A physiological evaluation of carbon sources for calcification in the octocoral Leptogorgia virgulata (Lamarck)

Extracellular carbonic anhydrase activity promotes a carbon concentration mechanism in metazoan calcifying cells

Many calcifying organisms utilize metabolic CO₂ to generate CaCO₃ minerals to harden their shells and skeletons. Carbonic anhydrases are evolutionary ancient enzymes that have been proposed to play a key role in the calcification process, with the underlying mechanisms being little understood. Here, we used the calcifying primary mesenchyme cells (PMCs) of sea urchin larva to study the role of cytosolic (iCAs) and extracellular carbonic anhydrases (eCAs) in the cellular carbon concentration mechanism (CCM). Molecular analyses identified iCAs and eCAs in PMCs and highlight the prominent expression of a glycosylphosphatidylinositol-anchored membrane-bound CA (Cara7). Intracellular pH recordings in combination with CO₂ pulse experiments demonstrated iCA activity in PMCs. iCA activity measurements, together with pharmacological approaches, revealed an opposing contribution of iCAs and eCAs on the CCM. H⁺-selective electrodes were used to demonstrate eCA-catalyzed CO₂ hydration rates at the cell surface. Knockdown of Cara7 reduced extracellular CO₂ hydration rates accompanied by impaired formation of specific skeletal segments. Finally, reduced pH_i regulatory capacities during inhibition and knockdown of Cara7 underscore a role of this eCA in cellular HCO₃^- uptake. This work reveals the function of CAs in the cellular CCM of a marine calcifying animal. Extracellular hydration of metabolic CO₂ by Cara7 coupled to HCO₃^- uptake mechanisms mitigates the loss of carbon and reduces the cellular proton load during the mineralization process. The findings of this work provide insights into the cellular mechanisms of an ancient biological process that is capable of utilizing CO₂ to generate a versatile construction material.

Biomineralization: Integrating mechanism and evolutionary history

Calcium carbonate (CaCO₃) biomineralizing organisms have played major roles in the history of life and the global carbon cycle during the past 541 Ma. Both marine diversification and mass extinctions reflect physiological responses to environmental changes through time. An integrated understanding of carbonate biomineralization is necessary to illuminate this evolutionary record and to understand how modern organisms will respond to 21st century global change. Biomineralization evolved independently but convergently across phyla, suggesting a unity of mechanism that transcends biological differences. In this review, we combine CaCO₃ skeleton formation mechanisms with constraints from evolutionary history, omics, and a meta-analysis of isotopic data to develop a plausible model for CaCO₃ biomineralization applicable to all phyla. The model provides a framework for understanding the environmental sensitivity of marine calcifiers, past mass extinctions, and resilience in 21st century acidifying oceans. Thus, it frames questions about the past, present, and future of CaCO₃ biomineralizing organisms.

The Biology and Evolution of Calcite and Aragonite Mineralization in Octocorallia

Octocorallia (class Anthozoa, phylum Cnidaria) is a group of calcifying corals displaying a wide diversity of mineral skeletons. This includes skeletal structures composed of different calcium carbonate polymorphs (aragonite and calcite). This represents a unique feature among anthozoans, as scleractinian corals (subclass Hexacorallia), main reef builders and focus of biomineralization research, are all characterized by an aragonite exoskeleton. From an evolutionary perspective, the presence of aragonitic skeletons in Octocorallia is puzzling as it is observed in very few species and has apparently originated during a Calcite sea (i.e., time interval characterized by calcite-inducing seawater conditions). Despite this, octocorals have been systematically overlooked in biomineralization studies. Here we review what is known about octocoral biomineralization, focusing on the evolutionary and biological processes that underlie calcite and aragonite formation. Although differences in research focus between octocorals and scleractinians are often mentioned, we highlight how strong variability also exists between different octocoral groups. Different main aspects of octocoral biomineralization have been in fact studied in a small set of species, including the (calcitic) gorgonian Leptogorgia virgulata and/or the precious coral Corallium rubrum. These include descriptions of calcifying cells (scleroblasts), calcium transport and chemistry of the calcification fluids. With the exception of few histological observations, no information on these features is available for aragonitic octocorals. Availability of sequencing data is also heterogeneous between groups, with no transcriptome or genome available, for instance, for the clade Calcaxonia. Although calcite represents by far the most common polymorph deposited by octocorals, we argue that studying aragonite-forming could provide insight on octocoral, and more generally anthozoan, biomineralization. First and foremost it would allow to compare calcification processes between octocoral groups, highlighting homologies and differences. Secondly, similarities (exoskeleton) between Heliopora and scleractinian skeletons, would provide further insight on which biomineralization features are driven by skeleton characteristics (shared by scleractinians and aragonitic octocorals) and those driven by taxonomy (shared by octocorals regardless of skeleton polymorph). Including the diversity of anthozoan mineralization strategies into biomineralization studies remains thus essential to comprehensively study how skeletons form and evolved within this ecologically important group of marine animals.

