Reduced Calcification Rates Research Articles

Macroalgal presence decreases coral calcification rates more than ocean acidification

Global coral reef degradation has precipitated phase shifts toward macroalgal-dominated communities. Despite the negative repercussions for reefscapes, higher abundances of primary producers have the potential to positively impact the physicochemical environment and mitigate negative impacts of ocean acidification (OA). In this study, we investigated the influence of macroalgal (cf. Sargassum vulgare) density on coral (Acropora millepora and A. hemprichii) calcification rates under current and future OA conditions. Corals were resistant to OA up to ~ 1100 µatm, with no changes in calcification rates. However, the presence of (low and high density) algae reduced calcification rates by ~ 41.8%, suggesting either a chemical defense response due to algal metabolites or potential physical impacts from shading or abrasion. Documented beneficial buffering effects of macroalgae in OA may also elicit negative impacts on coral calcification, suggesting further work is needed to elucidate how species interactions influence responses to projected climate change.

Seasonal drivers of productivity and calcification in the coral Platygyra carnosa in a subtropical reef

Scleractinian corals are increasingly subjected to local stressors combined with global changes. In subtropical areas, corals exhibit metabolic plasticity and resilience in response to variability and extremes in local temperature, salinity, and light; however, the physiological mechanisms by which corals acclimate or adapt to these changing conditions remain disputed. We assessed the physiological status of the coral Platygyra carnosa during a two-year in situ monitoring survey. To obtain metabolic rates (respiration and photosynthesis), photochemical efficiency (Fv / Fm), and biocalcification measurements, non-invasive techniques such as underwater respirometry, Pulse Amplitude Modulated (PAM) fluorometry, total alkalinity measurements, and digital photography were used. Our findings show clear seasonality in water quality parameters, which affected coral health. Elevated temperatures during the summer were below the maximum monthly mean < 31°C) but reduced the energetic productivity of corals (-44% relative to winter). Fluctuations in salinity (25–38 ppt) and pH (7.65–8.44) were linked to rainfall and reduced calcification rates. The conditions during the spring were favorable for coral metabolism and calcification (+20% relative to summer). Overall, our research demonstrates that the metabolic plasticity of P. carnosa in response to shifts in seawater quality allows this species to survive ongoing environmental change. Our in situ observations provide fundamental insights into coral response mechanisms under changing environmental conditions and contribute to projections of coral health under future scenarios of global change.

The History of Cenozoic Carbonate Flux in the Atlantic Ocean Constrained by Multiple Regional Carbonate Compensation Depth Reconstructions

AbstractThe Atlantic is the only ocean basin almost entirely surrounded by passive margins, and a major global long‐term sink of carbonate carbon that has evaded subduction. Quantifying the history of carbonate accumulation in the Atlantic has been limited by the absence of well‐defined regional carbonate compensation depth (CCD) models. We determine the CCD for the northern North Atlantic, central North Atlantic, and South Atlantic, and use these reconstructions to compute the carbonate carbon mass and carbonate carbon flux in a tectonic framework at 0.5 m.y. intervals since 66 Ma. We find that the total carbonate carbon mass of the Atlantic has grown 2.5‐fold from ∼1,500 Mt at 66 Ma to ∼3,800 Mt at present day. The overall Cenozoic increase in carbonate carbon flux toward the present day is punctuated by “carbonate crash” phases in the mid‐Eocene at ∼44–38 Ma and in the mid‐late Miocene at ∼19–8 Ma. During these times the flux decreases from ∼45 to ∼25 Mt C/yr, likely caused by carbonate dissolution and reductions in productivity. Reduced carbonate carbon flux in the mid‐Eocene also coincides with reduced calcification rates of small coccolithophores previously observed offshore Africa. After ∼8 Ma the carbonate carbon flux rises to a Cenozoic maximum of ∼75 Mt C/yr at ∼3 Ma, possibly driven by enhanced flux of nutrients into the ocean. Our CCD curves and the resulting carbonate accumulation history are useful for calibrating ocean chemistry models, and constraining global terrestrial weathering rates, climate perturbations, and carbon cycle models.

