in the period of the year when highest pCO2 (800-2300 µatm) are encountered, which is, at the same time, the main growth period due to highest phytoplankton densities. In laboratory experiments, calcification rates of M. edulis are maintained at elevated pCO2 levels which are expected to occur by the year 2300. Only at high pCO2 above 3000 µatm, calcification is significantly reduced. One possible reason for this tolerance is the fact that even under control conditions, the extracellular body fluids (haemolymph and extrapallial fluid, EPF) of M. edulis are characterized by low pH and [CO32-] and high pCO2. Therefore, the EPF which is in direct contact with the shell is undersaturated with calcium carbonate also at current, low seawater pCO2. Under elevated pCO2, mussels do not buffer the extracellular acidosis by means of bicarbonate accumulation. Thus haemolymph pH and [CO32-] are reduced even further. Calcification might not be affected by the extracellular acidosis, as an amorphous calcium carbonate (ACC) precursor is most probably formed in intracellular vesicles. Since mussels are able to efficiently regulate the intracellular pH, reduced extracellular pH might therefore have only little impact on the initial calcification process. On the other hand, the production of the organic shell components, e.g. the periostracum, consumes high amounts of energy. Especially in young thin shelled life stages with a higher organic shell content most of the energy allocated to growth is required for shell production. Under elevated pCO2, mussels initially (two months acclimation) up - regulate their metabolic rates which may indicate higher energy demand for ion regulatory processes. Long-term acclimated animals (12 months acclimation) probably switch to an energetically less expensive compensation and do not exhibit elevated aerobic metabolism. However, long-term acclimated mussels are characterized by lower filtration rates. As consequence, after both intermediate and long-term exposure, the scope for growth is reduced in high pCO2 acclimated animals. Additionally, after intermediate and also long-term acclimation to elevated pCO2, protein metabolism is increased, as indicated by an elevation of ammonia excretion rates. This mode of energy generation is less efficient than oxidation of lipid or carbohydrate and may contribute to lower energy availability for growth and calcification. Similar to other aquatic animals, ammonia excretion in mussels seems to be facilitated by NH3 diffusion through Rhesus (Rh) and ammonium transporter (Amt) protein channels and subsequent acid-trapping by separate proton excretion. In order to test the importance of energy supply and elevated pCO2 on mussel calcification, juvenile M. edulis were exposed to a crossed experimental design for seven weeks. Higher food supply enables mussels to calcify also under highly elevated pCO2. In general food supply is the most important factor which determines the growth rates of mussels whereas pCO2 has only a minor effect. In a simultaneous field study, mussels were transplanted to the energy rich high pCO2 inner fjord and to the outer parts of the fjord at lower pCO2 and particulated organic carbon concentrations. Similar to the laboratory experiment, mussels exhibit much higher growth rates in the high pCO2 inner fjord with its higher particulate organic carbon concentrations. This reveals the importance of energy availability impacting CO2 tolerance of M. edulis. Die steigenden CO2 Konzentration der Atmosphäre und die folgende Äquilibrierung mit dem Oberflächenwasser der Ozeane führen zu erhöhten pCO2 und sinkenden Karbonationen Konzentrationen ([CO32-]) und pH Werten. Die sogenannte Ozeanversauerung hat vermutlich weitreichende Auswirkungen auf die marinen Ökosysteme und führt möglicherweise dazu, dass die Kalzifizierungsraten insbesondere benthischer Wirbelloser abnimmt. Das Untersuchungsgebiet der Kieler Förde weist, durch den Auftrieb bodennahen Wassers mit niedriger O2 Sättigung und hohen CO2 Konzentrationen, bereits heutzutage hohe und variable pCO2 Werte auf. Innerhalb der letzten 50 Jahre hat die Eutrophierung zu einer deutlichen Erhöhung der pCO2 Werte in der Förde geführt. Damit ist sowohl das Ausmaß als auch die Geschwindigkeit der Versauerung höher, als es für die weltweiten Ozeane im Zuge des zukünftigen CO2 Anstiegs zu erwarten ist. Im Gegensatz