Habitat traits and food availability determine the response of marine invertebrates to ocean acidification

Energy availability and local adaptation are major components in mediating the effects of ocean acidification (OA) on marine species. In a long‐term study, we investigated the effects of food availability and elevated pCO₂ (ca. 400, 1000 and 3000 μatm) on growth of newly settled Amphibalanus (Balanus) improvisus to reproduction, and on their offspring. We also compared two different populations, which were presumed to differ in their sensitivity to pCO₂ due to differing habitat conditions: Kiel Fjord, Germany (Western Baltic Sea) with naturally strong pCO₂ fluctuations, and the Tjärnö Archipelago, Sweden (Skagerrak) with far lower fluctuations. Over 20 weeks, survival, growth, reproduction and shell strength of Kiel barnacles were all unaffected by elevated pCO₂, regardless of food availability. Moulting frequency and shell corrosion increased with increasing pCO₂ in adults. Larval development and juvenile growth of the F1 generation were tolerant to increased pCO₂, irrespective of parental treatment. In contrast, elevated pCO₂ had a strong negative impact on survival of Tjärnö barnacles. Specimens from this population were able to withstand moderate levels of elevated pCO₂ over 5 weeks when food was plentiful but showed reduced growth under food limitation. Severe levels of elevated pCO₂ negatively impacted growth of Tjärnö barnacles in both food treatments. We demonstrate a conspicuously higher tolerance to elevated pCO₂ in Kiel barnacles than in Tjärnö barnacles. This tolerance was carried over from adults to their offspring. Our findings indicate that populations from fluctuating pCO₂ environments are more tolerant to elevated pCO₂ than populations from more stable pCO₂ habitats. We furthermore provide evidence that energy availability can mediate the ability of barnacles to withstand moderate CO₂ stress. Considering the high tolerance of Kiel specimens and the possibility to adapt over many generations, near future OA alone does not seem to present a major threat for A. improvisus.     

Response to ocean acidification in larvae of a large tropical marine fish,Rachycentron canadum

Currently, ocean acidification is occurring at a faster rate than at any time in the last 300 million years, posing an ecological challenge to marine organisms globally. There is a critical need to understand the effects of acidification on the vulnerable larval stages of marine fishes, as there is potential for large ecological and economic impacts on fish populations and the human economies that rely on them. We expand upon the narrow taxonomic scope found in the literature today, which overlooks many life history characteristics of harvested species, by reporting on the larvae of Rachycentron canadum (cobia), a large, highly mobile, pelagic‐spawning, widely distributed species with a life history and fishery value contrasting other species studied to date. We raised larval cobia through the first 3 weeks of ontogeny under conditions of predicted future ocean acidification to determine effects on somatic growth, development, otolith formation, swimming ability, and swimming activity. Cobia exhibited resistance to treatment effects on growth, development, swimming ability, and swimming activity at 800 and 2100 μatm pCO₂. However, these scenarios resulted in a significant increase in otolith size (up to 25% larger area) at the lowest pCO₂ levels reported to date, as well as the first report of significantly wider daily otolith growth increments. When raised under more extreme scenarios of 3500 and 5400 μatm pCO₂, cobia exhibited significantly reduced size‐at‐age (up to 25% smaller) and a 2–3 days developmental delay. The robust nature of cobia may be due to the naturally variable environmental conditions this species currently encounters throughout ontogeny in coastal environments, which may lead to an increased acclimatization ability even during long‐term exposure to stressors.     