Quantifying skeletal density at high temporal resolution in massive scleractinian corals by using two-dimensional grid-scanning gamma densitometry

The quantification of skeletal density in massive scleractinians is necessary for a better understanding of skeletal growth in reef-forming corals. However, skeletal density is difficult to quantify and requires sophisticated analytical techniques. In this study, two-dimensional grid-scanning gamma densitometry is used for the first time, to quantify skeletal density fluctuations at higher temporal (intra-annual) resolution as compared to previous annual bulk densities determined with this approach. For testing this application and to evaluate its use for being a tool in coral sclerochronology, a colony of the widespread Atlantic massive coral Orbicella faveolata from the central Belize Barrier Reef (Central America) is herein investigated. In the studied coral, temporal resolution of individual density values corresponds to an approximately bi-weekly resolution. A long-term decline in (intra-)annual skeletal density is observed combined with reduced calcification rates. This indicates a limitation in the capability for skeletal formation in O. faveolata corals within the central Belize Barrier Reef, expressed in reduced skeletal carbonate accretion. In general, time series analyses and statistical correlations of the obtained high-resolution density datasets with skeletal growth patterns (linear extension rates, calcification rates) and geochemical (δ13C, δ18O) data reveal a complex interplay of environmental parameters, which might have controlled the skeletal density in the studied coral.

Colonies of Acropora formosa with greater survival potential have reduced calcification rates.

Coral reefs are facing increasingly devasting impacts from ocean warming and acidification due to anthropogenic climate change. In addition to reducing greenhouse gas emissions, potential solutions have focused either on reducing light stress during heating, or on the potential for identifying or engineering “super corals”. A large subset of these studies, however, have tended to focus primarily on the bleaching response of corals, and assume erroneously that corals that bleach earlier in a thermal event die first. Here, we explore how survival, observable bleaching, coral skeletal growth (as branch extension and densification), and coral tissue growth (protein and lipid concentrations) varies for conspecifics collected from distinctive reef zones at Heron Island on the Southern Great Barrier Reef. A reciprocal transplantation experiment was undertaken using the dominant reef building coral (Acropora formosa) between the highly variable reef flat and the less variable reef slope environments. Coral colonies originating from the reef flat had higher rates of survival and amassed greater protein densities but calcified at reduced rates compared to conspecifics originating from the reef slope. The energetics of both populations however potentially benefited from greater light intensity present in the shallows. Reef flat origin corals moved to the lower light intensity of the reef slope reduced protein density and calcification rates. For A. formosa, genetic differences, or long-term entrainment to a highly variable environment, appeared to promote coral survival at the expense of calcification. The response decouples coral survival from carbonate coral reef resilience, a response that was further exacerbated by reductions in irradiance. As we begin to discuss interventions necessitated by the CO2 that has already been released into the atmosphere, we need to prioritise our focus on the properties that maintain valuable carbonate ecosystems. Rapid and dense calcification by corals such as branching Acropora is essential to the ability of carbonate coral reefs to rebound following disturbance events and maintain 3D structure but may be the first property that is sacrificed to enable coral genet survival under stress.

Growth Response of Reef-Building Corals to Ocean Acidification Is Mediated by Interplay of Taxon-Specific Physiological Parameters

Ocean acidification (OA) poses a major threat to calcifying organisms such as reef-building corals, typically leading to reduced calcification rates. Mechanisms to compensate the effects of OA on coral growth may, however, involve processes other than calcification. Yet, the physiological patterns mediating coral growth under OA are not fully understood, despite an extensive body of literature characterizing physiological changes in corals under OA. Therefore, we conducted a three-month laboratory experiment with six scleractinian coral species (Acropora humilis, Acropora millepora, Pocillopora damicornis, Pocillopora verrucosa, Porites cylindrica, and Porites lutea) to assess physiological parameters that potentially characterize growth (calcification, volume, and surface area), maintenance (tissue biomass, and lipid and protein content), and cellular stress (apoptotic activity) response under ambient (pH 7.9) and low pH (pH 7.7). We identified genus- and species-specific physiological parameters potentially mediating the observed growth responses to low pH. We found no significant changes in calcification but species showed decreasing growth in volume and surface area, which occurred alongside changes in maintenance and cellular stress parameters that differed between genera and species. Acropora spp. showed elevated cellular stress and Pocillopora spp. showed changes in maintenance-associated parameters, while both genera largely maintained growth under low pH. Conversely, Porites spp. experienced the largest decreases in volume growth but showed no major changes in parameters related to maintenance or cellular stress. Our findings indicate that growth- and calcification-related responses alone may not fully reflect coral susceptibility to OA. They may also contribute to a better understanding of the complex physiological processes leading to differential growth changes of reef-building corals in response to low pH conditions.