zu anderen Gebieten, die vergleichbar erhöhten pCO2 Werten ausgesetzt sind, leben zahlreiche kalzifizierende Wirbellose in der Förde und die Miesmuschel Mytilus edulis dominiert die benthische Gemeinschaft. Die Larven der Muscheln siedeln insbesondere in der Jahreszeit in der die höchsten pCO2 Werte (800-2300 µatm) auftreten. Aufgrund der hohen Phytoplanktonkonzentrationen ist dies ebenfalls die Zeit der höchsten Wachstumsraten. In Laborversuchen ist M. edulis in Lage die Kalzifizierungsraten unter erhöhten pCO2 Werten, die für das Jahr 2300 erwartet werden, aufrechtzuerhalten. Extrazelluläre Flüssigkeiten (Hämolymphe und extrapalliale Flüssigkeit, EPF) weisen auch unter Kontrollbedingungen hohe pCO2 Werte und niedrige pH und [CO32-] auf. Die EPF, die in direktem Kontakt zur Schale steht, ist demnach auch bei niedrigem Meerwasser pCO2 mit Kalziumkarbonat untersättigt. Bei erhöhtem Meerwasser pCO2 säuert sich die Hämolymphe an und wird nicht durch Bikarbonat gepuffert. Die Ansäuerung des extrazellulären Raumes hat vermutlich deshalb nur geringe Auswirkungen auf die Kalzifizierung, da ein amorpher Kalziumkarbonatvorläufer bereits in intrazellulären Vesikeln gebildet wird. Muscheln können den intrazellulären pH weitestgehend unabhängig vom Außenmedium regulieren, weshalb die extrazelluläre Ansäuerung die Schalenbildung nur geringfügig beeinflusst. Allerdings benötigt die Synthese der organischen Schalenbestandteile sehr viel Energie. Insbesondere in jungen, dünnschaligen Lebensstadien, die Schaleen mit einem höheren Organikanteil aufweisen, wird der größte Teil der für das Wachstum benötigten Energie in die Schalenbildung investiert. Unter erhöhtem pCO2, weisen Muscheln zunächst höhere metabolische Raten auf, was möglicherweise auf einen erhöhten Energiebedarf für aktive Ionentransportprozesse hinweist. Nach langer Akklimation scheinen sie auf eine effizientere Regulation zu wechseln und der aerobe Stoffwechsel ist nicht mehr erhöht. Allerdings nimmt die Filtrationsleistung ab. In beiden Fällen ist demnach die Energie, die für das Wachstum zur Verfügung steht, reduziert. Außerdem ist nach mittlerer und Langzeit-Akklimierung der Proteinstoffwechsel erhöht, erkennbar an der höheren Ammoniumexkretion. Diese Art der Energiegewinnung ist weniger effizient als Lipid- und kohlenhydratstoffwechsel und könnte zu einer verminderten Energieverfügbarkeit beitragen. Wie auch in anderen aquatischen Tieren scheint die Ammoniumexkretion in Muscheln durch Rhesus (Rh) und Ammoniumtransporter (Amt) Kanalproteine und eine anschließende Protonierung gefördert zu werden. Die Bedeutung von Energieversorgung und erhöhtem pCO2 auf die Kalzifizerung ist in einem gekreuzten Versuchsansatz getestet worden. Höhere Futterzugabe ermöglicht den Muscheln die Kalzifizierung auch unter hohen pCO2. Generell ist die Futterzufuhr von größerer Bedeutung für das Wachstums während pCO2 nur einen geringen Effekt hat. In einem zeitgleich durchgeführten Feldexperiment sind junge Muscheln in die Innenförde mit ihren hohen pCO2 und in die Außenförde mit niedrigeren pCO2 und partikulären organischen Kohlenstoffkonzentrationen verpflanzt worden. In Übereinstimmung mit dem Laborexperiment, weisen die Tiere in der inneren Förde trotz des höheren pCO2 deutlich höhere Wachstumsraten als die in der Außenförde auf. Dieses Ergebnis betont die Bedeutung der Energieverfügbarkeit für die Toleranz von M. edulis gegenüber der Ozeanversauerung. Muscheln scheinen gegenüber erhöhten pCO2 Werten in Laborexperimenten aber auch innerhalb der Kieler Förde relativ tolerant zu sein. Die hohe Energieverfügbarkeit in dem eutrophierten Habitat der Kieler Förde könnte die Toleranz gegenüber hohen pCO2 Werten fördern. Allerdings werden die steigenden CO2 Konzentrationen der Atmosphäre die pCO2 Werte der Förde zukünftig deutlich erhöhen. Während die benthische Lebensphase gegenüber den erwarteten Werten tolerant zu sein scheint, könnten the planktischen Larven sensibler sein. Allerdings wies M. edulis eine hohe Anpassungsrate an die Versauerung der jüngeren Vergangenheit auf und könnte dementsprechend auch in der Lage sein, sich an zukünftige Bedingungen anzupassen. Um verlässlich vorherzusagen, ob M. edulis auch in Zukunft erfolgreich sein wird, ist es notwendig auch die Effekte erhöhter Temperaturen und die Auswirkungen solcher Bedingungen auf die Haupträuberorgansismen zu berücksichtigen

