Ocean acidification weakens the structural integrity of coralline algae

The uptake of anthropogenic emission of carbon dioxide is resulting in a lowering of the carbonate saturation state and a drop in ocean pH. Understanding how marine calcifying organisms such as coralline algae may acclimatize to ocean acidification is important to understand their survival over the coming century. We present the first long-term perturbation experiment on the cold-water coralline algae, which are important marine calcifiers in the benthic ecosystems particularly at the higher latitudes. Lithothamnion glaciale, after three months incubation, continued to calcify even in undersaturated conditions with a significant trend towards lower growth rates with increasing pCO2. However, the major changes in the ultra-structure occur by 589 μatm (i.e. in saturated waters). Finite element models of the algae grown at these heightened levels show an increase in the total strain energy of nearly an order of magnitude and an uneven distribution of the stress inside the skeleton when subjected to similar loads as algae grown at ambient levels. This weakening of the structure is likely to reduce the ability of the alga to resist boring by predators and wave energy with severe consequences to the benthic community structure in the immediate future (50 years).

Growth of cultured giant clams (Tridacna spp.) in low pH, high-nutrient seawater: species-specific effects of substrate and supplemental feeding under acidification

Four species of giant clams, Tridacna maxima, T. squamosa, T. derasa and T. crocea, were cultured in outdoor raceways for 364 days at the Waikīkī Aquarium and the Oceanic Institute on the island of O‘ahu, Hawai‘i, USA.  Growth of each species was compared among individuals grown with and without supplemental phytoplankton feeding, and directly on the substrate or mounted on concrete plugs in low pH, high nutrient seawater.  Among clams cultured with and without supplemental phytoplankton (Chaetoceros spp.), feeding resulted in significantly lower mortality in all species but T. deresa, whereas growth was significantly higher among fed clams for all species except T. squamosa. Tridacna derasa showed roughly a three-fold increase in growth when fed (88.5 g ± 4.4 SD) than when unfed (26.0 g ± 2.1 SD), whereas T. maxima growth was substantially lower, but nearly 10-fold greater in response to feeding (9.0 g ± 1.9 SD). The overall mortality rate of juvenile clams was significantly lower in the fed (44.4 ± 10.0%) than the unfed (71.8 ± 9.6%) trials, with the greatest effect observed in mortality of T. maxima (fed 15% versus unfed 80%) and T. squamosa (fed 65% versus unfed 95%). None of the T. squamosa remained on concrete plugs for the duration of the experiment. Among the remaining three species, there was no difference in either wet weight or shell length for T. maxima and for wet weight only in T. derasa on (186.5 g ± 16.1 SD) and off (147.0 g ± 6.0 SD) the concrete plugs.  In contrast, T. crocea had significantly greater shell growth off the plugs (14.3 mm ± 1.0 SD versus 8.5 mm ± 1.7 SD) but significantly greater gain in wet weight on the concrete plugs (26.3 g ± 1.5 SD versus 58.5 g ± 2.5 SD).  The seawater wells used for this study are well characterized with elevated levels of inorganic nutrients and higher pCO2 relative to tropical ocean waters, roughly approximating predictions for future oceanic conditions under IPCC IS92a emission scenarios. In comparison to previous studies in natural seawater, T. derasa had a significantly higher shell growth rate in the high-nutrient, low-pH well water.  In contrast, T. maxima and T. squamosa had significantly lower growth rates in low pH, whereas growth of T. crocea was not significantly different between low pH and ambient seawater.  These experiments demonstrate species-specific differences with each treatment, which cautions against making broad generalizations regarding the effects of substrate type, feeding effects, nutrient enrichment, and ocean acidification on tridacnid culture and survival.

