Posts Tagged 'calcification'

Ocean acidification exacerbates UVR-induced inhibition of photosystem II and I in Corallina officinalis

Published 16 July 2026 Science Closed
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Highlights

  • OA reduces calcification, compromising the UV shield and exacerbating photoinhibition in C. officinalis.
  • OA amplifies PSII donor-side damage, disrupts electron flow to PSI, and suppresses PSI function.
  • OA shifts PSII damage-repair balance toward irreversible photoinhibition by increasing damage and suppressing repair.

Abstract

Ocean acidification (OA) exerts diverse effects on marine macroalgae, with calcified species being particularly vulnerable. Due to calcified skeletons can contribute to physical screening against solar ultraviolet radiation (UVR), OA-driven calcification loss may increase exposure of the photosynthetic apparatus to UVR. Here, we cultured Corallina officinalis under ambient CO₂ (∼420 μatm) or elevated CO₂ (∼1000 μatm), with or without UVR, under natural solar radiation. Our results confirmed that OA reduced calcification and, under UVR, enhanced donor-side impairment of photosystem II (PSII), as evidenced by an increase in the relative K-step (Wk, an indicator of OEC damage) and a decrease in the maximum quantum yield of PSII (Fᵥ/Fₘ). This donor-side injury was accompanied by a reconfiguration of energy fluxes per PSII reaction center, particularly under combined OA and UVR. These impairments further extended to intersystem electron transport and limited the linear electron flow from PSII to the intersystem chain. Photosystem I (PSI) related electron transport was also functionally constrained, as evidenced by the reduced electron transfer probability and terminal reduction yield. This, together with the reduction of cyclic electron transport around PSI, resulted in over-reduction of the intersystem chain and making PSI the limiting photosystem. Together, these results indicate that OA amplified UVR-induced net photodamage and weakened PSII repair capacity in C. officinalis, while also constraining PSI-related electron transport. These findings highlight the potential vulnerability of calcified red algae under future high-CO₂, high-UVR coastal oceans.

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Strong effects of sun exposure on oyster shell corrosion and compensatory calcification: a factor confounding coastal acidification responses

Published 13 July 2026 Science Closed
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The dynamics of calcium carbonate structures in marine organisms (skeletons and shells) has become increasingly important due to heightened interest in marine environmental acidification. Research into molluscan shell corrosion and calcification in response to acidification is typically carried out in laboratory-controlled settings, which often overlooks the intricate interactions found in natural environments. Mollusks inhabiting intertidal zones are especially susceptible to intense shell weathering caused by tidal cycles of heating, cooling, wetting, and drying, exacerbated by solar radiation during periods of air exposure. We investigated the effect of sun exposure (solar radiative heating) on both outer shell corrosion and inner shell compensatory calcification in the tropical oyster, Saccostrea scyphophilla. Shell properties were compared between oysters from neighboring populations in sun-exposed and shaded habitats. Habitat temperatures were measured using iButtons, and right shell valve corrosion was quantified. Compensatory calcification was assessed through measurements of shell thickness, shell density, shell compression strength, and mineralogical properties. Our results revealed that oysters in the sun that experience global irradiance, higher temperature peaks and broader daily temperature ranges (averaging an increase of 10 °C) show considerably greater outer shell surface corrosion (87%) compared to shaded oysters (31%) that experience only diffuse irradiance. Sun-exposed shells also become thickened in the midsection and around the adductor muscle, and they are slightly stronger, indicating compensation for the outer shell loss. These findings highlight the need for caution when interpreting molluscan shell dynamics based on laboratory marine acidification protocols that fail to account for the many natural environmental factors influencing shell formation and dissolution.