Seed traits,seed‐reserve utilization and offspring performance across pre‐industrial to future CO2 concentrations in a Mediterranean community

Atmospheric CO₂ enrichment can affect plants directly via impacts on their performance, and indirectly, by environment‐specific traits passed down from the mother plant to the offspring. Such maternal effects can significantly alter plant species composition, especially in annual ecosystems where the entire community is recruited from seeds each year. This study assessed impacts of future, high CO₂ (440 and 600 ppm) and pre‐industrial, low CO₂ (280 ppm) on seed traits and offspring performance in three plant functional groups (grasses, legumes, forbs) comprising 17 annual species of a semi‐arid Mediterranean community. In grasses, seed size and seed‐reserve utilization as expressed by root elongation tended to be higher at high than at low maternal CO₂, but total seed protein concentration and protein pool decreased with increasing maternal CO₂. The response of seed size to high CO₂ increased with increasing leaf‐mass fraction in grasses, and decreased with decreasing concentration of leaf non‐structural carbohydrates in legumes. Offspring development was studied at ambient CO₂, and showed reduced emergence success of high‐CO₂ progeny compared with low‐CO₂ progeny in forbs. Total biomass was lower in high‐CO₂ than in low‐CO₂ offspring across all functional groups. The biomass response to high maternal CO₂ in legume offspring correlated inversely with seed size, resulting in up to 25% lower biomass in large‐seeded species. Under the scenario of maternal effects combined with projected changes in biomass and seed production under direct exposure to high CO₂, legumes might gain and forbs and grasses might lose from future CO₂ enrichment. Most changes in seed traits and offspring performance were greater between pre‐industrial and near‐future CO₂ than between near‐ and remote‐future CO₂ concentrations. Hence, maternal effects of increasing CO₂ may contribute to current changes in plant productivity and species composition, and they need to be considered when predicting impacts of global change on plant communities.     

Larval carry‐over effects from ocean acidification persist in the natural environment

An extensive body of work suggests that altered marine carbonate chemistry can negatively influence marine invertebrates, but few studies have examined how effects are moderated and persist in the natural environment. A particularly important question is whether impacts initiated in early life might be exacerbated or attenuated over time in the presence or absence of other stressors in the field. We reared Olympia oyster (Ostrea lurida) larvae in laboratory cultures under control and elevated seawater pCO₂ concentrations, quantified settlement success and size at metamorphosis, then outplanted juveniles to Tomales Bay, California, in the mid intertidal zone where emersion and temperature stress were higher, and in the low intertidal zone where conditions were more benign. We tracked survival and growth of outplanted juveniles for 4 months, halfway to reproductive age. Survival to metamorphosis in the laboratory was strongly affected by larval exposure to elevated pCO₂ conditions. Survival of juvenile outplants was reduced dramatically at mid shore compared to low shore levels regardless of the pCO₂ level that oysters experienced as larvae. However, juveniles that were exposed to elevated pCO₂ as larvae grew less than control individuals, representing a larval carry‐over effect. Although juveniles grew less at mid shore than low shore levels, there was no evidence of an interaction between the larval carry‐over effect and shore level, suggesting little modulation of acidification impacts by emersion or temperature stress. Importantly, the carry‐over effects of larval exposure to ocean acidification remained unabated 4 months later with no evidence of compensatory growth, even under benign conditions. This latter result points to the potential for extended consequences of brief exposures to altered seawater chemistry with potential consequences for population dynamics.     

Altered physiological and growth responses to elevated [CO2] in offspring from holm oak (Quercus ilex L.) mother trees with lifetime exposure to naturally elevated [CO2]

An important question with respect to plant performance in future climatic scenarios is whether the offspring of mature trees that have experienced lifelong exposure to elevated [CO₂] show altered physiological responses to elevated [CO₂] compared with those originating from current ambient CO₂ concentrations. To investigate this question, acorns were collected from two seed sources, denoted as ‘control’ and ‘spring’, from Quercus ilex mother trees grown at ambient (36 Pa) and at about twice ambient CO₂ concentrations, respectively, close to a natural CO₂ spring, Laiatico, central Italy. The seedlings were raised for 8 months under controlled conditions at ambient and elevated [CO₂] in a reciprocal experimental design and were used for the determination of biomass, photosynthesis and foliar carbohydrate concentrations, as well as the accumulation of structural biomass and lignin during leaf maturation. Under ambient [CO₂], biomass and foliar carbon acquisition in control progeny were not significantly different from spring progeny. However, under elevated [CO₂], spring seedlings showed less CO₂ acclimation than control seedlings but no significant differences in non‐structural carbohydrate concentrations and structural biomass per unit leaf dry mass. Developmental lignin accumulation in leaves was delayed under elevated [CO₂] compared with ambient [CO₂], but only in control progeny. Under elevated [CO₂], whole‐plant biomass, leaf area and stem diameter were significantly increased in Quercus ilex seedlings from both seed sources but with a higher stimulation of above‐ground biomass in spring than in control seedlings and a higher stimulation of below‐ground biomass in control seedlings. These results indicate that life history and/or progeny may determine the species‐specific CO₂ response and suggest that positive CO₂ acclimation is possible.     