Carbonic Anhydrase as a Biomarker of Global and Local Impacts: Insights from Calcifying Animals

The emission of greenhouse gases has grown in unprecedented levels since the beginning of the industrial era. As a result, global climate changes, such as heightened global temperature and ocean acidification, are expected to negatively impact populations. Similarly, industrial and urban unsustainable development are also expected to impose local impacts of their own, such as environmental pollution with organic and inorganic chemicals. As an answer, biomarkers can be used in environmental programs to assess these impacts. These tools are based in the quantification of biochemical and cellular responses of target species that are known to respond in a sensitive and specific way to such stresses. In this context, carbonic anhydrase has shown to be a promising biomarker candidate for the assessment of global and local impacts in biomonitoring programs, especially in marine zones, such as coral reefs, considering the pivotal role of this enzyme in the calcification process. Therefore, the aim of this review is to show the recent advances in the carbonic anhydrase research and the reasons why it can be considered as a promising biomarker to be used for calcifying organisms.

Physiological and biogeochemical traits of bleaching and recovery in the mounding species of coral Porites lobata: implications for resilience in mounding corals

Mounding corals survive bleaching events in greater numbers than branching corals. However, no study to date has determined the underlying physiological and biogeochemical trait(s) that are responsible for mounding coral holobiont resilience to bleaching. Furthermore, the potential of dissolved organic carbon (DOC) as a source of fixed carbon to bleached corals has never been determined. Here, Porites lobata corals were experimentally bleached for 23 days and then allowed to recover for 0, 1, 5, and 11 months. At each recovery interval a suite of analyses were performed to assess their recovery (photosynthesis, respiration, chlorophyll a, energy reserves, tissue biomass, calcification, δ¹³C of the skeletal, δ¹³C, and δ¹⁵N of the animal host and endosymbiont fractions). Furthermore, at 0 months of recovery, the assimilation of photosynthetically acquired and zooplankton-feeding acquired carbon into the animal host, endosymbiont, skeleton, and coral-mediated DOC were measured via ¹³C-pulse-chase labeling. During the first month of recovery, energy reserves and tissue biomass in bleached corals were maintained despite reductions in chlorophyll a, photosynthesis, and the assimilation of photosynthetically fixed carbon. At the same time, P. lobata corals catabolized carbon acquired from zooplankton and seemed to take up DOC as a source of fixed carbon. All variables that were negatively affected by bleaching recovered within 5 to 11 months. Thus, bleaching resilience in the mounding coral P. lobata is driven by its ability to actively catabolize zooplankton-acquired carbon and seemingly utilize DOC as a significant fixed carbon source, facilitating the maintenance of energy reserves and tissue biomass. With the frequency and intensity of bleaching events expected to increase over the next century, coral diversity on future reefs may favor not only mounding morphologies but species like P. lobata, which have the ability to utilize heterotrophic sources of fixed carbon that minimize the impact of bleaching and promote fast recovery.

A new coral carbonic anhydrase in Stylophora pistillata

Scleractinian corals are of particular interest due to their ability to establish an intracellular mutualistic symbiosis with phototrophic dinoflagellates and to deposit high rates of calcium carbonate in their skeleton. Carbonic anhydrases have been shown to play a crucial role in both processes. In this study, we report the molecular cloning and characterization of a novel α-CA in the coral Stylophora pistillata. This enzyme shares homologies with the human isoform CA II and is referred to as STPCA-2. STPCA-2 is 35.2 kDa and possesses all key amino acids for catalytic activity. With a ratio between catalytic and Michaelis constants (k(cat)/K(m)) of 8.3.10(7) M(-1)  s(-1) is considered as highly active. Owing to its intracellular localisation in the oral endoderm and in the aboral tissue, we propose that STPCA-2 is involved in pH regulation and/or inorganic carbon delivery to symbiont and calcification.