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Geographic variation in proteomic responses to ocean acidification in a cold-water coral (Balanophyllia elegans)

Published 16 June 2026 Science Closed
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In the face of a rapidly changing climate, assessing organismal responses to future stressors in the context of current, natural exposure to stress could provide key insights to understanding marine ecosystem resilience. I used Balanophyllia elegans, a cold-water, solitary, azooxanthellate coral as a model to better understand how varying oceanographic conditions across its geographic range have shaped its ability to tolerate and potentially adapt to current and future ocean acidification conditions. I collected B. elegans individuals from four sites across 2,500km of their range and subjected them to two pH treatments to investigate site-specific protein expression in response to low pH. Using proteomic analysis, I found that corals from each site responded differentially to low pH, mainly through changes in regulation of metabolism, calcification, and homeostasis-related proteins. Additionally, health condition varied significantly between sites after exposure to low pH, providing further evidence of site-specific responses. These results demonstrate site-specific variation in responses and tolerance to low pH, a pattern that could inform future investigations into environmental-driven adaptive expression. Such site-specific responses highlight the importance of multi-source studies for predicting a species’ ability to navigate future climate changes.

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Controls on boron isotope ratios in marine bivalve shells: insights from a controlled experiment across pH and temperature gradients

Published 8 May 2026 Science Closed
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Documenting spatial and temporal patterns of ocean acidification and understanding the way marine organisms build carbonate skeletons is critical to assessing their potential vulnerability to present and future stressors. The boron isotopic composition (δ11Bc) of many marine carbonates provides insight into the pH at the site of calcification within biocalcifiers and, by extension, the pH of ambient seawater when the carbonate formed. The modification of seawater carbonate chemistry at the site of calcification by marine calcifiers and the utility of different taxa as paleo-pH proxy archives remains an area of active research. Despite the significance of marine bivalves to ecosystem function, high-resolution paleoclimatic studies, and the shellfish industry, their biocalcification mechanisms, controls on internal pH, and potential for reconstructing records of past seawater pH remain unclear. To address these gaps, a 20.5-week flowthrough tank experiment was conducted in which four species of commercially important bivalves from the northwest Atlantic Ocean were grown in tanks with controlled pHT (pH 7.4 to 8.0) and temperature conditions (6 to 12 °C). A total of 106 shell samples from 99 individuals of adult and juvenile Arctica islandica (ocean quahog), juvenile Mercenaria mercenaria (northern quahog or hard clam), juvenile Mya arenaria (soft-shell clam) and juvenile Placopecten magellanicus (Atlantic sea scallop) were analyzed from this controlled experiment to assess the seawater pH, temperature, and growth rate controls on shell δ11Bc.These four bivalve species, grown under identical, controlled conditions, showed differential responses to the same seawater temperature and pH, likely due to differences in how they regulate the pH of their internal fluids. Juvenile P. magellanicus and juvenile M. mercenaria demonstrated significant relationships (R≥0.60; p-value <0.006) between tank pHT and δ11Bc, suggesting potential utility as proxies for past ambient seawater pH. Conversely, the δ11Bc of juvenile A. islandica and juvenile M. arenaria did not yield a strong relationship with seawater pHT but instead yielded significant relationships with shell growth rate (linear extension), with a positive relationship for M. arenaria and a negative relationship for juvenile A. islandica. The δ11B results from the few (n=9) adult A. islandica shells measured show the most variability across the range of pH and temperatures (range of 16‰) and no significant relationship was found with seawater pH or growth rate. Despite rigorous oxidative cleaning of samples, the data suggest that adult A. islandica shells contain boron-rich organic phases resistant to traditional cleaning techniques. This suggests that the next step in the development of boron-based pH proxies in A. islandica requires additional research into robust cleaning and sampling methods of periostracum and other organics. Despite the need for further investigations to constrain growth rate effects and cleaning techniques in A. islandica and M. arenaria, there is potential for developing paleo-pH proxies from P. magellanicus and M. mercenaria to better understand spatial and temporal patterns of past, present and future ocean acidification.

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Unravelling marine benthic functioning shifts under ocean acidification

Published 10 April 2026 Science Closed
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Ocean acidification (OA) driven by increasing atmospheric CO2 is altering marine biodiversity. However, impacts of OA on ecosystem functioning at the community level, including calcification, primary production and nutrient uptake, remain largely unknown. Here, we conducted community transplant experiments at natural CO2 vents to assess how declining pH affects marine community species composition, biomass, and key ecosystem processes over time. Our results indicate that community shifts caused by declining pH lead to decreased biomass and calcification rates, while photosynthesis and nutrient uptake rates increased. By leveraging OA field model systems and in situ measurements of ecosystem functioning, this study provides critical insights into how OA-induced biodiversity loss reshapes the structure and functioning of temperate marine coastal ecosystems.

