SYNOPSIS. Photosynthesis and calcification in zooxanthellate scleractinian corals and coral reefs are reviewed at several scales: cellular (pathways and transport mechanisms of inorganic carbon and calcium), organismal (interaction between photosynthesis and calcification, effect of light) and ecosystemic (community primary production and calcification, and air-sea CO2 exchanges).

The coral host plays a major role in supplying carbon for the photosynthesis by the algal symbionts through a system similar to the carbon-concentrating mechanism described in free living algal cells. The details of carbon supply to the calcification process are almost unknown, but metabolic CO2 seems to be a significant source. Calcium supply for calcification is diffusional through oral layers, and active membrane transport only occurs between the calicoblastic cells and the site of calcification. Photosynthesis and calcification are tightly coupled in zooxanthellate scleractinian corals and coral reef communities. Calcification is, on average, three times higher in light than in darkness. The recent suggestion that calcification is dark-repressed rather than light-enhanced is not supported by the literature. There is a very strong correlation between photosynthesis and calcification at both the organism and community levels, but the ratios of calcification to gross photosynthesis (0.6 in corals and 0.2 in reef communities) differ from unity, and from each other as a function of level.

The potential effect of global climatic changes ( p CO2 and temperature) on the rate of calcification is also reviewed. In various calcifying photosynthetic organisms and communities, the rate of calcification decreases as a function of increasing p CO2 and decreasing calcium carbonate saturation state. The calculated decrease in CaCO3, production, estimated using the scenarios considered by the International Panel on Climate Change (IPCC), is 10% between 1880 and 1990, and 9–30% (mid estimate: 22%) from 1990 to 2100. Inadequate understanding of the mechanism of calcification and its interaction with photosynthesis severely limits the ability to provide an accurate prediction of future changes in the rate of calcification.

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Photosynthesis and Calcification at Cellular, Organismal and Community Levels in Coral Reefs: A Review on Interactions and Control by Carbonate Chemistry

A characteristic of nerve cells is their ability to propagate an action potential and to communicate through specialized contact zones, called synapses. Synapses have received much attention from neurobiologists over the last 40 years, after their existence was ascertained by electron microscopy (De Robertis and Bennett 1954; Palade and Palay 1954). At the synapse, information is transferred from one cell to the contiguous one by neurotransmitters, viz. acetylcholine, biogenic amines and amino acids, or neuromodulators. These are mostly small peptides acting either alone or in combination (Furness et al. 1988; H6kfelt 1992). Thus, nerve cells communicate mostly by the intermediary of chemical signals. These important concepts have had a tremendous impact on the \"Zeitgeist\" of the neurosciences and have led many scientists to try to unravel the exact mechanisms of synaptic transmission. Furthermore, the discovery that the symptoms in Parkinson's disease are the consequence of a decrease in the concentration of the neurotransmitter dopamine in the basal ganglia, and that an improvement can be achieved by a dopamine precursor that passes through the blood-brain barrier (Hornykiewicz 1966), has helped to shape the opinion that many neurological and psychiatric diseases can be explained as defects in neurotransmitter systems. In the meantime, major efforts have been directed at understanding the way in which neurotransmitters

Calcium-binding proteins: selective markers of nerve cells.

Calcium pumps of the plasma membrane (also known as plasma membrane Ca2+-ATPases or PMCAs) are responsible for the expulsion of Ca2+ from the cytosol of all eukaryotic cells. Together with Na+/Ca2+ exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca2+concentration. Like the Ca2+ pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca2+ from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological need...

https://www.physiology.org/doi/pdf/10.1152/physrev.2001.81.1.21

Role of alternative splicing in generating isoform diversity among plasma membrane calcium pumps.

Supramolekulare Chemie ist die Chemie der intermolekularen Bindung und beschaftigt sich mit Strukturen und Funktionen von Einheiten, die durch Assoziation von zwei oder mehr chemischen Spezies gebildet werden. Molekulare Erkennung in Ubermolekulen, die bei der Rezeptor/Substrat-Bindung entstehen, beruht auf dem Prinzip der molekularen Komplementaritat, wie es bei der Erkennung spharischer, tetraedrischer und linearer Substrate durch Rezeptoren, Corezeptoren, Metallorezeptoren und amphiphile Rezeptoren vorgefunden wird. Supramolekulare Katalyse mit Rezeptoren, die reaktive Gruppen tragen, bewirkt Bindungsspaltungen und -knupfungen durch Cokatalyse. Lipophile Rezeptoren konnen als selektive Carrier fur verschiedenartige Substrate verwendet werden, und sie ermoglichen den Aufbau von Transportsystemen, die mit einem Elektronen- oder Protonengradienten oder mit einem Photoprozes gekoppelt sind. Wahrend Endorezeptoren Substrate durch „konvergente Wechselwirkungen” in Molekulhohlraumen binden, werden bei Exorezeptoren die Substrate durch Wechselwirkungen zwischen den Ausenflachen von Rezeptor und Substrat gebunden. Demgemas lassen sich neue Typen von Rezeptoren, z. B. die Metallonucleate, entwickeln. In polymolekularen Aggregaten konnen Rezeptoren, Carrier und Katalysatoren zu molekularen und supramolekularen Funktionseinheiten fuhren, die strukturell organisierte und funktionell integrierte chemische Systeme sind („supramolekulare Architektur”). Erkennungs-, Translokations- und Transformationsprozesse mit molekularen Funktionseinheiten werden unter dem Gesichtspunkt analysiert, ob sie durch Photonen, Elektronen oder Ionen ausgelost werden konnen. Auf diese Weise lassen sich die Gebiete der molekularen Photonik, Elektronik und Ionik definieren. Fuhrt man photosensitive Gruppen ein, ergeben sich photoaktive Rezeptoren, die sich zur Lichtumwandlung und Ladungstrennung eignen. Redoxaktive, langkettige Polyolefine – „molekulare Drahte” – konnen Elektronen, z. B. durch Membranen, ubertragen. Tubulare Mesophasen, die durch Stapelung geeigneter makrocyclischer Rezeptoren entstehen, konnen Ionenkanale bilden. Bei acyclischen Liganden gibt es das Phanomen der molekularen Selbstorganisation, was zu Komplexen mit doppelt-helicaler Struktur fuhrt. Derartige Entwicklungen im Bereich des „molekularen und supramolekularen Designs und Engineerings” lassen auf photonische, elektronische und ionische molekulare Funktionseinheiten hoffen, die hochselektive Erkennungs-, Umwandlungs- und Ubertragungsprozesse – Verarbeitung von Signalen und Informationen auf molekularer Ebene – ausfuhren konnen.