Is the relative thickness of ammonoid septa influenced by ocean acidification, phylogenetic relationships and palaeogeographic position?

The impact of increasing atmospheric CO2 and the resulting decreasing pH of seawater are in the focus of current environmental research. These factors cause problems for marine calcifiers such as reduced calcification rates and the dissolution of calcareous skeletons. While the impact on recent organisms is well established, little is known about long-term evolutionary consequences. Here, we assessed whether ammonoids reacted to environmental change by changing septal thickness. We measured the septal thickness of ammonoid phragmocones through ontogeny in order to test the hypothesis that atmospheric pCO2, seawater pH and other factors affected aragonite biomineralisation in ammonoids. Particularly, we studied septal thickness of ammonoids before and after the ocean acidification event in the latest Triassic until the Early Cretaceous. Early Jurassic ammonoid lineages had thinner septa relative to diameter than their Late Triassic relatives, which we tentatively interpret as consequence of a positive selection for reduced shell material as an evolutionary response to this ocean acidification event. This response was preserved within several lineages among the Early Jurassic descendants of these ammonoids. By contrast, we did not find a significant correlation between septal thickness and long-term atmospheric pCO2 or seawater pH, but we discovered a correlation with palaeolatitude.

Tolerance to a highly variable environment does not infer resilience to future ocean warming and acidification in a branching coral

AbstractCoral populations from reef habitats that experience extreme daily abiotic fluctuations have been suggested as candidates to survive and proliferate under future climate change. Few studies, however, have exposed corals from dynamic environments to the synergistic effects of ocean warming and acidification to investigate whether tolerance of present‐day environmental variability is maintained under future climate stress. This study assessed the impact of RCP2.6 (+0.8°C and +25 ppm) and RCP4.5 (+1.3°C and +66 ppm) ocean warming and acidification on the survivorship, primary calcification (i.e., extension), secondary calcification (i.e., densification), and protein densities of Isopora palifera originating from two distinct reef habitats (abiotically variable reef flat vs. stable reef slope) over 9 weeks. Temperature and pCO2 were offset from a reef slope baseline temperature of 26.0°C ± 0.6°C and pCO2 concentration of 559 ± 56 ppm, incorporating natural diurnal variability. A trade‐off was observed in I. palifera originating from the reef flat, which significantly increased tissue protein densities but reduced densification relative to corals from the reef slope. Survivorship nor extension rates differed between corals originating from the variable or stable reef habitats. Mortality increased under RCP4.5 as extension rates increased, indicating that energetic investment in skeletal expansion becomes unsustainable under future climate stress. Increasing temperature and CO2 reduced calcification rates irrespective of the corals originating reef habitat suggesting with future climate change, exposure to greater environmental variability may not benefit coral calcification. These results demonstrate that tolerance to present‐day abiotic variability does not necessarily infer resilience to moderate future ocean warming and acidification conditions.

Decoupling salinity and carbonate chemistry: low calcium ion concentration rather than salinity limits calcification in Baltic Sea mussels