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Physiological responses of Swedish maerl to ocean acidification and warming

Published 17 March 2026 Science Closed
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Maerl, (Corallinales, Rhodophyta), are free-living calcareous algae found in coastal ecosystems. They form biogenic beds with complex structures in which other species can find refuge or on which other species can settle, which highlights their importance as an ecosystem. While many species have been investigated worldwide, maerl from the Swedish west coast are poorly studied. This report investigated both acidification and warming impacts on different physiological functions of Swedish maerl, including photosynthesis, respiration and calcification. The maerl were exposed to different pH levels and temperatures in both light and dark conditions to determine their physiological thresholds, where photosynthesis and respiration were measured via oxygen fluctuations, photosynthetic efficiency via PAM fluorometry and calcification via alkalinity titrations. It was found that neither photosynthetic nor respiratory oxygen exchange showed positive or negative trends when exposed to changes in pH. On the contrary, photosynthesis peaked at the natural ambient temperature of 16°C and respiration increased with increasing temperature. Photosynthetic efficiency also did not show any trends to pH changes. However, calcification showed a significant (p < 0.05) negative response to pH in both light and dark conditions, with the response more severe in dark conditions. This suggests that decreasing pH may induce skeletal dissolution, and that photosynthesis could help buffer internal responses to external conditions. Carbonate production at ambient conditions in the light was calculated to be 556 ± 54 g CaCO3 m-2 yr-1, showing that Swedish maerl are just as, if not more, productive than maerl found elsewhere. Overall, this report showed that photosynthetic and respiratory thresholds may not be reached with acidification and that temperature increases could instead have much more severe consequences. It also showed that calcification thresholds will be met sooner rather than later, depending on acidification rates, in darker conditions for maerl found in temperate and possibly polar regions.

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Eco-evolutionary dynamics of planktonic calcifying communities under ocean acidification

Published 12 March 2026 Science Closed
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Increasing emissions of CO2 into the atmosphere are causing ocean acidification, threatening calcifying organisms. In this study, we model the physiological responses of coccolithophorids to acidification to understand the ecological and evolutionary outcomes of a system in interaction with zooplankton. Assuming a trade-off between growth and protection against grazing, we show that calcification has bivalent effects on transfers between two trophic levels and that acidity can strongly alter energy transfers. Taking into account the evolution of calcifying phenotypes in response to acidification, we show that the system outcome contrasts with previous results. While the effect of evolution depends on how calcification affects grazing, it nevertheless follows that acidification leads to a decrease in calcifying capacity. This evolutionary decrease may be progressive, but can also lead to tipping points where abrupt shifts may occur. Such a counter-selection of calcification in turn affects ecosystem functioning, enhancing energy transfers within the system and modifying carbon fluxes. We discuss how such eco-evolutionary changes may impact food webs integrity, carbon sequestration into the deep ocean and therefore endanger the carbon pump stability.

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Ocean acidification modulates material flux linked with coral calcification and photosynthesis

Published 14 January 2026 Science Closed
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Coral reefs are essential for the foundation of marine ecosystems. However, ocean acidification (OA), driven by rising atmospheric carbon dioxide (CO2) threatens coral growth and biological homeostasis. This study examines two Hawaiian coral species—Montipora capitata and Pocillopora acuta to elevated pCO2 simulating OA. Utilizing pH and O2 microsensors under controlled light and dark conditions, this work characterized interspecific concentration boundary layer (CBL) traits and quantified material fluxes under ambient and elevated pCO2. The results of this study revealed that under increased pCO2, P. acuta showed a significant reduction in dark proton efflux, followed by an increase in light O2 flux, suggesting reduced calcification and enhanced photosynthesis. In contrast, M. capitata did not show any robust evidence of changes in either flux parameters under similar increased pCO2 conditions. Statistical analyses using linear models revealed several significant interactions among species, treatment, and light conditions, identifying physical, chemical, and biological drivers of species responses to increased pCO2. This study also presents several conceptual models that correlate the CBL dynamics measured here with calcification and metabolic processes, thereby justifying our findings. We indicate that elevated pCO2 exacerbates microchemical gradients in the CBL and may threaten calcification in vulnerable species such as P. acuta, while highlighting the resistance of M. capitata. Therefore, this study advances our understanding of how interspecific microenvironmental processes could influence coral responses to changing ocean chemistry.