The effect of chronic and acute low pH on the intracellular DMSP production and epithelial cell morphology of red coralline algae

The release of dimethylsulphoniopropionate (DMSP) by marine algae has major impacts on the global sulphur cycle and may influence local climate through the formation of dimethylsulphide (DMS). However, the effect of global change on DMSP/DMS (DMS(P)) production by algae is not well understood. This study examined the effect of low pH on DMS(P) production and epithelial cell morphology of the free-living red coralline alga Lithothamnion glaciale. Three pH treatments were used in the 80-day experiment: (1) current pH level (8.18, control), (2) low, chronic pH representing a 2100 ocean acidification (OA) scenario (7.70) and (3) low, acute pH (7.75, with a 3-day spike to 6.47), representing acute variable conditions that might be associated with leaks from carbon capture and storage infrastructure, at CO₂ vent sites or in areas of upwelling. DMS(P) production was not significantly enhanced under low, stable pH conditions, indicating that red coralline algae may have some resilience to OA. However, intracellular and water column DMS(P) concentrations were significantly higher than the control when pH was acutely spiked. Cracks were observed between the cell walls of the algal skeleton in both low pH treatments. It is proposed that this structural change may cause membrane damage that allows DMS(P) to leak from the cells into the water column, with subsequent implications for the cycling of DMS(P) in coralline algae habitats.

Marine bivalve shell geochemistry and ultrastructure from modern low pH environments: environmental effect versus experimental bias (update)

Bivalve shells can provide excellent archives of past environmental change but have not been used to interpret ocean acidification events. We investigated carbon, oxygen and trace element records from different shell layers in the mussels Mytilus galloprovincialis combined with detailed investigations of the shell ultrastructure. Mussels from the harbour of Ischia (Mediterranean, Italy) were transplanted and grown in water with mean pHT 7.3 and mean pHT 8.1 near CO2 vents on the east coast of the island. Most prominently, the shells recorded the shock of transplantation, both in their shell ultrastructure, textural and geochemical record. Shell calcite, precipitated subsequently under acidified seawater responded to the pH gradient by an in part disturbed ultrastructure. Geochemical data from all test sites show a strong metabolic effect that exceeds the influence of the low-pH environment. These field experiments showed that care is needed when interpreting potential ocean acidification signals because various parameters affect shell chemistry and ultrastructure. Besides metabolic processes, seawater pH, factors such as salinity, water temperature, food availability and population density all affect the biogenic carbonate shell archive.

Impacts of ocean acidification on respiratory gas exchange and acid–base balance in a marine teleost, Opsanus beta

The oceanic carbonate system is changing rapidly due to rising atmospheric CO₂, with current levels expected to rise to between 750 and 1,000 μatm by 2100, and over 1,900 μatm by year 2300. The effects of elevated CO₂ on marine calcifying organisms have been extensively studied; however, effects of imminent CO₂ levels on teleost acid–base and respiratory physiology have yet to be examined. Examination of these physiological processes, using a paired experimental design, showed that 24 h exposure to 1,000 and 1,900 μatm CO₂ resulted in a characteristic compensated respiratory acidosis response in the gulf toadfish (Opsanus beta). Time course experiments showed the onset of acidosis occurred after 15 min of exposure to 1,900 and 1,000 μatm CO₂, with full compensation by 2 and 4 h, respectively. 1,900-μatm exposure also resulted in significantly increased intracellular white muscle pH after 24 h. No effect of 1,900 μatm was observed on branchial acid flux; however, exposure to hypercapnia and HCO₃ ⁻ free seawater compromised compensation. This suggests branchial HCO₃ ⁻ uptake rather than acid extrusion is part of the compensatory response to low-level hypercapnia. Exposure to 1,900 μatm resulted in downregulation in branchial carbonic anhydrase and slc4a2 expression, as well as decreased Na⁺/K⁺ ATPase activity after 24 h of exposure. Infusion of bovine carbonic anhydrase had no effect on blood acid–base status during 1,900 μatm exposures, but eliminated the respiratory impacts of 1,000 μatm CO₂. The results of the current study clearly show that predicted near-future CO₂ levels impact respiratory gas transport and acid–base balance. While the full physiological impacts of increased blood HCO₃ ⁻ are not known, it seems likely that chronically elevated blood HCO₃ ⁻ levels could compromise several physiological systems and furthermore may explain recent reports of increased otolith growth during exposure to elevated CO₂