The benthic foraminiferal community in a naturally CO2-rich coastal habitat in the southwestern Baltic Sea

It is expected that the calcification of foraminifera will be negatively affected by the ongoing acidification of the oceans. Compared to the open oceans, these organisms are subjected to much more adverse carbonate system conditions in coastal and estuarine environments such as the southwestern Baltic Sea, where benthic foraminifera are abundant. This study documents the seasonal changes of carbonate chemistry and the ensuing response of the foraminiferal community with bi-monthly resolution in Flensburg Fjord. In comparison to the surface pCO2, which is close to equilibrium with the atmosphere, we observed large seasonal fluctuations of pCO2 in the bottom and sediment pore waters. The sediment pore water pCO2 was constantly high during the entire year ranging from 1244 to 3324 μatm. Nevertheless, in contrast to the bottom water, sediment pore water was slightly supersaturated with respect to calcite as consequence of higher alkalinity (AT) for the most time of the year. Foraminiferal assemblages were dominated by two calcareous species, Ammonia aomoriensis and Elphidium incertum, and the agglutinated Ammotium cassis. The one year-cycle was characterized by seasonal community shifts. Our results revealed that there is no dynamic response of foraminiferal population density and diversity to elevated sediment pore water pCO2. Surprisingly, the fluctuations of sediment pore water undersaturation (Ωcalc) co-vary with the population densities of living Ammonia aomoriensis. Further, we observed that most of the tests of living calcifying specimens were intact. Only Ammonia aomorienis showed dissolution and recalcification structures on the tests, especially at undersaturated conditions. Therefore, the benthic community is subjected to constantly high pCO2 and tolerates elevated levels as long as sediment pore water remains supersaturated. Model calculations inferred that increasing atmospheric CO2 concentrations will finally lead to a perennial undersaturation in sediment pore waters. Whereas benthic foraminifera indeed may cope with a high sediment pore water pCO2, the steady undersaturation of sediment pore waters would likely cause a significant higher mortality of the dominating Ammonia aomoriensis. This shift may eventually lead to changes in the benthic foraminiferal communities in Flensburg Fjord, as well as in other regions experiencing naturally undersaturated Ωcalc levels

Impact of seawater carbonate chemistry on the calcification of marine bivalves

Bivalve calcification, particularly of the early larval stages, is highly sensitive to the change in ocean carbonate chemistry resulting from atmospheric CO2 uptake. Earlier studies suggested that declining seawater [CO32−] and thereby lowered carbonate saturation affect shell production. However, disturbances of physiological processes such as acid-base regulation by adverse seawater pCO2 and pH can affect calcification in a secondary fashion. In order to determine the exact carbonate system component by which growth and calcification are affected it is necessary to utilize more complex carbonate chemistry manipulations. As single factors, pCO2 had no effects and [HCO3-] and pH had only limited effects on shell growth, while lowered [CO32−] strongly impacted calcification. Dissolved inorganic carbon (CT) limiting conditions led to strong reductions in calcification, despite high [CO32−], indicating that [HCO3-] rather than [CO32−] is the inorganic carbon source utilized for calcification by mytilid mussels. However, as the ratio [HCO3-] / [H+] is linearly correlated with [CO32−] it is not possible to differentiate between these under natural seawater conditions. An equivalent of about 80 μmol kg−1 [CO32−] is required to saturate inorganic carbon supply for calcification in bivalves. Below this threshold biomineralization rates rapidly decline. A comparison of literature data available for larvae and juvenile mussels and oysters originating from habitats differing substantially with respect to prevailing carbonate chemistry conditions revealed similar response curves. This suggests that the mechanisms which determine sensitivity of calcification in this group are highly conserved. The higher sensitivity of larval calcification seems to primarily result from the much higher relative calcification rates in early life stages. In order to reveal and understand the mechanisms that limit or facilitate adaptation to future ocean acidification, it is necessary to better understand the physiological processes and their underlying genetics that govern inorganic carbon assimilation for calcification

Sour times: seawater acidification effects on growth, feeding behaviour and acid–base status of Asterias rubens and Carcinus maenas

The impact of seawater acidification on calcifying organisms varies at the species level. If the impact differs between predator and prey in strength and/or sign, trophic interactions may be altered. In the present study, we investigated the impact of 3 different seawater pCO2 levels (650, 1250 and 3500 µatm) on the acid–base status or the growth of 2 predatory species, the common sea star Asterias rubens and the shore crab Carcinus maenas, and tested whether the quantity or size of prey consumed is affected. We exposed both the predators and their prey, the blue mussel Mytilus edulis, over a time span of 10 wk and subsequently performed feeding experiments. Intermediate acidification levels had no significant effect on growth or consumption in either predator species. The highest acidification level reduced feeding and growth rates in sea stars by 56%, while in crabs a 41% decrease in consumption rates of mussels could be demonstrated over the 10 wk experimental period but not in the subsequent shorter feeding assays. Because only a few crabs moulted in the experiment, acidification effects on crab growth could not be investigated. Active extracellular pH compensation by means of bicarbonate accumulation was observed in C. maenas, whereas the coelomic fluid pH in A. rubens remained uncompensated. Acidification did not provoke a measurable shift in prey size preferred by either predator. Mussels exposed to elevated pCO2 were preferred by previously untreated A. rubens but not by C. maenas. The observed effects on species interactions were weak even at the high acidification levels expected in the future in marginal marine habitats such as the Baltic Sea. Our results indicate that when stress effects are similar (and weak) on interacting species, biotic interactions may remain unaffected