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The role of heterotrophy in the response of Oculina arbuscula to ocean acidification

Published 31 December 2025 Science Closed
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On both tropical and temperate reefs, the calcium carbonate skeletons produced by scleractinian corals provide habitat that supports a high biodiversity of fishes and invertebrates. Ocean acidification (OA), driven by excess anthropogenic CO2 uptake, causes declines in seawater pH and carbonate ion concentration and can compromise coral calcification by causing increased energetic demands. Deciphering how corals meet this increased energetic demand is critical to predicting their future persistence. Oculina arbuscula is a facultatively symbiotic temperate coral common on subtropical reefs of the South Atlantic Bight. This coral has demonstrated calcification resilience to reduced pH conditions in both symbiotic and aposymbiotic forms, despite aposymbiotic colonies lacking access to photosynthetically-derived energy. I hypothesized that energy acquired through heterotrophy is a mechanism by which O. arbuscula obtains the resources necessary to overcome the heightened energy demand created by ocean acidification. To investigate the role of heterotrophy, a 90-day laboratory experiment was conducted exposing aposymbiotic O. arbuscula fragments to a pH of either 7.7 or 8.0 under three different feeding levels of Artemia spp. nauplii. Although fragments with greater food consumption showed significantly higher calcification rates, this effect was independent of pH. Similarly, biochemical analyses indicated that total protein and total carbohydrate stores increased with higher food consumption but were unaffected by pH exposure. In contrast, total lipid stores decreased during the experiment, regardless of pH exposure or food level, suggesting the heterotrophic contribution to lipid stores was deficient. Together, these results indicate that while heterotrophically-derived energy may not be a primary mechanism underlying the ability of O. arbuscula to sustain calcification rates under OA stress, this coral species should continue to thrive in an increasingly acidifying ocean as long as heterotrophic food resources are in abundance.

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A global meta-analysis reveals consistently negative effects of ocean acidification on marine cultured bivalves: implications for future bivalve aquaculture

Published 16 December 2025 Science Closed
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The exponential rise in atmospheric CO₂ driven by human activities is accelerating climate change and causing ocean acidification (OA). While the effects of elevated CO₂ on a wide range of marine species have been well documented, the implications of OA for bivalve aquaculture have received comparatively little attention. Using a multi-level meta-analytical approach, we evaluated the impacts of two elevated pCO₂ levels—classified as high and extreme—on cultured bivalves, based on 266 observations from 24 species across tropical and temperate regions. Overall, both elevated pCO₂ levels negatively affected bivalves, reducing survival, growth, feeding rates, development, and calcification. Larvae were generally more vulnerable than juveniles and adults. Our analyses further indicated that temperate bivalves were more sensitive to OA than tropical and subtropical counterparts. Among taxa, clams were the most vulnerable under high CO₂ emission scenarios, whereas scallops were the most sensitive under extreme pCO₂ levels. We also discuss potential mitigation strategies for the bivalve aquaculture industry. With advancements in local and regional monitoring, coupled with targeted measures such as buffering sites, selective breeding, and integrated multi-trophic aquaculture, the adverse effects of OA on bivalve farming could be mitigated.

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Asymmetric effects of acidification and warming on foundation species and their predators in the California rocky intertidal zone