Abstract. The Baltic Sea has a salinity gradient decreasing from fully marine (> 25) in the west to below 7 in the central Baltic Proper. Habitat-forming and ecologically dominant mytilid mussels exhibit decreasing growth when salinity < 11; however, the mechanisms underlying reduced calcification rates in dilute seawater are not fully understood. Both [HCO3-] and [Ca2+] also decrease with salinity, challenging calcifying organisms through CaCO3 undersaturation (Ω≤1) and unfavourable ratios of calcification substrates ([Ca2+] and [HCO3-]) to the inhibitor (H+), expressed as the extended substrate–inhibitor ratio (ESIR). This study combined in situ monitoring of three southwest Baltic mussel reefs with two laboratory experiments to assess how various environmental conditions and isolated abiotic factors (salinity, [Ca2+], [HCO3-] and pH) impact calcification in mytilid mussels along the Baltic salinity gradient. Laboratory experiments rearing juvenile Baltic Mytilus at a range of salinities (6, 11 and 16), HCO3- concentrations (300–2100 µmol kg−1) and Ca2+ concentrations (0.5–4 mmol kg−1) reveal that as individual factors, low [HCO3-], pH and salinity cannot explain low calcification rates in the Baltic Sea. Calcification rates are impeded when Ωaragonite ≤ 1 or ESIR ≤ 0.7 primarily due to [Ca2+] limitation which becomes relevant at a salinity of ca. 11 in the Baltic Sea. Field monitoring of carbonate chemistry and calcification rates suggest increased food availability may be able to mask the negative impacts of periodic sub-optimal carbonate chemistry, but not when seawater conditions are permanently adverse, as observed in two Baltic reefs at salinities < 11. Regional climate models predict a rapid desalination of the southwest and central Baltic over the next century and potentially a reduction in [Ca2+] which may shift the distribution of marine calcifiers westward. It is therefore vital to understand the mechanisms by which the ionic composition of seawater impacts bivalve calcification for better predicting the future of benthic Baltic ecosystems.

The Impact of Life on Climate Stabilization Over Different Timescales

AbstractSurface life has been argued to be crucial in keeping a planet habitable in the long term. Biologically enhanced weathering compensates for increasing solar luminosity, and temperature‐dependent plant productivity weakens climate perturbations. Furthermore, a reduced calcification rate of marine organisms provides a negative feedback to rising atmospheric CO2. Here, I present a model of the long‐term carbon cycle including biological enhancement of weathering and marine calcification. Climate oscillations of periods from thousands to millions of years arise from a simple model of mountain uplift and erosion. I systematically study the influence of the biologically driven feedbacks on damping these oscillations. For oscillations of periods <2 Myr, the marine calcification feedback yields surface temperature amplitudes that are approximately 1/3 lower compared to a model that ignores this feedback. Abiotic oceans, however, would not necessarily imply a less stable climate on short timescales, due to a higher pH and thereby a higher buffer capacity. On longer timescales, the higher ocean pH would reduce climate stability, since the seafloor weathering feedback would work less efficiently. Biological enhancement of weathering helps stabilizing the climate against oscillations of periods >0.5 Myr. These findings are sensitive to the ratio of land to oceans, however. Furthermore, the mantle carbon degassing rate plays a role, since the temperature dependence of biological primary productivity may be smaller at higher temperatures. Altogether, life can be argued to stabilize the climate on timescales longer than some 100 kyr, while details depend on the geological state of the planet.

Meta-Analysis Reveals Reduced Coral Calcification Under Projected Ocean Warming but Not Under Acidification Across the Caribbean Sea

Ocean acidification and warming are two of the many threats to coral reefs worldwide, and Caribbean reef-building corals are especially vulnerable. However, even within the Caribbean, experimental acidification and warming studies reveal a wide array of coral calcification responses across reef systems and among species, complicating efforts to predict how corals will respond to these global-scale stressors. We conducted a meta-analysis to investigate the calcification responses of Caribbean corals to experimentally induced seawater ocean acidification, ocean warming, and the combination of both stressors. Calcification rates were reduced for corals reared under warming alone, but acidification and the combination of both stressors did not clearly reduce calcification rates. Calcification responses of corals collected from the Florida Keys and Belize were compared for regional differences since a greater number of studies were performed on corals collected from these two regions. Notably, corals from the Florida Keys did not exhibit reduced calcification under acidification, warming, or the combination of both stressors, while corals from Belize exhibited reduced calcification under warming alone. Further investigation of these regional trends suggests that the warming and acidification treatments employed dictated calcification responses, rather than collection region. Results from this meta-analysis are constrained by the very few studies that have been conducted within the Caribbean to assess ocean acidification and warming and the large variation in experimental procedure among studies. This meta-analysis reveals existing gaps in our understanding of how corals will likely respond to projected acidification and warming and highlights ways to improve comparability among experimental studies conducted on corals within the same region to better predict coral calcification response under global change.