The physiological response of marine diatoms to ocean acidification: differential roles of seawater pCO2 and pH

Although increasing the pCO₂ for diatoms will presumably down‐regulate the CO₂‐concentrating mechanism (CCM) to save energy for growth, different species have been reported to respond differently to ocean acidification (OA). To better understand their growth responses to OA, we acclimated the diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum, and Chaetoceros muelleri to ambient (pCO₂ 400 μatm, pH 8.1), carbonated (pCO₂ 800 μatm, pH 8.1), acidified (pCO₂ 400 μatm, pH 7.8), and OA (pCO₂ 800 μatm, pH 7.8) conditions and investigated how seawater pCO₂ and pH affect their CCMs, photosynthesis, and respiration both individually and jointly. In all three diatoms, carbonation down‐regulated the CCMs, while acidification increased both the photosynthetic carbon fixation rate and the fraction of CO₂ as the inorganic carbon source. The positive OA effect on photosynthetic carbon fixation was more pronounced in C. muelleri, which had a relatively lower photosynthetic affinity for CO₂, than in either T. pseudonana or P. tricornutum. In response to OA, T. pseudonana increased respiration for active disposal of H⁺ to maintain its intracellular pH, whereas P. tricornutum and C. muelleri retained their respiration rate but lowered the intracellular pH to maintain the cross‐membrane electrochemical gradient for H⁺ efflux. As the net result of changes in photosynthesis and respiration, growth enhancement to OA of the three diatoms followed the order of C. muelleri > P. tricornutum > T. pseudonana. This study demonstrates that elucidating the separate and joint impacts of increased pCO₂ and decreased pH aids the mechanistic understanding of OA effects on diatoms in the future, acidified oceans.     

Differential proteomic responses of selectively bred and wild‐type Sydney rock oyster populations exposed to elevated CO2

Previous work suggests that larvae from Sydney rock oysters that have been selectively bred for fast growth and disease resistance are more resilient to the impacts of ocean acidification than nonselected, wild‐type oysters. In this study, we used proteomics to investigate the molecular differences between oyster populations in adult Sydney rock oysters and to identify whether these form the basis for observations seen in larvae. Adult oysters from a selective breeding line (B2) and nonselected wild types (WT) were exposed for 4 weeks to elevated pCO₂ (856 μatm) before their proteomes were compared to those of oysters held under ambient conditions (375 μatm pCO₂). Exposure to elevated pCO₂ resulted in substantial changes in the proteomes of oysters from both the selectively bred and wild‐type populations. When biological functions were assigned, these differential proteins fell into five broad, potentially interrelated categories of subcellular functions, in both oyster populations. These functional categories were energy production, cellular stress responses, the cytoskeleton, protein synthesis and cell signalling. In the wild‐type population, proteins were predominantly upregulated. However, unexpectedly, these cellular systems were downregulated in the selectively bred oyster population, indicating cellular dysfunction. We argue that this reflects a trade‐off, whereby an adaptive capacity for enhanced mitochondrial energy production in the selectively bred population may help to protect larvae from the effects of elevated CO₂, whilst being deleterious to adult oysters.     