Roles of amyloid beta-peptide-associated oxidative stress and brain protein modifications in the pathogenesis of Alzheimer's disease and mild cognitive impairment

Oxidative stress has been implicated to play a crucial role in the pathogenesis of a number of diseases, including neurodegenerative disorders, cancer, and ischemia, just to name a few. Alzheimer disease (AD) is an age-related neurodegenerative disorder that is recognized as the most common form of dementia. AD is histopathologically characterized by the presence of extracellular amyloid plaques, intracellular neurofibrillary tangles, the presence of oligomers of amyloid beta-peptide (Abeta), and synapse loss. In this review we discuss the role of Abeta in the pathogenesis of AD and also the use of redox proteomics to identify oxidatively modified brain proteins in AD and mild cognitive impairment. In addition, redox proteomics studies in in vivo models of AD centered around human Abeta(1-42) are discussed.

Localization and diurnal variations of carbonic anhydrase mRNA expression in the inner ear of the rainbow trout Oncorhynchus mykiss

Physiological studies have suggested that carbonic anhydrase (CA) plays a central role in otolith biomineralization via ion transport. However, the presence and exact function of CA in the inner ear have not been determined. In the present study, to investigate the localization of CA and its involvement in otolith calcification, we cloned two cDNAs encoding CAs from the rainbow trout sacculus. These two cDNAs, designated rainbow trout CAa (rtCAa) and rtCAb, both had an open reading frame encoding 260 amino acids with a sequence identity of 78%. Remarkably, rtCAb has a high degree of homology (82%) with "high activity CA" in the zebrafish, and its mRNA expression showed variation in the range 1.9-11.4 x 10(4) copies/ng total RNA in the sacculus. In contrast, rtCAa mRNA was constantly expressed at approximately 3 x 10(4) copies/ng total RNA. In situ hybridization revealed that rtCAb mRNA was strongly expressed in the distal squamous epithelial cells and transitional epithelial cells, except the mitochondria-rich cells, whereas, rtCAa was localized in extrasaccular tissue. These results suggest that the rtCAb isozyme is involved in the daily increment formation and calcification of otoliths via phase and spatial differences of the bicarbonate supply to the endolymph.

Two atypical carbonic anhydrase homologs from the planula larva of the scleractinian coral Fungia scutaria

In cnidarians, the enzyme carbonic anhydrase (CA) is important to inorganic carbon (Ci) flux in processes including calcification and dinoflagellate symbiont photosynthesis. Although CA is known to function in Ci delivery to symbionts in adults with mature symbioses, it is not known when CA becomes active in this capacity during the onset of symbiosis in developing hosts. We identified two CA cDNA sequences from the planula larvae of the Hawaiian scleractinian coral Fungia scutaria. Expression of these larval CAs did not differ between infected and uninfected larvae or vary over the course of infection. Bioinformatic analyses of the two homologs showed that the sequences are unusually short and are missing some residues that support active site structure in other CAs. This is the first description of a short form of CA. Phylogenetic analyses of the larval CAs grouped them with membrane-bound homologs from vertebrates. Studies in other calcifying cnidarians have identified membrane-associated CAs as functioning in calcification, and therefore the two larval CAs could play a role in the onset of calcification during metamorphosis. A longer CA isoform was amplified from adult F. scutaria cDNA but not from larvae, suggesting that the longer form is not expressed in larvae. The longer form grouped with cytosolic CAs including a symbiotic anemone homolog implicated in Ci delivery to dinoflagellate symbionts. The apparent absence of this "symbiosis" CA in larvae suggests that the Ci supply mechanism is not active during the initial onset of the association.

