Mixed Effects of Elevated pCO2 on Fertilisation,Larval and Juvenile Development and Adult Responses in the Mobile Subtidal Scallop Mimachlamys asperrima (Lamarck, 1819)

Ocean acidification is predicted to have severe consequences for calcifying marine organisms especially molluscs. Recent studies, however, have found that molluscs in marine environments with naturally elevated or fluctuating CO₂ or with an active, high metabolic rate lifestyle may have a capacity to acclimate and be resilient to exposures of elevated environmental pCO₂. The aim of this study was to determine the effects of near future concentrations of elevated pCO₂ on the larval and adult stages of the mobile doughboy scallop, Mimachlamys asperrima from a subtidal and stable physio-chemical environment. It was found that fertilisation and the shell length of early larval stages of M. asperrima decreased as pCO₂ increased, however, there were less pronounced effects of elevated pCO₂ on the shell length of later larval stages, with high pCO₂ enhancing growth in some instances. Byssal attachment and condition index of adult M. asperrima decreased with elevated pCO₂, while in contrast there was no effect on standard metabolic rate or pH_e. The responses of larval and adult M. asperrima to elevated pCO₂ measured in this study were more moderate than responses previously reported for intertidal oysters and mussels. Even this more moderate set of responses are still likely to reduce the abundance of M. asperrima and potentially other scallop species in the world’s oceans at predicted future pCO₂ levels.     

Imperceptible senescence: Ageing in the ocean quahog Arctica islandica

The ocean quahog Arctica islandica is the longest-lived of all bivalve and molluscan species on earth. Animals close to 400 years are common and reported maximum live span around Iceland is close to 400 years. High and stable antioxidant capacities are a possible strategy to slow senescence and extend lifespan and this study has investigated several antioxidant parameters and a mitochondrial marker enzyme in a lifetime range spanning from 4–200 years in the Iceland quahog. In gill and mantle tissues of 4–192 year old A. islandica, catalase, citrate synthase activity and glutathione concentration declined rapidly within the first 25 years, covering the transitional phase of rapid somatic growth and sexual maturation to the outgrown mature stages (～32 years). Thereafter all three parameters kept rather stable levels for > 150 years. In contrast, superoxide dismutase activities maintained high levels throughout life time. These findings support the ‘Free Radical-Rate of Living theory’, antioxidant capacities of A.islandica are extraordinarily high and thus may explain the species long life span.     

Food supply and seawater pCO2 impact calcification and internal shell dissolution in the blue mussel Mytilus edulis

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.     

Elemental stoichiometry of the key calcifying marine phytoplankton Emiliania huxleyi under ocean climate change: A meta-analysis

The elemental composition of marine microorganisms (their C:N:P ratio, or stoichiometry) is central to understanding the biotic and biogeochemical processes underlying key marine ecosystem functions. Phytoplankton C:N:P is species specific and flexible to changing environmental conditions. However, bulk or fixed phytoplankton stoichiometry is usually assumed in biogeochemical and ecological models because more realistic, environmentally responsive C:N:P ratios have yet to be defined for key functional groups. Here, a comprehensive meta-analysis of experimental laboratory data reveals the variable C:N:P stoichiometry of Emiliania huxleyi, a globally significant calcifying phytoplankton species. Mean C:N:P of E. huxleyi is 124C:16N:1P under control conditions (i.e. growth not limited by one or more environmental stressors) and shows a range of responses to changes in nutrient and light availability, temperature and pCO₂. Macronutrient limitation caused strong shifts in stoichiometry, increasing N:P and C:P under P deficiency (by 305% and 493% respectively) and doubling C:N under N deficiency. Responses to light, temperature and pCO₂ were mixed but typically shifted cellular elemental content and C:N:P stoichiometry by ca. 30% or less. Besides these independent effects, the interactive effects of multiple environmental changes on E. huxleyi stoichiometry under future ocean conditions could be additive, synergistic or antagonistic. To synthesise our meta-analysis results, we explored how the cellular elemental content and C:N:P stoichiometry of E. huxleyi may respond to two hypothetical future ocean scenarios (increased temperature, irradiance and pCO₂ combined with either N deficiency or P deficiency) if an additive effect is assumed. Both future scenarios indicate decreased calcification (which is predominantly sensitive to elevated pCO₂), increased C:N, and up to fourfold shifts in C:P and N:P. Our results strongly suggest that climate change will significantly alter the role of E. huxleyi (and potentially other calcifying phytoplankton species) in marine biogeochemical processes.     