Supramolekulare Chemie – Moleküle, Übermoleküle und molekulare Funktionseinheiten (Nobel-Vortrag)†

In the course of evolution, Ca2+ has emerged as the most versatile intracellular messenger. Its concentration within cells is controlled by reversible binding to specific classes of proteins that act as Ca2+ sensors to decode its information before passing it on to targets. The decoding operation is based on specific conformational changes in the sensor proteins. Other proteins intrinsic to membranes simply control Ca2+ concentration without processing its message, by transporting it across membrane boundaries. They are located in the plasma membrane and in the membranes of the organelles (the endo(sarco)plasmic reticulum, the mitochondria, the nuclear envelope), which play distinctive roles in the cellular homeostasis of Ca2+. Ca2+ is an ambivalent signaling agent. It carries information to virtually all processes important to cell life (e.g., it couples excitation to contraction, secretion, gene transcription, and controls enzyme activity through protein phosphorylation-dephosphorylation), but also transmits signals that promote the programmed demise of cells. When escaping control, Ca2+ also precipitates toxic cell death.

Generation, control, and processing of cellular calcium signals.

The calcium signal.

The primary structure of a region of the erythrocyte plasma membrane calcium pump which is phosphorylated by the cAMP-dependent protein kinase has been determined. The sequence is A-P-T-K-R-N-S-S(P)-P-P-P-S-P-D. The site is located between the calmodulin binding domain and the C-terminus of the ATPase. The ATPase is phosphorylated only at this site by the cAMP-dependent protein kinase, and the phosphorylation is inhibited by calmodulin. The effect of the phosphorylation is to decrease the Km for Ca2+ of the purified ATPase from about 10 microM to about 1.4 microM and to increase the Vmax of ATP hydrolysis about 2-fold.

Primary structure of the cAMP-dependent phosphorylation site of the plasma membrane calcium pump

The amino acid sequence of the phosphorylation domain of the erythrocyte Ca2+ ATPase.

The highly purified Ca2+-pumping ATPase from human erythrocyte membranes displays two p-nitrophenylphosphatase (NPPase) activities: one of these requires calmodulin and low concentrations of Ca2+, while the other requires ATP and higher Ca2+ concentrations. The free Ca2+ concentrations required for the expression of the two NPPase activities differed very substantially. Both activities required high free Mg2+ concentrations and displayed simple hyperbolic kinetics toward p-nitrophenyl phosphate (NPP) with a Km in the range of 5-20 mM. Study of the dependence of the calmodulin-stimulated NPPase on Mg2+ and NPP indicated that the Mg-NPP complex is not the substrate of the enzyme. Under conditions optimal for ATP-requiring NPPase (1 mM free Ca2+), the Ca2+-ATPase displayed simple hyperbolic kinetics with a low Km for ATP. NPP competitively inhibited this activity, and the apparent Ki for NPP was less than 1 mM, much lower than the Km for NPP as a substrate. If NPP were inhibiting the ATPase by binding at the same site at which NPP is hydrolyzed, the apparent Ki for NPP as inhibitor would be the same as the Km for NPP as substrate. (Under these circumstances, the apparent Ki and the Km can be directly compared, since NPP was being hydrolyzed under both circumstances.) Since Ki was much lower than Km, NPP must have been inhibiting at another site; thus, these data show the existence of two types of NPP sites on the enzyme, one at which NPP is hydrolyzed and the other at which it inhibits ATP hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Two Ca2+-requiring p-nitrophenylphosphatase activities of the highly purified Ca2+-pumping adenosinetriphosphatase of human erythrocyte membranes, one requiring calmodulin and the other ATP.

John T. Penniston

Papers

The calcium signal.

Primary structure of the cAMP-dependent phosphorylation site of the plasma membrane calcium pump

The amino acid sequence of the phosphorylation domain of the erythrocyte Ca2+ ATPase.

Two Ca2+-requiring p-nitrophenylphosphatase activities of the highly purified Ca2+-pumping adenosinetriphosphatase of human erythrocyte membranes, one requiring calmodulin and the other ATP.

Comparison of rates of proton ejection and oxygen consumption within 300 msec after oxygenation of beef heart mitochondria.