Calcification in the planula and polyp of the hydroidHydractinia symbiolongicarpus(Cnidaria, Hydrozoa)

This study examines calcification in planulae and polyps of the hydroid Hydractinia symbiolongicarpus. We observed that established colonies produce a crystalline mat on their substratum and that crystals visible by polarized light microscopy occur in the vacuoles of the gastrodermal cells of both polyps and planulae. The crystalline mat was found by infrared spectroscopy to contain calcium carbonate in the form of aragonite. The composition of the vacuolar crystals and the cellular mechanisms for manufacturing them were explored by alteration of calcium levels in the environment and by the use of pharmacological agents (acetazolamide, caffeine, DIDS, diltiazem, nifedipine, procaine, Ruthenium Red, ryanodine and verapamil) that affect cellular uptake and transport of calcium and bicarbonate. The results indicated that the crystals in the vacuoles contained calcium carbonate. The gastrodermal cells are hypothesized to serve as a physiological sink for excess calcium that enters the organism during motility, secretion and metamorphosis of the planula, and to create a crystalline substratum for the colony of polyps.

Effects of enzyme and anion transport inhibitors on in vitro incorporation of inorganic carbon and calcium into endolymph and otoliths in salmon Oncorhynchus masou

The transepithelial transport of inorganic carbon to endolymph and its subsequent deposition on otoliths were pharmacologically examined by incubating the sacculus containing an otolith with NaH(14)CO(3). Calcium incorporation was also studied. Carbon incorporation into endolymph and otoliths was saturated with increased concentrations of bicarbonate ions in the incubation medium and was followed by the Michaelis-Menten equation with a K(m) of 26.3 mM and 0.4 mM, respectively. Carbon incorporation decreased with an increase in chloride concentrations in the medium. Calcium incorporation was not affected by chloride and bicarbonate ions up to 10 mM. Higher concentrations of bicarbonate ions reduced calcium incorporation into both fractions. Carbon incorporation into endolymph and otoliths was inhibited by acetazolamide, disulfonate stilbenes (DIDS and SITS), thiocyanate, and ouabain. Calcium incorporation was not affected by these inhibitors. Amiloride inhibited carbon incorporation into otoliths alone. These results suggest that HCO(3)(-)-ATPase and Cl(-)/HCO(3)(-)-exchangers are involved in the transepithelial transport of bicarbonate ions to the endolymph. Carbonic anhydrase was also suggested to play a role in carbonate production for otolith calcification.

Sources and mechanisms of inorganic carbon transport for coral calcification and photosynthesis

The sources and mechanisms of inorganic carbon transport for scleractinian coral calcification and photosynthesis were studied using a double labelling technique with H(14)CO(3) and (45)Ca. Clones of Stylophora pistillata that had developed into microcolonies were examined. Compartmental and pharmacological analyses of the distribution of(45)Ca and H(14)CO(3) in the coelenteron, tissues and skeleton were performed in dark or light conditions or in the presence of various seawater HCO(3)(−) concentrations. For calcification, irrespective of the lighting conditions, the major source of dissolved inorganic carbon (DIC) is metabolic CO(2) (70–75% of total CaCO(3) deposition), while only 25–30% originates from the external medium (seawater carbon pool). These results are in agreement with the observation that metabolic CO(2) production in the light is at least six times greater than is required for calcification. This source is dependent on carbonic anhydrase activity because it is sensitive to ethoxyzolamide. Seawater DIC is transferred from the external medium to the coral skeleton by two different pathways: from sea water to the coelenteron, the passive paracellular pathway is largely sufficient, while a DIDS-sensitive transcellular pathway appears to mediate the flux across calicoblastic cells. Irrespective of the source, an anion exchanger performs the secretion of DIC at the site of calcification. Furthermore, a fourfold light-enhanced calcification of Stylophora pistillata microcolonies was measured. This stimulation was only effective after a lag of 10 min. These results are discussed in the context of light-enhanced calcification. Characterisation of the DIC supply for symbiotic dinoflagellate photosynthesis demonstrated the presence of a DIC pool within the tissues. The size of this pool was dependent on the lighting conditions, since it increased 39-fold after 3 h of illumination. Passive DIC equilibration through oral tissues between sea water and the coelenteric cavity is insufficient to supply this DIC pool, suggesting that there is an active transepithelial absorption of inorganic carbon sensitive to DIDS, ethoxyzolamide and iodide. These results confirm the presence of CO(2)-concentrating mechanisms in coral cells. The tissue pool is not, however, used as a source for calcification since no significant lag phase in the incorporation of external seawater DIC was measured.