Sensitivity of planktic foraminiferal test bulk density to ocean acidification

The anthropogenic CO2 accumulating in the ocean is lowering seawater carbonate ion concentration and may reduce calcification rates of marine calcareous organisms. Several proxies based on test weights of planktic foraminifera have been used to evaluate the impact of ocean acidification on these organisms. Unfortunately, because of the absence of a method to evaluate the bulk density of a test, the impact of seawater carbonate chemistry on test calcification is still not fully understood. In this study, we measured bulk densities of living Globigerina bulloides (planktic foraminifera) tests with an X-ray micro-computed tomography (XMCT) scanner and compared them with ambient seawater characteristics. Results demonstrated that test bulk densities were controlled by ambient seawater carbonate ion concentrations and that changes of test bulk densities were accompanied by changes in micron to submicron scale porosity of internal ultrastructure. These results suggest that alteration of the bulk density of foraminiferal tests due to acidification of ambient seawater can be directly observed by XMCT scanning. A useful metric of calcification intensity would therefore be physical measurements of test densities with XMCT.

The effect of ocean acidification on tropical coral calcification: Insights from calcification fluid DIC chemistry

Ocean acidification typically reduces calcification in tropical marine corals but the mechanism for this process is not understood. We use skeletal boron geochemistry (B/Ca and δ11B) to reconstruct the calcification fluid DIC of corals cultured over both high and low seawater pCO2 (180, 400 and 750 μatm). We observe strong positive correlations between calcification fluid pH and concentrations of the DIC species potentially implicated in aragonite precipitation (be they CO32−, HCO3− or HCO3− + CO32−). Similarly, with the exception of one outlier, the fluid concentrations of precipitating DIC species are strongly positively correlated with coral calcification rate. Corals cultured at high seawater pCO2 usually have low calcification fluid pH and low concentrations of precipitating DIC, suggesting that a reduction in DIC substrate at the calcification site is responsible for decreased calcification. The outlier coral maintained high pHCF and DICCF at high seawater pCO2 but exhibited a reduced calcification rate indicating that the coral has a limited energy budget to support proton extrusion from the calcification fluid and meet other calcification demands. We find no evidence that increasing seawater pCO2 enhances diffusion of CO2 into the calcification site. Instead the overlying [CO2] available to diffuse into the calcification site appears broadly comparable between seawater pCO2 treatments, implying that metabolic activity (respiration and photosynthesis) generates a similar [CO2] in the vicinity of the calcification site regardless of seawater pCO2.

A shell-formation related carbonic anhydrase in Crassostrea gigas modulates intracellular calcium against CO2 exposure: Implication for impacts of ocean acidification on mollusk calcification

Ocean acidification (OA) could decrease the shells and skeletons formation of mollusk by reducing the availability of carbonate ions at calcification sites. Carbonic anhydrases (CAs) convert CO2 to HCO3− and play important roles in biomineralization process from invertebrate to vertebrate. In the present study, a CA (designated as CgCA) was identified and characterized in Pacific oyster C. gigas. The cDNA of CgCA was of 927bp encoding a predicted polypeptide of 308 amino acids with a signal peptide and a CA catalytic function domain. The mRNA transcripts of CgCA were constitutively expressed in all tested tissues with the highest levels in mantle and hemocytes. During the early development period, the mRNA transcripts of CgCA could be detected in all the stages with the highest level in D-veliger larvae. Elevated CO2 increased the mRNA transcripts of CgCA in muscle, mantle, hepatopancreas, gill and hemocytes significantly (p<0.05) and induced the translocation of CgCA in hemocytes and mantle. Moreover, elevated CO2 also caused the decrease of intracellular Ca2+ in hemocytes (p<0.05). The inhibition of CA by acetazolamide and suppression of CgCA gene via RNA interference could increase the intracellular Ca2+ in hemocytes (p<0.05). Besides, the decrease of intracellular Ca2+ content caused by Ca2+ reagent ionomycin could affect localization of CgCA in mantle tissue. The results indicated CgCA played essential roles in calcification and elevated CO2 accelerated the mutual modulation between calcium and CgCA, implying reduced calcification rate and dissolved shells under OA.