Published 11 December 2025 Science Closed
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The effects of climate change on marine organisms act through multiple pathways, as ocean warming and acidification can affect both their physiology and interspecies interactions. Asymmetries in species-specific physiological responses to climate change may alter the strength of interactions, such as those between predator and prey, which will have cascading effects on ecosystem structure. How foundation species and their interactions are affected by climate change will profoundly affect their community due to their dominance. I assessed the physiological responses of two common California rocky intertidal consumer–resource pairs across multiple trophic levels. I measured metabolic rates after four weeks of exposure to a range of nine pH levels (7.2–8.0) at two temperature levels (ambient, +4°C). At the lowest trophic level, I examined the effects of climate change on a primary producer foundation species, Silvetia compressa (golden rockweed), and its herbivore, Tegula eiseni, under differing upwelling regimes in early and late spring. Rockweed responded more to acidification than warming, decreasing photosynthetic rates in early spring and increasing rates during late spring. Their snail consumer, however, responded most strongly to temperature—increasing both respiration rates and calcification under warm conditions in late spring. In addition to species specific responses to climate stressors, the rockweed–snail pair had context-dependent responses based on background environmental conditions. Greater upwelling during late spring, combined with a younger snail population could explain differences in responses between early and late spring. Next, I examined asymmetries between a calcifying foundation species, Mytilus californianus, and its whelk predator, Nucella emarginata. Specifically, mussels were generally resistant to acute exposure to ocean warming and acidification, while whelks were highly sensitive to temperature. Whelks decreased their calcification, respiration, shell extension, and probability of drilling a mussel under warmer conditions. Across both experiments, I observed asymmetries in response to changes in pH and temperature between consumer and resource, which can shift ecosystems between bottom-up and top-down processes. Overall, I showed that mesopredators, such as herbivorous and carnivorous snails, appeared to be the most sensitive to changes in temperature relative to their foundation species prey. Climate change may reshape rocky intertidal communities by altering predation patterns on foundation species, which could either facilitate or threaten the survival of other associated species in a changing environment.

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A synthesis of Eastern oyster (Crassostrea virginica) growth and calcification responses under changing environmental conditions

Published 21 November 2025 Science Closed
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Eastern oysters (Crassostrea virginica) are foundational reef builders and ecosystem engineers that provide habitat complexity, enhance biodiversity, and influence biogeochemical cycles by shifting local carbonate chemistry in estuaries along the U.S. Atlantic coast and Gulf of Mexico. However, the environmental ranges governing oyster shell growth and calcification rates remain poorly constrained because available studies vary in the metrics quantified, experimental settings, and spatial coverage. We synthesized existing literature on C. virginica growth and calcification, assessing directional responses to changing environmental conditions. Variability in ecological, spatial, and temporal scales among studies and disparities between laboratory and field-based measurements complicate direct comparisons. Despite heterogeneity in the synthesized data, consistent patterns emerged; shell growth limitations were common at salinities below ~ 12 in U.S. Gulf of Mexico populations, and calcification declines were frequently observed under acidified conditions (pH < 7.7) in U.S. Atlantic populations. By summarizing patterns across life stages, regions, and study types, we highlight environmental stressors likely to impair oyster reef resilience and function. A more integrative research approach, incorporating both individual- and reef-scale processes across experimental and natural settings, is critical for refining predictions of oyster reef resilience. Standardized methodologies and interdisciplinary frameworks will enhance our ability to quantify the role of oyster reefs in carbon cycling and assess their response to future environmental stressors.

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Environmental stressors interplay with top-down and bottom-up effects upon shell structure and function of an intertidal marine snail

Published 17 November 2025 Science Closed
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Highlights

  • Environmental stressors affect shell properties varied across the trophic network.
  • OA, OW and predator cues, reduced snail’s shell growth and calcification.
  • OA and OW influenced shell structure and resistance more than predator risk.
  • Food quality modulates periostracum organic content under OA and OW conditions.
  • Biopolymer plasticity aids shell resistance, reducing climate stress vulnerability.

Abstract

Mollusc gastropods have evolved complex shells to protect their soft tissues from biotic and abiotic stress, but the impact of biological and environmental interactions on shell properties is not well understood. This study assessed how the individual and combined effects of increased temperature and pCO2 affect the structural and functional properties in shells of the intertidal snail Tegula atra, considering predator risk from the crab Homalaspis plana and changes in the nutritional quality of its food source, the brown kelp Lessonia spicata. Ocean acidification (OA) and ocean warming (OW) significantly affected growth rate and calcification of snails, with greater impacts under predator risk (top-down) than food quality (bottom-up) influences. FTIR-ATR analyses of the organic composition of shell periostracum indicated that OA conditions increased total organic matter, while polysaccharides, and carbonate content signals showed complex interactive effects under OA and OW conditions, with minor predator cue effects, while the nutritional value of the food source alters polysaccharides and lipids signals. Functional properties (resistance) of the shell material were affected by OA, OW, and predator cues but not by food quality source. These findings provides a novel understanding of how interacting climate stressors and trophic dynamics shape the structural (biomineralization) and functional (biomechanical) resilience of intertidal gastropods.