Ocean Acidification and Increased Temperature Have Both Positive and Negative Effects on Early Ontogenetic Traits of a Rocky Shore Keystone Predator Species

The combined effect of ocean acidification and warming is expected to have significant effects on several traits of marine organisms. The gastropod Concholepas concholepas is a rocky shore keystone predator characteristic of the south-eastern Pacific coast of South America and an important natural resource exploited by small-scale artisanal fishermen along the coast of Chile and Peru. In this study, we used small juveniles of C. concholepas collected from the rocky intertidal habitats of southern Chile (39°S) to evaluate under laboratory conditions the potential consequences of projected near-future levels of ocean acidification and warming for important early ontogenetic traits. The individuals were exposed long-term (5.8 months) to contrasting pCO₂ (ca. 500 and 1400 μatm) and temperature (15 and 19°C) levels. After this period we compared body growth traits, dislodgement resistance, predator-escape response, self-righting and metabolic rates. With respect to these traits there was no evidence of a synergistic interaction between pCO₂ and temperature. Shell growth was negatively affected by high pCO₂ levels only at 15°C. High pCO₂ levels also had a negative effect on the predator-escape response. Conversely, dislodgement resistance and self-righting were positively affected by high pCO₂ levels at both temperatures. High tenacity and fast self-righting would reduce predation risk in nature and might compensate for the negative effects of high pCO₂ levels on other important defensive traits such as shell size and escape behaviour. We conclude that climate change might produce in C. concholepas positive and negative effects in physiology and behaviour. In fact, some of the behavioural responses might be a consequence of physiological effects, such as changes in chemosensory capacity (e.g. predator-escape response) or secretion of adhesive mucous (e.g. dislodgement resistance). Moreover, we conclude that positive behavioural responses may assist in the adaptation to negative physiological impacts, and that this may also be the case for other benthic organisms.     

Shallow Water Marine Sediment Bacterial Community Shifts Along a Natural CO2 Gradient in the Mediterranean Sea Off Vulcano,Italy

The effects of increasing atmospheric CO₂ on ocean ecosystems are a major environmental concern, as rapid shoaling of the carbonate saturation horizon is exposing vast areas of marine sediments to corrosive waters worldwide. Natural CO₂ gradients off Vulcano, Italy, have revealed profound ecosystem changes along rocky shore habitats as carbonate saturation levels decrease, but no investigations have yet been made of the sedimentary habitat. Here, we sampled the upper 2 cm of volcanic sand in three zones, ambient (median pCO₂ 419 μatm, minimum Ω_arag 3.77), moderately CO₂-enriched (median pCO₂ 592 μatm, minimum Ω_arag 2.96), and highly CO₂-enriched (median pCO₂ 1611 μatm, minimum Ω_arag 0.35). We tested the hypothesis that increasing levels of seawater pCO₂ would cause significant shifts in sediment bacterial community composition, as shown recently in epilithic biofilms at the study site. In this study, 454 pyrosequencing of the V1 to V3 region of the 16S rRNA gene revealed a shift in community composition with increasing pCO₂. The relative abundances of most of the dominant genera were unaffected by the pCO₂ gradient, although there were significant differences for some 5 % of the genera present (viz. Georgenia, Lutibacter, Photobacterium, Acinetobacter, and Paenibacillus), and Shannon Diversity was greatest in sediments subject to long-term acidification (>100 years). Overall, this supports the view that globally increased ocean pCO₂ will be associated with changes in sediment bacterial community composition but that most of these organisms are resilient. However, further work is required to assess whether these results apply to other types of coastal sediments and whether the changes in relative abundance of bacterial taxa that we observed can significantly alter the biogeochemical functions of marine sediments.     

Ocean Acidification Disrupts Prey Responses to Predator Cues but Not Net Prey Shell Growth in Concholepas concholepas (loco)

Background

Most research on Ocean Acidification (OA) has largely focused on the process of calcification and the physiological trade-offs employed by calcifying organisms to support the building of calcium carbonate structures. However, there is growing evidence that OA can also impact upon other key biological processes such as survival, growth and behaviour. On wave-swept rocky shores the ability of gastropods to self-right after dislodgement, and rapidly return to normal orientation, reduces the risk of predation.