Coralline algal structure is more sensitive to rate,rather than the magnitude,of ocean acidification

Marine pCO₂ enrichment via ocean acidification (OA), upwelling and release from carbon capture and storage (CCS) facilities is projected to have devastating impacts on marine biomineralisers and the services they provide. However, empirical studies using stable endpoint pCO₂ concentrations find species exhibit variable biological and geochemical responses rather than the expected negative patterns. In addition, the carbonate chemistry of many marine systems is now being observed to be more variable than previously thought. To underpin more robust projections of future OA impacts on marine biomineralisers and their role in ecosystem service provision, we investigate coralline algal responses to realistically variable scenarios of marine pCO₂ enrichment. Coralline algae are important in ecosystem function; providing habitats and nursery areas, hosting high biodiversity, stabilizing reef structures and contributing to the carbon cycle. Red coralline marine algae were exposed for 80 days to one of three pH treatments: (i) current pH (control); (ii) low pH (7.7) representing OA change; and (iii) an abrupt drop to low pH (7.7) representing the higher rates of pH change observed at natural vent systems, in areas of upwelling and during CCS releases. We demonstrate that red coralline algae respond differently to the rate and the magnitude of pH change induced by pCO₂ enrichment. At low pH, coralline algae survived by increasing their calcification rates. However, when the change to low pH occurred at a fast rate we detected, using Raman spectroscopy, weaknesses in the calcite skeleton, with evidence of dissolution and molecular positional disorder. This suggests that, while coralline algae will continue to calcify, they may be structurally weakened, putting at risk the ecosystem services they provide. Notwithstanding evolutionary adaptation, the ability of coralline algae to cope with OA may thus be determined primarily by the rate, rather than magnitude, at which pCO₂ enrichment occurs.     

SHORT‐ VERSUS LONG‐TERM RESPONSES TO CHANGING CO2 IN A COASTAL DINOFLAGELLATE BLOOM: IMPLICATIONS FOR INTERSPECIFIC COMPETITIVE INTERACTIONS AND COMMUNITY STRUCTURE

Increasing pCO₂ (partial pressure of CO₂) in an “acidified” ocean will affect phytoplankton community structure, but manipulation experiments with assemblages briefly acclimated to simulated future conditions may not accurately predict the long‐term evolutionary shifts that could affect inter‐specific competitive success. We assessed community structure changes in a natural mixed dinoflagellate bloom incubated at three pCO₂ levels (230, 433, and 765 ppm) in a short‐term experiment (2 weeks). The four dominant species were then isolated from each treatment into clonal cultures, and maintained at all three pCO₂ levels for approximately 1 year. Periodically (4, 8, and 12 months), these pCO₂‐conditioned clones were recombined into artificial communities, and allowed to compete at their conditioning pCO₂ level or at higher and lower levels. The dominant species in these artificial communities of CO₂‐conditioned clones differed from those in the original short‐term experiment, but individual species relative abundance trends across pCO₂ treatments were often similar. Specific growth rates showed no strong evidence for fitness increases attributable to conditioning pCO₂ level. Although pCO₂ significantly structured our experimental communities, conditioning time and biotic interactions like mixotrophy also had major roles in determining competitive outcomes. New methods of carrying out extended mixed species experiments are needed to accurately predict future long‐term phytoplankton community responses to changing pCO₂.     

Ocean acidification and warming scenarios increase microbioerosion of coral skeletons