Bioerosion: the other ocean acidification problem

Bioerosion of calcium carbonate is the natural counterpart of biogenic calcification. Both are affected by ocean acidification (OA). We summarize definitions and concepts in bioerosion research and knowledge in the context of OA, providing case examples and meta-analyses. Chemically mediated bioerosion relies on energy demanding, biologically controlled undersaturation or acid regulation and increases with simulated OA, as does passive dissolution. Through substrate weakening both processes can indirectly enhance mechanical bioerosion, which is not directly affected by OA. The low attention and expert knowledge on bioerosion produced some ambiguous views and approaches, and limitations to experimental studies restricted opportunities to generalize. Comparability of various bioerosion and calcification rates remains difficult. Physiological responses of bioeroders or interactions of environmental factors are insufficiently studied. We stress the importance to foster and advance high quality bioerosion research as global trends suggest the following: (i) growing environmental change (eutrophication, coral mortality, OA) is expected to elevate bioerosion in the near future; (ii) changes harmful to calcifiers may not be as severe for bioeroders (e.g. warming); and (iii) factors facilitating bioerosion often reduce calcification rates (e.g. OA). The combined result means that the natural process bioerosion has itself become a “stress factor”&amp;#x94; for reef health and resilience.

Physiological response of the cold-water coral Desmophyllum dianthus to thermal stress and ocean acidification.

Rising temperatures and ocean acidification driven by anthropogenic carbon emissions threaten both tropical and temperate corals. However, the synergistic effect of these stressors on coral physiology is still poorly understood, in particular for cold-water corals. This study assessed changes in key physiological parameters (calcification, respiration and ammonium excretion) of the widespread cold-water coral Desmophyllum dianthus maintained for ∼8 months at two temperatures (ambient 12 °C and elevated 15 °C) and two pCO2 conditions (ambient 390 ppm and elevated 750 ppm). At ambient temperatures no change in instantaneous calcification, respiration or ammonium excretion rates was observed at either pCO2 levels. Conversely, elevated temperature (15 °C) significantly reduced calcification rates, and combined elevated temperature and pCO2 significantly reduced respiration rates. Changes in the ratio of respired oxygen to excreted nitrogen (O:N), which provides information on the main sources of energy being metabolized, indicated a shift from mixed use of protein and carbohydrate/lipid as metabolic substrates under control conditions, to less efficient protein-dominated catabolism under both stressors. Overall, this study shows that the physiology of D. dianthus is more sensitive to thermal than pCO2 stress, and that the predicted combination of rising temperatures and ocean acidification in the coming decades may severely impact this cold-water coral species.

Mitochondrial electron transport activity and metabolism of experimentally bleached hermatypic corals

Bleached corals (Porites cylindrica and Galaxea fascicularis) were obtained through extended incubation (over 45days) under light depletion and privation: low light and dark conditions, and heat stress (32°C). The colonies in the different treatments became bleached and had reduced metabolic rates, photosynthesis, calcification and respiration; reduced biomass, zooxanthellae density, and chlorophyll a concentrations; and reduced mitochondrial electron transport system activity, which represent potential respiration rates. The most important reduction in mitochondrial electron transport activity was shown when the activities were normalized by the unit of surface and not by the unit of host protein. This result indicates that the reduction in activity could be mainly explained by the reduction of biomass and tissue thickness. However, increased Manganese Superoxide dismutase (Mn SOD) activity, a mitochondrial SOD, suggests that ROS production occurs in the mitochondria under heat stress with the consequence of potentially damaging the electron transport system. The reduced calcification rates observed are hypothesized to be the results of a decrease in the energy available for calcification due to the reduced photosynthetic rates, limiting the availability of substrates for respiration and therefore the energy production, and the decreased in the number of active mitochondrial electron transport system. Electron transport system activity associated with respiration is the basis of all metabolic processes and is not biased by incubation like traditional measurements of respiration in an aquarium. Therefore, ETSA could be used as an overall indicator of coral health, especially for host animal health.