Methodology/Principal Findings

The impacts of OA on this self-righting behaviour and other important parameters such as growth, survival, shell dissolution and shell deposition in Concholepas concholepas (loco) were investigated under contrasting pCO₂ levels. Although no impacts of OA on either growth or net shell calcification were found, the results did show that OA can significantly affect self-righting behaviour during the early ontogeny of this species with significantly faster righting times recorded for individuals of C. concholepas reared under increased average pCO₂ concentrations (± SE) (716±12 and 1036±14 µatm CO₂) compared to those reared at concentrations equivalent to those presently found in the surface ocean (388±8 µatm CO₂). When loco were also exposed to the predatory crab Acanthocyclus hassleri, righting times were again increased by exposure to elevated CO₂, although self-righting times were generally twice as fast as those observed in the absence of the crab.

Conclusions and Significance

These results suggest that self-righting in the early ontogeny of C. concholepas will be positively affected by pCO₂ levels expected by the end of the 21st century and beginning of the next one. However, as the rate of self-righting is an adaptive trait evolved to reduce lethal predatory attacks, our result also suggest that OA may disrupt prey responses to predators in nature.     

The Mitochondrial Genome of Arctica islandica; Phylogeny and Variation

Arctica islandica is known as the longest-lived non-colonial metazoan species on earth and is therefore increasingly being investigated as a new model in aging research. As the mitochondrial genome is associated with the process of aging in many species and bivalves are known to possess a peculiar mechanism of mitochondrial genome inheritance including doubly uniparental inheritance (DUI), we aimed to assess the genomic variability of the A. islandica mitochondrial DNA (mtDNA). We sequenced the complete mitochondrial genomes of A. islandica specimens from three different sites in the Western Palaearctic (Iceland, North Sea, Baltic Sea). We found the A. islandica mtDNA to fall within the normal size range (18 kb) and exhibit similar coding capacity as other animal mtDNAs. The concatenated protein sequences of all currently known Veneroidea mtDNAs were used to robustly place A. islandica in a phylogenetic framework. Analysis of the observed single nucleotide polymorphism (SNP) patterns on further specimen revealed two prevailing haplotypes. Populations in the Baltic and the North Sea are very homogenous, whereas the Icelandic population, from which exceptionally old individuals have been collected, is the most diverse one. Homogeneity in Baltic and North Sea populations point to either stronger environmental constraints or more recent colonization of the habitat. Our analysis lays the foundation for further studies on A. islandica population structures, age research with this organism, and for phylogenetic studies. Accessions for the mitochondrial genome sequences: KC197241 Iceland; KF363951 Baltic Sea; KF363952 North Sea; KF465708 to KF465758 individual amplified regions from different speciemen     

Consideration of carbon dioxide release during shell production in LCA of bivalves

Purpose

Life cycle assessment (LCA) can be used to understand the environmental impacts of the shellfish aquaculture and wild harvest industries. To date, LCA of shellfish exclude carbon dioxide (CO₂) release from bivalve shell production when quantifying global warming potential per functional unit. In this study, we explain the rationale for including CO₂ released during shell production in LCA of bivalves, demonstrate a method for estimating this CO₂ release, and apply the method to previous studies to demonstrate the importance of including CO₂ from shell production in LCA.

Methods

A simple approach for calculating CO₂ from bivalve shell production was developed utilizing the seacarb package in R statistical software. The approach developed allows for inclusion of site-specific environmental parameters such as water temperature, salinity, pH, and pCO₂ when calculating CO₂ release from shell production. We applied the method to previously published LCA of bivalve production systems to assess the impact of including this CO₂ source in the LCA. The past studies include aquaculture and wild harvest production strategies and multiple bivalve species.

Results and discussion

When we recalculated the total kg CO₂ released in past studies including CO₂ release from shell production, the additional CO₂ release increased the total global warming impact category (CO₂ equivalents) in cradle-to-gate studies by approximately 250% of the original reported value. Discussion of our results focuses on the importance of different components of our calculations and site-specific environmental parameters. We make predictions on how the magnitude and importance of CO₂ released during shell production could change due to climate change and ocean acidification, and provide suggestions on how CO₂ release from shell production can be reduced through careful selection of aquaculture facility location and aquaculture practices.