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Integrative analysis of coral plasticity and adaptations reveals key proteins driving resilience to changes in ocean carbonate chemistry

Published 14 November 2025 Science Closed
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Understanding how corals adapt to changes in seawater carbonate chemistry is crucial for developing effective coral conservation strategies. Research to date has mostly focused on short-term experiments, overlooking long-term evolutionary effects. Here, we investigated the link between short-term stress responses and long-term genetic adaptations in the coral species Porites pukoensis through experiments under varying CO2 and alkalinity conditions. Our results showed that alkalinity enrichment significantly increased coral calcification rates by 35%-45% compared to high CO2 treatment, highlighting the potential of alkalinity enrichment to mitigate acidification impacts. Corals modulated relative expression levels of basic and acidic proteins in response to changes in seawater carbonate chemistry in the stress experiments. Genomic data revealed that this mechanism has been evolutionarily fixed in various organisms adapting to seawater carbonate chemistry. Additionally, both experimental and genomic results showed that extracellular matrix proteins, like collagen with von Willebrand factor type A domain, were modified in response to distinct carbonate environments. Molecular dynamics simulations and in-vitro experiments demonstrated that the structural stability of these proteins contributes to coral resilience under acidified conditions. This study established an integrated framework combining stress experiments, multi-omics analyses, molecular simulations, and in-vitro validation to identify key proteins involved in coral adaptation to acidification.

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Sediment topography enhances the response of coral reef carbonate sediment dissolution to ocean acidification

Published 13 November 2025 Science Closed
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The interaction between water flow and sediment topography (e.g., surface ripples) in shallow, permeable coral reef carbonate sediments establishes pressure gradients that increase the rate of sediment–water solute exchange relative to water flow along a flat bottom. It is unknown how this effect from surface ripples may modify the rate at which the sediment porewater is exposed to future chemical changes in the overlying water column, such as elevated pCO2 that is causing ocean acidification (OA). To address this question, this study used a series of 22-h incubations in flume aquaria with live permeable calcium carbonate sediment communities and examined the interactive effect of pCO2 (400 and 1000 µatm) and surface topography (flat and rippled sediments) on invertebrate infaunal activity, carbonate sediment microbial metabolism, and inorganic carbonate dissolution. Results show that the introduction of oxygen into flat sediments was largely driven by infaunal activity, whereas introduction of oxygen into rippled sediments was largely driven by physical flow processes. Rippled sediments exhibited rates of respiration and gross primary production that were ~ 45% and ~ 50% higher, respectively, than flat sediments. An increase in pCO2 shifted the sediments in the flat flumes from net calcifying to net dissolving, an effect that was amplified an additional ~ 60% in rippled sediments. These results suggest that current estimates of coral reef carbonate sediment calcification may be underestimating the dissolution response to OA where the carbonate sediment environment exhibits ripples in the topography.

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Coral calcification resistance to acidification is physiologically linked with complex intracellular calcium ion dynamics between host and symbiont cells

Published 8 October 2025 Science Closed
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Coral calcification is a highly complex process with numerous caveats regarding the mechanisms that dictate productivity and function. Ion homeostasis, however, is the foremost physiological process unanimously shared among Scleractinia and essential for calcification. Consequently, changes to the seawater environment may elicit adverse effects on ion homeostasis. With increasing climate shifts, the physicochemical regime of our global ocean is changing rapidly. Responses of coral calcification to physicochemical change prevail in having little uniformity on an unambiguous mechanism of resistance. Therefore, this study chose a relatively tolerant Hawaiian coral, Montipora capitata to focus efforts on understanding ion homeostasis under chemical seawater manipulation designed to limit calcification. Results indicate a physiological hormesis (two-phase adaptive response) of overall coral host gene expression that was not shared with algal symbionts and decoupled from calcification rates. The sole ion homeostatic mechanism shown was calcium ion regulation by both the host and symbiont cells. Calcium ion homeostasis was also found to be mechanistically different between winter and summer seasons. Thus, potentially indicating complex interactions between host and symbiont cells, as well as the ability for M. capitata to promote calcification under stress. Putatively synthesized here are the physiological cascades and mechanisms of resistance to environmental triggers of acidosis and seasonal change. This work provides insight into linking calcium ion homeostasis with coral resistance and aims to suggest this mechanism as biomolecular indicator used in future assessments to compare tolerance.