Effects of CO2-induced pH reduction on the exoskeleton structure and biophotonic properties of the shrimp Lysmata californica.

The anticipated effects of CO2-induced ocean acidification on marine calcifiers are generally negative, and include dissolution of calcified elements and reduced calcification rates. Such negative effects are not typical of crustaceans for which comparatively little ocean acidification research has been conducted. Crustaceans, however, depend on their calcified exoskeleton for many critical functions. Here, we conducted a short-term study on a common caridean shrimp, Lysmata californica, to determine the effect of CO2-driven reduction in seawater pH on exoskeleton growth, structure, and mineralization and animal cryptic coloration. Shrimp exposed to ambient (7.99 ± 0.04) and reduced pH (7.53 ± 0.06) for 21 days showed no differences in exoskeleton growth (percent increase in carapace length), but the calcium weight percent of their cuticle increased significantly in reduced pH conditions, resulting in a greater Ca:Mg ratio. Cuticle thickness did not change, indicating an increase in the mineral to matrix ratio, which may have mechanical consequences for exoskeleton function. Furthermore, there was a 5-fold decrease in animal transparency, but no change in overall shrimp coloration (red). These results suggest that even short-term exposure to CO2-induced pH reduction can significantly affect exoskeleton mineralization and shrimp biophotonics, with potential impacts on crypsis, physical defense, and predator avoidance.

The Omega myth: what really drives lower calcification rates in an acidifying ocean

Abstract The literature on ocean acidification (OA) contains a prevalent misconception that reduced organismal calcification rates in an acidifying ocean are driven by a reduction in carbonate ion (CO32−) substrate availability (e.g. Omega or Ω). However, recent research in diverse organisms suggests that a reduction in seawater pH (i.e. increasing proton concentrations, [H+]) is the most likely driver of reduced calcification rates in these organisms. OA leads to higher [H+] in seawater which alters the proton gradient between internal cellular reservoirs and external bulk seawater, making it difficult for organisms to maintain pH homeostasis. Biologically mediated calcification is a complex process, so it is unlikely that simple CO32− substrate limitation is responsible for the observed decreases in calcification rates under OA conditions. Despite these inherent complexities, current predictions concerning the fate of calcifying organisms in an acidifying ocean have relied on the relationship between calcification rates and Ω. To more accurately predict how OA will affect the calcification of marine organisms, and consequently the global carbon cycle, we need to further elucidate the mechanisms driving observed decreases in calcification under acidified conditions.

Low calcification rates and calcium carbonate production in Porites panamensis at its northernmost geographic distribution

AbstractPorites panamensis is a hermatypic coral present in the eastern Pacific Ocean. Skeletal growth parameters have been reported, but studies of the relationship between annual calcification rates and environmental controls are scarce. In this study, we investigated three aspects of the annual calcification rates of P. panamensis: growth parameters among three P. panamensis populations; the sea surface temperature as a calcification rate control spanning a latitudinal gradient; and calcium carbonate production among three sites. Growth parameters varied among the sites due to the colony growth form. Massive colonies in the north showed a higher calcification rate than encrusting colonies in the south (mean: 1.22–0.49 g CaCO3 · cm−2 · yr−1), where variations in calcification rates were related to growth rate (0.91–0.38 cm · yr−1) rather than to skeletal density differences (overall mean ± SD, 1.31 ± 0.04 g CaCO3 · cm−3). Our results showed a positive linear relationship between annual calcification rates and sea surface temperatures within these P. panamensis populations. Differences were related to distinct oceanographic environments (within and at the entrance of the Gulf of California) with different sea surface temperature regimes and other chemical properties. Different populations calcified under different environmental conditions. Calcium carbonate production was dependent upon the calcification rate and coral cover and so carbonate production was higher in the north (coral cover 12%) than in the south (coral cover 3.5). Thus, the studied sites showed low calcium carbonate production (0.25–0.43 kg CaCO3 · m−2 · yr−1). Our results showed reduced calcification rates, regional temperature regime control over calcification rates, different growth forms, low coral cover and low calcium carbonate production rates in P. panamensis.