Conclusions

We provide a method for including CO₂ from shell release in LCA of bivalves and recommend that future LCA of bivalves include this CO₂ as part of the global warming impact category.

     

Stable Photosymbiotic Relationship under CO2-Induced Acidification in the Acoel Worm Symsagittifera Roscoffensis

As a consequence of anthropogenic CO₂ emissions, oceans are becoming more acidic, a phenomenon known as ocean acidification. Many marine species predicted to be sensitive to this stressor are photosymbiotic, including corals and foraminifera. However, the direct impact of ocean acidification on the relationship between the photosynthetic and nonphotosynthetic organism remains unclear and is complicated by other physiological processes known to be sensitive to ocean acidification (e.g. calcification and feeding). We have studied the impact of extreme pH decrease/pCO₂ increase on the complete life cycle of the photosymbiotic, non-calcifying and pure autotrophic acoel worm, Symsagittifera roscoffensis. Our results show that this species is resistant to high pCO₂ with no negative or even positive effects on fitness (survival, growth, fertility) and/or photosymbiotic relationship till pCO₂ up to 54 K µatm. Some sub-lethal bleaching is only observed at pCO₂ up to 270 K µatm when seawater is saturated by CO₂. This indicates that photosymbiosis can be resistant to high pCO₂. If such a finding would be confirmed in other photosymbiotic species, we could then hypothesize that negative impact of high pCO₂ observed on other photosymbiotic species such as corals and foraminifera could occur through indirect impacts at other levels (calcification, feeding).     

Octocoral Tissue Provides Protection from Declining Oceanic pH

Increase in anthropogenic pCO₂ alters seawater chemistry and could lead to reduced calcification or skeleton dissolution of calcifiers and thereby weaken coral-reef structure. Studies have suggested that the complex and diverse responses in stony coral growth and calcification, as a result of elevated pCO₂, can be explained by the extent to which their soft tissues cover the underlying skeleton. This study compared the effects of decreased pH on the microstructural features of both in hospite (within the colony) and isolated sclerites (in the absence of tissue protection) of the zooxanthellate reef-dwelling octocoral Ovabunda macrospiculata. Colonies and isolated sclerites were maintained under normal (8.2) and reduced (7.6 and 7.3) pH conditions for up to 42 days. Both in hospite and isolated sclerites were then examined under SEM and ESEM microscopy in order to detect any microstructural changes. No differences were found in the microstructure of the in hospite sclerites between the control and the pH treatments. In stark contrast, the isolated sclerites revealed dissolution damage related to the acidity of the water. These findings suggest a protective role of the octocoral tissue against adverse pH conditions, thus maintaining them unharmed at high pCO₂. In light of the competition for space with the less resilient reef calcifiers, octocorals may thus have a significant advantage under greater than normal acidic conditions.     

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.     

Catabolic Activities of Neisseria meningitidis: Utilization of Succinate

Two strains of Saccharomycopsis guttulata, JB-1 and JB-3, isolated from stomach contents of domestic rabbits, were grown under different gas phases, and their growth rates were compared. Strain JB-1 grew exponentially at a maximal growth rate under a continuous gas phase of 15% CO₂, 2% O₂ in nitrogen. High cell yields with low cell granulation were obtained. The growth rates were almost the same between oxygen concentrations of 0.25 and 20% at 15% CO₂. Poor growth and early cell granulation occurred in the absence of oxygen at 15% CO₂. Growth increased at 2% O₂ in direct proportion to the carbon dioxide concentration up to 10 to 15% CO₂. A very high carbon dioxide content (e.g. 98%) was somewhat inhibitory. Cell granulation always occurred during the maximal stationary phase in media at pH 4, but was relatively slight at pH 5.6 or higher. Strain JB-3 responded to various gas phases in a similar manner except that it grew slowly in the absence of oxygen at 15% CO₂ (pH 4). The effect of an optimal gas phase on the growth of strain JB-1 was examined in relation to other environmental conditions. In the presence of 15% CO₂, 2% O₂, this strain grew exponentially in yeast autolysate-Proteose Peptone-glucose medium at 37 C at pH 2, 4, and 5.6 at approximately the same rate; the growth rate was somewhat lower at pH 6.2. Under similar conditions, strain JB-1 grew at 30 C and pH 4 at one-sixth its maximal growth rate. Cell granulation was greatly reduced at this temperature. With adequate CO₂ strain JB-1 also grew at a reduced rate in a yeast autolysate medium previously reported not to support growth. Results indicate that continuous gassing with an optimal gas phase increases the growth rate to the extent that the growth rate surpasses the death rate by a significant margin; as a result, granulated cells can be avoided almost entirely in the log phase.     