Biological mediation of carbonate dissolution represents a fundamental component of the destructive forces acting on coral reef ecosystems. Whereas ocean acidification can increase dissolution of carbonate substrates, the combined impact of ocean acidification and warming on the microbioerosion of coral skeletons remains unknown. Here, we exposed skeletons of the reef‐building corals, Porites cylindrica and Isopora cuneata, to present‐day (Control: 400 μatm – 24 °C) and future pCO₂–temperature scenarios projected for the end of the century (Medium: +230 μatm – +2 °C; High: +610 μatm – +4 °C). Skeletons were also subjected to permanent darkness with initial sodium hypochlorite incubation, and natural light without sodium hypochlorite incubation to isolate the environmental effect of acidic seawater (i.e., Ω_aragonite <1) from the biological effect of photosynthetic microborers. Our results indicated that skeletal dissolution is predominantly driven by photosynthetic microborers, as samples held in the dark did not decalcify. In contrast, dissolution of skeletons exposed to light increased under elevated pCO₂–temperature scenarios, with P. cylindrica experiencing higher dissolution rates per month (89%) than I. cuneata (46%) in the high treatment relative to control. The effects of future pCO₂–temperature scenarios on the structure of endolithic communities were only identified in P. cylindrica and were mostly associated with a higher abundance of the green algae Ostreobium spp. Enhanced skeletal dissolution was also associated with increased endolithic biomass and respiration under elevated pCO₂–temperature scenarios. Our results suggest that future projections of ocean acidification and warming will lead to increased rates of microbioerosion. However, the magnitude of bioerosion responses may depend on the structural properties of coral skeletons, with a range of implications for reef carbonate losses under warmer and more acidic oceans.     

Elevated pCO2 increases sperm limitation and risk of polyspermy in the red sea urchin Strongylocentrotus franciscanus

Anthropogenic carbon dioxide (CO₂) emissions and the resultant acidification of surface ocean waters are predicted to have far‐reaching consequences for biological processes in the marine environment. For example, because changes in pH and pCO₂ can alter sperm performance, ocean acidification may be accompanied by reductions in the success of fertilization in marine broadcast spawners. Several studies have attempted to determine the effects of elevated pCO₂ on marine invertebrate fertilization success, albeit with differing results. These conflicts may stem from the use of inappropriate sperm–egg contact times and, in several cases, the lack of measurements over a range of sperm concentrations extending from sperm‐limited conditions to polyspermy scenarios. In our study, we used biologically realistic sperm–egg contact times and a full range of sperm concentrations to assess the effect of elevated pCO₂ on fertilization in the broadcast spawning sea urchin, Strongylocentrotus franciscanus. Fertilization experiments were carried out in seawater bubbled with CO₂ to 400 (control), 800, and 1800 ppm. Using a fertilization kinetics model, we estimate that elevated pCO₂ levels both increased sperm limitation and reduced the efficiency of fast blocks to polyspermy. Thus, elevated pCO₂ decreased the range of sperm concentrations over which high fertilization success was likely. Given the inherent difficulties in achieving high fertilization success in broadcast spawners, raised pCO₂ levels are likely to exacerbate low fertilization success in low‐density populations or in areas with high water turbulence.     

Synergistic effects of ocean acidification and warming on overwintering pteropods in the Arctic

Ocean acidification and warming will be most pronounced in the Arctic Ocean. Aragonite shell‐bearing pteropods in the Arctic are expected to be among the first species to suffer from ocean acidification. Carbonate undersaturation in the Arctic will first occur in winter and because this period is also characterized by low food availability, the overwintering stages of polar pteropods may develop into a bottleneck in their life cycle. The impacts of ocean acidification and warming on growth, shell degradation (dissolution), and mortality of two thecosome pteropods, the polar Limacina helicina and the boreal L. retroversa, were studied for the first time during the Arctic winter in the Kongsfjord (Svalbard). The abundance of L. helicina and L. retroversa varied from 23.5 to 120 ind m^?2 and 12 to 38 ind m^?2, and the mean shell size ranged from 920 to 981 μm and 810 to 823 μm, respectively. Seawater was aragonite‐undersaturated at the overwintering depths of pteropods on two out of ten days of our observations. A 7‐day experiment [temperature levels: 2 and 7 °C, pCO₂ levels: 350, 650 (only for L. helicina) and 880 μatm] revealed a significant pCO₂ effect on shell degradation in both species, and synergistic effects between temperature and pCO₂ for L. helicina. A comparison of live and dead specimens kept under the same experimental conditions indicated that both species were capable of actively reducing the impacts of acidification on shell dissolution. A higher vulnerability to increasing pCO₂ and temperature during the winter season is indicated compared with a similar study from fall 2009. Considering the species winter phenology and the seasonal changes in carbonate chemistry in Arctic waters, negative climate change effects on Arctic thecosomes are likely to show up first during winter, possibly well before ocean acidification effects become detectable during the summer season.     

Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2

Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short‐term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long‐term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO₂ conditions (1,000 μatm). High CO₂‐selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO₂ (400 μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO₂‐selected cell lines. All of these trait changes persisted when high CO₂‐grown populations were moved back to ambient CO₂ conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short‐term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences.     

INTERACTIONS BETWEEN OCEAN ACIDIFICATION AND WARMING ON THE MORTALITY AND DISSOLUTION OF CORALLINE ALGAE1

Coralline algae are among the most sensitive calcifying organisms to ocean acidification as a result of increased atmospheric carbon dioxide (pCO₂). Little is known, however, about the combined impacts of increased pCO₂, ocean acidification, and sea surface temperature on tissue mortality and skeletal dissolution of coralline algae. To address this issue, we conducted factorial manipulative experiments of elevated CO₂ and temperature and examined the consequences on tissue survival and skeletal dissolution of the crustose coralline alga (CCA) Porolithon (=Hydrolithon) onkodes (Heydr.) Foslie (Corallinaceae, Rhodophyta) on the southern Great Barrier Reef (GBR), Australia. We observed that warming amplified the negative effects of high pCO₂ on the health of the algae: rates of advanced partial mortality of CCA increased from <1% to 9% under high CO₂ (from 400 to 1,100 ppm) and exacerbated to 15% under warming conditions (from 26°C to 29°C). Furthermore, the effect of pCO₂ on skeletal dissolution strongly depended on temperature. Dissolution of P. onkodes only occurred in the high‐pCO₂ treatment and was greater in the warm treatment. Enhanced skeletal dissolution was also associated with a significant increase in the abundance of endolithic algae. Our results demonstrate that P. onkodes is particularly sensitive to ocean acidification under warm conditions, suggesting that previous experiments focused on ocean acidification alone have underestimated the impact of future conditions on coralline algae. Given the central role that coralline algae play within coral reefs, these conclusions have serious ramifications for the integrity of coral‐reef ecosystems.     

Nitrogen source and pCO2 synergistically affect carbon allocation,growth and morphology of the coccolithophore Emiliania huxleyi: potential implications of ocean acidification for the carbon cycle

Coccolithophores are unicellular phytoplankton that produce calcium carbonate coccoliths as an exoskeleton. Emiliania huxleyi, the most abundant coccolithophore in the world's ocean, plays a major role in the global carbon cycle by regulating the exchange of CO₂ across the ocean‐atmosphere interface through photosynthesis and calcium carbonate precipitation. As CO₂ concentration is rising in the atmosphere, the ocean is acidifying and ammonium (NH₄⁺) concentration of future ocean water is expected to rise. The latter is attributed to increasing anthropogenic nitrogen (N) deposition, increasing rates of cyanobacterial N₂ fixation due to warmer and more stratified oceans, and decreased rates of nitrification due to ocean acidification. Thus, future global climate change will cause oceanic phytoplankton to experience changes in multiple environmental parameters including CO₂, pH, temperature and nitrogen source. This study reports on the combined effect of elevated pCO₂ and increased NH₄⁺ to nitrate (NO₃^?) ratio (NH₄⁺/NO₃^?) on E. huxleyi, maintained in continuous cultures for more than 200 generations under two pCO₂ levels and two different N sources. Herein, we show that NH₄⁺ assimilation under N‐replete conditions depresses calcification at both low and high pCO₂, alters coccolith morphology, and increases primary production. We observed that N source and pCO₂ synergistically drive growth rates, cell size, and the ratio of inorganic to organic carbon. These responses to N source suggest that, compared to increasing CO₂ alone, a greater disruption of the organic carbon pump could be expected in response to the combined effect of increased NH₄⁺/NO₃^? ratio and CO₂ level in the future acidified ocean. Additional experiments conducted under lower nutrient conditions are needed prior to extrapolating our findings to the global oceans. Nonetheless, our results emphasize the need to assess combined effects of multiple environmental parameters on phytoplankton biology to develop accurate predictions of phytoplankton responses to ocean acidification.     