Response of two marine bacterial isolates to high CO2 concentration

Experimental results related to the effects of ocean acidification on planktonic marine microbes are still rather inconsistent and occasionally contradictory. Moreover, laboratory or field experiments that address the effects of changes in CO₂ concentrations on heterotrophic microbes are very scarce, despite the major role of these organisms in the marine carbon cycle. We tested the direct effect of an elevated CO₂ concentration (1000 ppmv) on the biomass and metabolic rates (leucine incorporation, CO₂ fixation and respiration) of 2 isolates belonging to 2 relevant marine bacterial families, Rhodobacteraceae (strain MED165) and Flavobacteriaceae (strain MED217). Our results demonstrate that, contrary to some expectations, high pCO₂ did not negatively affect bacterial growth but increased growth efficiency in the case of MED217. The elevated partial pressure of CO₂ ( pCO₂) caused, in both cases, higher rates of CO₂ fixation in the dissolved fraction and, in the case of MED217, lower respiration rates. Both responses would tend to increase the pH of seawater acting as a negative feedback between elevated atmospheric CO₂ concentrations and ocean acidification.

Prolonged exposure to elevated CO2 promotes growth of the algal symbiont Symbiodinium muscatinei in the intertidal sea anemone Anthopleura elegantissima

Some photosynthetic organisms benefit from elevated levels of carbon dioxide, but studies on the effects of elevated PCO₂ on the algal symbionts of animals are very few. This study investigated the impact of hypercapnia on a photosynthetic symbiosis between the anemone Anthopleura elegantissima and its zooxanthella Symbiodinium muscatinei. Anemones were maintained in the laboratory for 1 week at 37 Pa PCO₂ and pH 8.1. Clonal pairs were then divided into two groups and maintained for 6 weeks under conditions naturally experienced in their intertidal environment, 45 Pa PCO₂, pH 8.1 and 231 Pa PCO₂, pH 7.3. Respiration and photosynthesis were measured after the 1-week acclimation period and after 6 weeks in experimental conditions. Density of zooxanthellal cells, zooxanthellal cell size, mitotic index and chlorophyll content were compared between non-clonemate anemones after the 1-week acclimation period and clonal anemones at the end of the experiment. Anemones thrived in hypercapnia. After 6 weeks, A. elegantissima exhibited higher rates of photosynthesis at 45 Pa (4.2 µmol O₂ g⁻¹ h⁻¹) and 231 Pa (3.30 µmol O₂ g⁻¹ h⁻¹) than at the initial 37 Pa (1.53 µmol O₂ g⁻¹ h⁻¹). Likewise, anemones at 231 Pa received more of their respiratory carbon from zooxanthellae (CZAR  = 78.2%) than those at 37 Pa (CZAR  = 66.6%) but less than anemones at 45 Pa (CZAR  = 137.3%). The mitotic index of zooxanthellae was significantly greater in the hypercapnic anemones than in anemones at lower PCO₂. Excess zooxanthellae were expelled by their hosts, and cell densities, cell diameters and chlorophyll contents were not significantly different between the groups. The response of A. elegantissima to hypercapnic acidification reveals the potential adaptation of an intertidal, photosynthetic symbiosis for high PCO₂.