Physiological Response of Crocosphaera watsonii to Enhanced and Fluctuating Carbon Dioxide Conditions

We investigated the effects of elevated pCO₂ on cultures of the unicellular N₂-fixing cyanobacterium Crocosphaera watsonii WH8501. Using CO₂-enriched air, cultures grown in batch mode under high light intensity were exposed to initial conditions approximating current atmospheric CO₂ concentrations (∼400 ppm) as well as CO₂ levels corresponding to low- and high-end predictions for the year 2100 (∼750 and 1000 ppm). Following acclimation to CO₂ levels, the concentrations of particulate carbon (PC), particulate nitrogen (PN), and cells were measured over the diurnal cycle for a six-day period spanning exponential and early stationary growth phases. High rates of photosynthesis and respiration resulted in biologically induced pCO₂ fluctuations in all treatments. Despite this observed pCO₂ variability, and consistent with previous experiments conducted under stable pCO₂ conditions, we observed that elevated mean pCO₂ enhanced rates of PC production, PN production, and growth. During exponential growth phase, rates of PC and PN production increased by ∼1.2- and ∼1.5-fold in the mid- and high-CO₂ treatments, respectively, when compared to the low-CO₂ treatment. Elevated pCO₂ also enhanced PC and PN production rates during early stationary growth phase. In all treatments, PC and PN cellular content displayed a strong diurnal rhythm, with particulate C:N molar ratios reaching a high of 22∶1 in the light and a low of 5.5∶1 in the dark. The pCO₂ enhancement of metabolic rates persisted despite pCO₂ variability, suggesting a consistent positive response of Crocosphaera to elevated and fluctuating pCO₂ conditions.     

Long-Term m5C Methylome Dynamics Parallel Phenotypic Adaptation in the Cyanobacterium Trichodesmium

A major challenge in modern biology is understanding how the effects of short-term biological responses influence long-term evolutionary adaptation, defined as a genetically determined increase in fitness to novel environments. This is particularly important in globally important microbes experiencing rapid global change, due to their influence on food webs, biogeochemical cycles, and climate. Epigenetic modifications like methylation have been demonstrated to influence short-term plastic responses, which ultimately impact long-term adaptive responses to environmental change. However, there remains a paucity of empirical research examining long-term methylation dynamics during environmental adaptation in nonmodel, ecologically important microbes. Here, we show the first empirical evidence in a marine prokaryote for long-term m5C methylome modifications correlated with phenotypic adaptation to CO₂, using a 7-year evolution experiment (1,000+ generations) with the biogeochemically important marine cyanobacterium Trichodesmium. We identify m5C methylated sites that rapidly changed in response to high (750 µatm) CO₂ exposure and were maintained for at least 4.5 years of CO₂ selection. After 7 years of CO₂ selection, however, m5C methylation levels that initially responded to high-CO₂ returned to ancestral, ambient CO₂ levels. Concurrently, high-CO₂ adapted growth and N₂ fixation rates remained significantly higher than those of ambient CO₂ adapted cell lines irrespective of CO₂ concentration, a trend consistent with genetic assimilation theory. These data demonstrate the maintenance of CO₂-responsive m5C methylation for 4.5 years alongside phenotypic adaptation before returning to ancestral methylation levels. These observations in a globally distributed marine prokaryote provide critical evolutionary insights into biogeochemically important traits under global change.     

