The zooxanthelle (alga of the kind Symbiodinium ) is a unicellular alga in Symbiose with the Corail. In the surface layers of the hot seas, deprived of the base of the marine food chain that is the Plancton, the zooxanthelles ones develop by absorbing the Carbon dioxide released by the corals and provide in various return Nutriment S to their host.

At the hard corals builders of reef (Scléractiniaires or Madréporaires) and at unquestionable Actiniaires, Corallimorphaires, Zoanthaires and Octocoralliaires ( Alcyonacea and Gorgonacea ) the endoderm of the polyps contain, without exception of the unicellular algae.

Introduction

These algae are brown Dinoflagellés gilded kind Symbiodinium . It are generally brown, the color most adapted to the absorption of the blue light. The zooxanthelles ones, which lives in symbiosis with the Corail, play a very important part in the metabolism of the animal. This association between the alga and the coral constitutes a true symbiosis because association benefits the two partners. Thus the metabolisms of the host (the Coral) and of the symbion (the alga) strongly interfere.

The zooxanthelles ones are localized in endoderm and are intracellular. They are already present in egg right after fecundation, in the ectoderm of the planula which they leave to gain the endoderm when the larva fixed itself. They multiply by scissiparity in the adult and are present in all the endodermal but more abundant cellular layers in the oral disc, the tentacles and the coenosarque one (zones more exposed to the light). This vegetable biomass stored by the polyps varies according to the species and can reach 45 to 60% of the protein biomass of the coral. The density of zooxanthelles and their distribution in fabrics depend on physiology on the polyp, of the conditions of illumination. The light seems to be the ecological factor most important. A lack of light involves a stronger dispersion of zooxanthelles and a reduction in their number. Contrary too much light can lead to an abundance of zooxanthelles and a regression of the polyps. When the conditions are bad, the zooxanthelles ones degenerate and are rejected: the corals bleach. This situation is however reversible if the conditions become again favorable.

Pigments

Like all the photosynthetic plants, the zooxanthelles ones contain pigments as the Chlorophylle S has and C , of the Caroténoïde S and the Xanthophylle S. These pigments are the molecules responsible for the Photosynthèse.

The fact of having several different pigments makes it possible zooxanthelles to adapt to the various conditions of illumination in term of quality and quantity and this whatever the depth. This famous photosynthesis which can be summarized simply with the manufacture of complex carbonaceous compounds starting from water and carbonic gas under the energy action of the light is not without influence on the metabolism of the host who lodges the algae. Thus the respiratory exchanges, the general Metabolism as well as the process of calcification are closely related to the metabolism of the zooxanthelles ones.

The corals synthesize also toxic substances. One supposes that these substances protect them from the pédateurs, even of the bacterial infections. Among these substances one finds quinoneimines, highly coloured, like the ascididemine, the calliactine, the preclinic kuanoniamine, currently object of tests like anti cancer patients. It is thought that the zooxanthelles ones take part partially even completely in the synthesis of these substances.

Respiratory exchanges

One of under product of photosynthesis is gas oxygen and this one can diffuse alga towards the cytoplasm of the cells of the polyp thus constituting a considerable contribution for the Respiration of the polyp.

The day the polyp absorbs oxygen of sea water and receives in more that produced by the zooxanthelles ones whereas the night only oxygen coming from sea water can be used since photosynthesis is stopped. This implies a diffusion of gases through the membrane of the zooxanthelles ones.

All in all, it results during the day a positive respiratory assessment i.e. the production of oxygen of association coral-zooxanthelle is higher than consumption. In general the oxygen uptake is strong but varies from one species to another. For example, Acropora are large-scale consumers what explains their low resistance to a rise in temperature making fall the oxygen concentration dissolves.

Metabolic roles

The zooxanthelles ones practice photosynthesis and zooxanthelles corals seem to have a photosynthetic output as good as the free Dinoflagellé S. In addition to the production of gas oxygen the zooxanthelles ones manufacture other molecules which will be advantageous with the polyps: glycerol, glucose, amino-acids, peptides etc

These molecules, to be used by the polyps, must imperatively cross the membrane of the alga to find in the Cytoplasme cells of the polyp. The transfer is supported by the digestive enzymes secreted by the coral, which make the walls cellular permeable algae to the metabolites. the compounds produced by the zooxanthelles ones are largely used by the polyp for its clean metabolisms of the Glucide S, Protide S and Lipide S. Finalement the symbiotic algae can be regarded as the organic carbon tank of the coral.

Contribution of the coral to the metabolism of the zooxanthelles ones

Nitrogenized and phosphorated waste of the coral is used partly by the alga. It was thought first of all that the carbonic gas produced by the breathing of the polyp was absorbed by the alga for photosynthesis but in fact it would seem rather than the source of CO2 used by the alga comes from bicarbonates of sea water. Several work clearly showed that the phosphates (phosphorated compounds) are used by the zooxanthelles ones for their metabolism protidic. In the same way at certain coral species ammonia (nitrogenized compound) is excreted ata rate ten times less at the light than with the darkness indicating than the photosynthesis of zooxanthelles uses ammonia. The nitrites, nitrates and amino-acids can also be used to a lesser extent.

External bonds

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