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Using boron isotopes to examine calcification fluid pH changes in marine calcifiers under environmental change

Published 23 September 2025 Science Closed
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CO₂-driven ocean acidification (OA) decreases seawater pH and carbonate ion concentrations, which can impact the calcification and physiology of marine calcifiers. These organisms form calcium carbonate skeletons and shells from a specialized calcification fluid that is, to varying degrees, isolated from surrounding seawater. The carbonate structures serve as archives, preserving the chemical signature of the calcification fluid, which can be analyzed using geochemical proxies. In the following thesis, I examine how different taxa respond to future ocean changes by exposing them to predicted future acidification scenarios. Additionally, I aim to understand if an organism’s resilience to the impacts of ocean acidification is linked to their ability to regulate their calcification fluid chemistry using geochemical proxies.

In Chapter 1, I investigate the geochemistry of three reservoirs important for biomineralization – seawater, the extrapallial calcification fluid (EPF), and the shell – of two commercially important bivalve species: Crassostrea virginica and Arctica islandica to understand if the boron isotope proxy is probing calcification fluid pH. Additionally, I examined the effects of three ocean acidification conditions (ambient: 500 ppm, moderate: 900 ppm, and high: 2800 ppm CO2) on the calcification and chemistry of the calcification fluid of the same three reservoirs for C. virginica. Comparisons of seawater and extrapallial fluid geochemistry indicated that the EPF has a distinct composition that differs from seawater. Additionally, our OA experiments show that EPF chemistry is significantly affected by ocean acidification, demonstrating that the biological pathways regulating or storing these ions are impacted by ocean acidification. I also found that shell δ11B does not faithfully record seawater pH, but rather was correlated with EPF pH, despite an offset from in situ microelectrode pH measurements. However, the δ11B-calculated pH values were consistently higher than microelectrode pH measurements, indicating that the shell δ11B may reflect pH at a more localized site of calcification, rather than pH of the bulk EPF.

In Chapter 2, I investigate the effects of four different seawater pH levels (8.03, 7.93, 7.83, and 7.63) on seven complexes of temperate coralline algae collected from New Zealand. I examined the photophysiology, calcification, and geochemical proxies to probe the internal carbonate chemistry of seven different species of coralline algae under simulated end-of-century ocean acidification scenarios. Under ambient conditions we found clear physiological differences between branching and encrusting species. We found that OA treatments only had a significant effect on calcification of three of the seven species, Corallina berteroi, Corallina spp., and Jania “bottlebrush.” Additionally, OA only affected the calcification fluid pH (pHCF) of two species, decreasing pHCF for both Corallina beteroi and Jania “feather.” Nonetheless, for all species pHCF was constantly upregulated compared to seawater pH, indicating a strong control over calcifying fluid chemistry. My results underscore the high resilience of coralline algae calcification under the different end-of-century ocean acidification scenarios. This tolerance to OA is related to the species’ ability to maintain a stable carbonate chemistry to support calcification as seawater pH declined.

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Coping with ocean acidification: metabolic shifts in Porites corals from the Palau Archipelago