Low hydrogen peroxide production in mitochondria of the long‐lived Arctica islandica: underlying mechanisms for slow aging

The observation of an inverse relationship between lifespan and mitochondrial H₂O₂ production rate would represent strong evidence for the disputed oxidative stress theory of aging. Studies on this subject using invertebrates are surprisingly lacking, despite their significance in both taxonomic richness and biomass. Bivalve mollusks represent an interesting taxonomic group to challenge this relationship. They are exposed to environmental constraints such as microbial H₂S, anoxia/reoxygenation, and temperature variations known to elicit oxidative stress. Their mitochondrial electron transport system is also connected to an alternative oxidase that might improve their ability to modulate reactive oxygen species (ROS) yield. Here, we compared H₂O₂ production rates in isolated mantle mitochondria between the longest‐living metazoan—the bivalve Arctica islandica—and two taxonomically related species of comparable size. In an attempt to test mechanisms previously proposed to account for a reduction of ROS production in long‐lived species, we compared oxygen consumption of isolated mitochondria and enzymatic activity of different complexes of the electron transport system in the two species with the greatest difference in longevity. We found that A. islandica mitochondria produced significantly less H₂O₂ than those of the two short‐lived species in nearly all conditions of mitochondrial respiration tested, including forward, reverse, and convergent electron flow. Alternative oxidase activity does not seem to explain these differences. However, our data suggest that reduced complex I and III activity can contribute to the lower ROS production of A. islandica mitochondria, in accordance with previous studies. We further propose that a lower complex II activity could also be involved.     

Ocean acidification and temperature increase impact mussel shell shape and thickness: problematic for protection?

Ocean acidification threatens organisms that produce calcium carbonate shells by potentially generating an under‐saturated carbonate environment. Resultant reduced calcification and growth, and subsequent dissolution of exoskeletons, would raise concerns over the ability of the shell to provide protection for the marine organism under ocean acidification and increased temperatures. We examined the impact of combined ocean acidification and temperature increase on shell formation of the economically important edible mussel Mytilus edulis. Shell growth and thickness along with a shell thickness index and shape analysis were determined. The ability of M. edulis to produce a functional protective shell after 9 months of experimental culture under ocean acidification and increasing temperatures (380, 550, 750, 1000 μatm pCO ₂, and 750, 1000 μatm pCO ₂ + 2°C) was assessed. Mussel shells grown under ocean acidification conditions displayed significant reductions in shell aragonite thickness, shell thickness index, and changes to shell shape (750, 1000 μatm pCO ₂) compared to those shells grown under ambient conditions (380 μatm pCO ₂). Ocean acidification resulted in rounder, flatter mussel shells with thinner aragonite layers likely to be more vulnerable to fracture under changing environments and predation. The changes in shape presented here could present a compensatory mechanism to enhance protection against predators and changing environments under ocean acidification when mussels are unable to grow thicker shells. Here, we present the first assessment of mussel shell shape to determine implications for functional protection under ocean acidification.     

Light availability determines susceptibility of reef building corals to ocean acidification

Elevated seawater pCO₂, and in turn ocean acidification (OA), is now widely acknowledged to reduce calcification and growth of reef building corals. As with other environmental factors (e.g., temperature and nutrients), light availability fundamentally regulates calcification and is predicted to change for future reef environments alongside elevated pCO₂ via altered physical processes (e.g., sea level rise and turbidity); however, any potential role of light in regulating the OA-induced reduction of calcification is still unknown. We employed a multifactorial growth experiment to determine how light intensity and pCO₂ together modify calcification for model coral species from two key genera, Acropora horrida and Porites cylindrica, occupying similar ecological niches but with different physiologies. We show that elevated pCO₂ (OA)-induced losses of calcification in the light (G _L) but not darkness (G _D) were greatest under low-light growth conditions, in particular for A. horrida. High-light growth conditions therefore dampened the impact of OA upon G _L but not G _D. Gross photosynthesis (P _G) responded in a reciprocal manner to G _L suggesting OA-relieved pCO₂ limitation of P _G under high-light growth conditions to effectively enhance G _L. A multivariate analysis of past OA experiments was used to evaluate whether our test species responses were more widely applicable across their respective genera. Indeed, the light intensity for growth was identified as a significant factor influencing the OA-induced decline of calcification for species of Acropora but not Porites. Whereas low-light conditions can provide a refuge for hard corals from thermal and light stress, our study suggests that lower light availability will potentially increase the susceptibility of key coral species to OA.