Mussel larvae modify calcifying fluid carbonate chemistry to promote calcification

Understanding mollusk calcification sensitivity to ocean acidification (OA) requires a better knowledge of calcification mechanisms. Especially in rapidly calcifying larval stages, mechanisms of shell formation are largely unexplored—yet these are the most vulnerable life stages. Here we find rapid generation of crystalline shell material in mussel larvae. We find no evidence for intracellular CaCO3 formation, indicating that mineral formation could be constrained to the calcifying space beneath the shell. Using microelectrodes we show that larvae can increase pH and [CO32−] beneath the growing shell, leading to a ~1.5-fold elevation in calcium carbonate saturation state (Ωarag). Larvae exposed to OA exhibit a drop in pH, [CO32−] and Ωarag at the site of calcification, which correlates with decreased shell growth, and, eventually, shell dissolution. Our findings help explain why bivalve larvae can form shells under moderate acidification scenarios and provide a direct link between ocean carbonate chemistry and larval calcification rate

Salinity dependence of recruitment success of the sea star Asterias rubens in the brackish western Baltic Sea

Salinity strongly influences development and distribution of the sea star Asterias rubens. In Kiel Fjord, located in the western Baltic Sea, A. rubens is the only echinoderm species and one of the main benthic predators controlling blue mussel (Mytilus edulis) abundance. However, Kiel Fjord with an average salinity of about 15 is located close to the eastern distribution boundary of A. rubens in the Baltic Sea. In this study, we combined field and laboratory investigations to test whether the salinity of Kiel Fjord is high enough to enable successful development of A. rubens. Sea star eggs were fertilized in vitro, and development was monitored in the laboratory at four salinities (9, 12, 15 and 18) for 10 weeks. At a salinity of 9, development ceased prior to the blastula stage. At a salinity of 12, no larvae reached metamorphosis. At higher salinities, larvae developed normally and metamorphosed into juvenile sea stars. Abundances of A. rubens larvae and settled juveniles were also observed in Kiel Fjord and correlated to salinity values measured from March until June during 6 years (2005–2010). Results revealed high A. rubens settlement rates only in 2009, the year when salinity was the highest and least variable during the period of spawning and larval development. It appears that only years with high and stable salinities permit recruitment of A. rubens in Kiel Fjord. Projected desalination of the Baltic Sea could shift the distribution of A. rubens in the western Baltic Sea north-westwards and may lead to local extinction of a keystone species of the benthic ecosystem

Conditions of Mytilus edulis extracellular body fluids and shell composition in a pH-treatment experiment: Acid-base status, trace elements and delta B-11

Mytilus edulis were cultured for 3 months under six different seawater pCO(2) levels ranging from 380 to 4000 mu atm. Specimen were taken from Kiel Fjord (Western Baltic Sea, Germany) which is a habitat with high and variable seawater pCO(2) and related shifts in carbonate system speciation (e. g., low pH and low CaCO3 saturation state). Hemolymph (HL) and extrapallial fluid (EPF) samples were analyzed for pH and total dissolved inorganic carbon (C-T) to calculate pCO(2) and [HCO3-]. A second experiment was conducted for 2 months with three different pCO(2) levels (380, 1400 and 4000 mu atm). Boron isotopes (delta B-11) were investigated by LA-MC-ICP-MS (Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry) in shell portions precipitated during experimental treatment time. Additionally, elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF of specimen from the second experiment were measured via ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry). Extracellular pH was not significantly different in HL and EPF but systematically lower than ambient water pH. This is due to high extracellular pCO(2) values, a prerequisite for metabolic CO2 excretion. No accumulation of extracellular [HCO3-] was measured. Elemental ratios (B/Ca, Mg/Ca and Sr/Ca) in the EPF increased slightly with pH which is in accordance with increasing growth and calcification rates at higher seawater pH values. Boron isotope ratios were highly variable between different individuals but also within single shells. This corresponds to a high individual variability in fluid B/Ca ratios and may be due to high boron concentrations in the organic parts of the shell. The mean delta B-11 value shows no trend with pH but appears to represent internal pH (EPF) rather than ambient water pH