Published 9 September 2025 Science Closed
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Increased atmospheric CO2 levels lead to ocean acidification, threatening coral reefs. However, certain coral species thrive in naturally acidified environments, offering unique opportunities to explore potential acclimatization or adaptation strategies. We assessed the physiological and biochemical parameters of Porites cf. lobata. colonies from control and acidified sites in the Palau Archipelago. Using a holistic approach, we compared markers related to trophic state, symbiotic state, physiology, energy storage, and redox status, along with calcification and oxidative metabolism. Our findings indicate that these colonies can acclimatize to low-pH conditions by utilizing CO2 more effectively. The increased passive diffusion of CO2 through their tissues enables them to maintain photosynthesis and calcification rates by reallocating energy that would typically go toward bicarbonate uptake. However, this energy reallocation cannot maintain skeleton density. Corals expend energy to elevate pH in the extracellular calcifying fluid, which is highly energy-demanding and reduces lipid reserves, potentially compromising long-term resilience. Despite the heightened energy production requirements, oxidative stress does not appear to worsen; the colonies exhibited lower antioxidant defenses and protein damage under low-pH conditions. The absence of metabolic suppression due to stable respiration rates and increased biomass suggests modifications in metabolic pathways, likely shifting toward a Warburg-like effect. These findings highlight the potential for some corals to tolerate near-future ocean acidification, the trade-offs associated with this resilience, and the potential for cascading effects on reef ecosystems. Further research should explore corals metabolic pathways as potential coping mechanisms.

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Projected cooling and pCO2 conditions in upwelling zones and their influence on a prominent rocky shore ecosystem engineer

Published 3 September 2025 Science Closed
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HIGHLIGHTS

  • End-of-century projections point to intensified OA and cooling in upwelling zones
  • Projected OA enhanced growth, calcification, and byssus production in P. purpuratus
  • However, projected cooling reversed OA effects on most of these biological traits
  • These findings highlight the relevance of cooling and its strong interaction with OA

ABSTRACT

By the end of the century, upwelling zones are expected to undergo distinct changes due to the accumulation of greenhouse gases in the atmosphere. These changes include an intensification of the winds causing upwelling, further reducing sea surface temperatures (cooling), and an intensification of ocean acidification (OA). While only a few studies have evaluated the influence of cooling conditions in these systems, even fewer have assessed the combined effects of cooling and projected OA. This study addressed this gap by exposing juveniles of the intertidal purple mussel (Perumytilus purpuratus), a prominent intertidal ecosystem engineer, to distinct temperatures and pCO2 levels. Using a mesocosm system and a 2×2 factorial design, groups of purple mussels were exposed to current (15°C) and projected cooling conditions (10°C), and current and projected pCO2 levels (500 and 1500 μatm, respectively). After two months, we quantified mussel growth, calcification, byssus thread production, clearance, and metabolic rates. Growth, calcification, and byssus thread production rates were consistently affected by temperature and by the interaction between temperature and pCO2: At current temperatures (15°C) all these variables increased in response to OA, but when exposed to projected cooling conditions (10°C), these trends reversed and declined with OA. Mussel clearance rates followed the same trend, but in this case the only significant factor was the interaction between variables. Meanwhile, metabolic rates declined with temperature. A close examination of the variation among treatments suggests that the main changes were consistently associated with a sharp decline in most response variables to a combination of cooling and high pCO2 conditions. Hence, projected end of the century cooling and OA are likely to have direct (negative) effects on this habitat-forming species. Indirectly, the combination of these stressors may weaken mussel bed structure and reduce habitat complexity, thereby halting the benefits provided to associated intertidal communities.

Continue reading ‘Projected cooling and pCO2 conditions in upwelling zones and their influence on a prominent rocky shore ecosystem engineer’

Multidecadal decoupling between coral calcifying fluid and seawater saturation states

Published 1 September 2025 Science Closed
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Ocean acidification poses a threat to coral skeleton formation via reductions in the saturation state of aragonite (ΩAr) in seawater. Given that corals precipitate their skeletons from a calcifying fluid supplied by seawater, reductions in seawater ΩAr should, in theory, confound calcification. Here, we reconstruct up to 200 years of coral calcifying fluid ΩAr, using Raman spectroscopy techniques, at approximately monthly resolution in two Porites sp. skeletal cores from the Coral Sea region to investigate (i) the regulation of coral calcifying fluid ΩAr and (ii) the skeletal calcification response to industrial-era ocean acidification. Our results reveal a significant increase in calcifying fluid ΩAr, suggesting that some corals may adjust to the pace of acidification in the wild more effectively than suggested by short-term laboratory studies.

Continue reading ‘Multidecadal decoupling between coral calcifying fluid and seawater saturation states’

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