Food availability outweighs ocean acidification effects in juvenile Mytilus edulis: laboratory and field experiments

Ocean acidification is expected to decrease calcification rates of bivalves. Nevertheless, in many coastal areas high pCO₂ variability is encountered already today. Kiel Fjord (Western Baltic Sea) is a brackish (12–20 g kg^?1) and CO₂ enriched habitat, but the blue mussel Mytilus edulis dominates the benthic community. In a coupled field and laboratory study we examined the annual pCO₂ variability in this habitat and the combined effects of elevated pCO₂ and food availability on juvenile M. edulis growth and calcification. In the laboratory experiment, mussel growth and calcification were found to chiefly depend on food supply, with only minor impacts of pCO₂ up to 3350 μatm. Kiel Fjord was characterized by strong seasonal pCO₂ variability. During summer, maximal pCO₂ values of 2500 μatm were observed at the surface and >3000 μatm at the bottom. However, the field growth experiment revealed seven times higher growth and calcification rates of M. edulis at a high pCO₂ inner fjord field station (mean pCO₂ ca. 1000 μatm) in comparison to a low pCO₂ outer fjord station (ca. 600 μatm). In addition, mussels were able to out‐compete the barnacle Amphibalanus improvisus at the high pCO₂ site. High mussel productivity at the inner fjord site was enabled by higher particulate organic carbon concentrations. Kiel Fjord is highly impacted by eutrophication, which causes bottom water hypoxia and consequently high seawater pCO₂. At the same time, elevated nutrient concentrations increase the energy availability for filter feeding organisms such as mussels. Thus, M. edulis can dominate over a seemingly more acidification resistant species such as A. improvisus. We conclude that benthic stages of M. edulis tolerate high ambient pCO₂ when food supply is abundant and that important habitat characteristics such as species interactions and energy availability need to be considered to predict species vulnerability to ocean acidification.     

Ecophysiology matters: linking inorganic carbon acquisition to ecological preference in four species of microalgae (Chlorophyceae)

The effect of CO₂ supply is likely to play an important role in algal ecology. Since inorganic carbon (C_i) acquisition strategies are very diverse among microalgae and C_i availability varies greatly within and among habitats, we hypothesized that C_i acquisition depends on the pH of their preferred natural environment (adaptation) and that the efficiency of C_i uptake is affected by CO₂ availability (acclimation). To test this, four species of green algae originating from different habitats were studied. The pH‐drift and C_i uptake kinetic experiments were used to characterize C_i acquisition strategies and their ability to acclimate to high and low CO₂ conditions and high and low pH was evaluated. Results from pH drift experiments revealed that the acidophile and acidotolerant Chlamydomonas species were mainly restricted to CO₂, whereas the two neutrophiles were efficient bicarbonate users. CO₂ compensation points in low CO₂‐acclimated cultures ranged between 0.6 and 1.4 μM CO₂ and acclimation to different culture pH and CO₂ conditions suggested that CO₂ concentrating mechanisms were present in most species. High CO₂ acclimated cultures adapted rapidly to low CO₂ condition during pH‐drifts. C_i uptake kinetics at different pH values showed that the affinity for C_i was largely influenced by external pH, being highest under conditions where CO₂ dominated the C_i pool. In conclusion, C_i acquisition was highly variable among four species of green algae and linked to growth pH preference, suggesting that there is a connection between C_i acquisition and ecological distribution.