High CO2 and silicate limitation synergistically increase the toxicity of Pseudo-nitzschia fraudulenta

Anthropogenic CO₂ is progressively acidifying the ocean, but the responses of harmful algal bloom species that produce toxins that can bioaccumulate remain virtually unknown. The neurotoxin domoic acid is produced by the globally-distributed diatom genus Pseudo-nitzschia. This toxin is responsible for amnesic shellfish poisoning, which can result in illness or death in humans and regularly causes mass mortalities of marine mammals and birds. Domoic acid production by Pseudo-nitzschia cells is known to be regulated by nutrient availability, but potential interactions with increasing seawater CO₂ concentrations are poorly understood. Here we present experiments measuring domoic acid production by acclimatized cultures of Pseudo-nitzschia fraudulenta that demonstrate a strong synergism between projected future CO₂ levels (765 ppm) and silicate-limited growth, which greatly increases cellular toxicity relative to growth under modern atmospheric (360 ppm) or pre-industrial (200 ppm) CO₂ conditions. Cellular Si:C ratios decrease with increasing CO₂, in a trend opposite to that seen for domoic acid production. The coastal California upwelling system where this species was isolated currently exhibits rapidly increasing levels of anthropogenic acidification, as well as widespread episodic silicate limitation of diatom growth. Our results suggest that the current ecosystem and human health impacts of toxic Pseudo-nitzschia blooms could be greatly exacerbated by future ocean acidification and ‘carbon fertilization’ of the coastal ocean.

Impacts of seawater acidification on mantle gene expression patterns of the Baltic Sea blue mussel: implications for shell formation and energy metabolism

Marine organisms have to cope with increasing CO₂ partial pressures and decreasing pH in the oceans. We elucidated the impacts of an 8-week acclimation period to four seawater pCO₂ treatments (39, 113, 243 and 405 Pa/385, 1,120, 2,400 and 4,000 μatm) on mantle gene expression patterns in the blue mussel Mytilus edulis from the Baltic Sea. Based on the M. edulis mantle tissue transcriptome, the expression of several genes involved in metabolism, calcification and stress responses was assessed in the outer (marginal and pallial zone) and the inner mantle tissues (central zone) using quantitative real-time PCR. The expression of genes involved in energy and protein metabolism (F-ATPase, hexokinase and elongation factor alpha) was strongly affected by acclimation to moderately elevated CO₂ partial pressures. Expression of a chitinase, potentially important for the calcification process, was strongly depressed (maximum ninefold), correlating with a linear decrease in shell growth observed in the experimental animals. Interestingly, shell matrix protein candidate genes were less affected by CO₂ in both tissues. A compensatory process toward enhanced shell protection is indicated by a massive increase in the expression of tyrosinase, a gene involved in periostracum formation (maximum 220-fold). Using correlation matrices and a force-directed layout network graph, we were able to uncover possible underlying regulatory networks and the connections between different pathways, thereby providing a molecular basis of observed changes in animal physiology in response to ocean acidification.

Reduced calcification decreases photoprotective capability in the coccolithophorid Emiliania huxleyi

Intracellular calcification of coccolithophores generate CO₂ and consumes additional energy for acquisition of calcium and bicarbonate ions, therefore, it may correlate with photoprotective processes by influencing the energetics. To address this hypothesis, a calcifying Emiliania huxleyi strain (CS-369) was grown semi-continuously at reduced (0.1 mM, LCa) and ambient Ca²⁺ concentrations (10 mM, HCa) for 150 days (>200 generations). The HCa-grown cells had higher photosynthetic and calcification rates and higher contents of chl a and carotenoids compared to the naked (bearing no coccoliths) LCa-grown cells. When exposed to stress-full levels of PAR, LCa-grown cells displayed lower photochemical yield and less efficient non-photochemical quenching (NPQ). When the LCa or HCa-grown cells were inversely shifted to their counterpart medium, LCa to HCa transfer increased photosynthetic carbon fixation (P), calcification rate (C), C/P ratio, NPQ and pigments contents, while those shifted from HCa to LCa exhibited the opposite performance. Increased NPQ, carotenoids and quantum yield clearly linked with increased or sustained calcification in E. huxleyi. The calcification must have played a role in dissipating excessive energy or as an additional drainage of electrons absorbed by the photosynthetic antennae. This phenomenon was further supported by testing two noncalcifying strains, which showed insignificant changes photosynthetic carbon fixation and NPQ when transferred to LCa condition.