Corals are colonies of small marine animals called polyps. They secrete calcium carbonate which is hardened to form skeleton like porous structure which eventually builds up coral reefs. It may take centuries to form a coral reef. Skeleton provides support and protection for every polyp residing in its pores. Polyps as such don't leave alone, they are accompanied by eukaryotic and prokaryotic organisms such as fishes, echinoderms, worms, crustaceans, mollusks, sponges, tunicates, dinoflagellates, diatoms, macroalgae, cyanobacteria or blue green algae, bacteria, methanogenic archaea and viruses. Coral and associated living forms constitute a complex system which is termed as holobiont. Their interactions make the coral a unique and very diverse ecosystem on the Earth. About 20% marine flora and fauna population is known to be resident of coral reefs. Corals are found principally in Indian, Pacific and Atlantic oceans and Red Sea; warm, shallow and sunny marine waters are ideal for coral growth. Coral reefs have also been detected in deep and cold regions of Pacific Ocean. Islands like Maldives, Bahamas, Bermuda, Andros, Bay and Fiji; Great Barrier, Belize Barrier are some of the examples of coral reefs inhabited by human population.
Each coral has its specific bacterial community. Photosynthetic and heterotrophic bacteria are found to inhabit porous skeletonous structures of corals. Both types of bacteria are able to fix atmospheric and aquatic nitrogen (N2) into utilizable form (ammonia) for polyps. Polyps in absence of nitrogen fixing bacteria would otherwise have been devoid of nitrogen nutrition. Heterotrophic bacteria are also used as food by other organisms like sponges or mollusks of holobiont. Phototrophic bacteria are usually cyanobacteria like Symbiodinium which by the process of photosynthesis fix carbon dioxide to carbohydrates which is principle nutritional requirement of polyps. Many species of Symbiodinium are present in holobiont. Carbohydrates formed during photosynthesis are major constituents of mucus secreted by polyps; actually this mucus supports the growth of heterotrophic bacteria. Polyps also secrete slime which contains sugars like mucus as well as calcium carbonate, nucleic acids, proteins and fatty substances. This organic material forms the basis of nutrition of polyps and its microflora. Photosynthetic macroalgae or zooxanthellae are also present as holobiont community members. Besides photosynthesis, they control the supply of carbon dioxide required in calcium carbonate secretion and coral build up. Healthy coral represents such symbiotic interactions of all living members.
Man-made pollution such as sewage disposal in aquatic bodies and harmful excess application of fertilizers and pesticides have resulted in increase in concentration of nitrogen and phosphorus in ocean waters which are generally present in limiting amounts. Imbalance of ocean ecosystems including coral community is one of the bad effects of such pollution. It has caused increase in growth of heterotrophic bacteria with certain harmful effects. The climate change and global warming are responsible for increase in temperature of ocean water, exerting limits beyond the tolerance of corals. As a result corals worldover are in stressed condition, similar to our body's immunocompromised or immunodeficient state. Under such stress conditions, coral are prone to infections by opportunistic heterotrophic bacteria residing them. Some serious bacterial diseases of corals are described here.
Coral bleaching: It indicates dissociation of coral and cyanobacterial relationship. Bleaching or loss of color of coral, which is actually a color of photosynthetic pigment of associated cyanobacteria, is caused by pathogenic Vibrio shiloi and V. coralliilyticus bacteria. Disease is severe in hot weather such as in Indian Ocean and Red Sea; pathogens attack and kill cyanobacteria (Symbiodinium spp.) thus depriving coral of nutrition, in result coral also die. Disease has been native to Mediterranean coral reefs.
Yellow blotch disease: It is caused by Vibrio alginolyticus and infectious spread in coral reef is dependent on temperature. Appearance of yellow blotches or stains on coral tissue alongwith bleaching is evident by this infection. It has been detected in Caribbean corals.
Black band disease: Appearance of black band across coral colonies during summer is caused by bacterial consortia. Consortium may contain phototrophic toxin producing cyanobacteria. It is not known to cause by single bacterium. Different species of sulfur reducing (Desulfovibrio spp.) and sulfur oxidizing (Beggiatoa spp.) bacteria along with cyanobacteria (Oscillatoria spp.) have been detected in some consortia. They produce lethal anoxic and toxic sulfur rich environment for growth of corals. Disease is predominant in corals Diploria strigosa of Belize reefs.
White pox disease: Serratia marcescens is the causative agent of white pox. This bacterium is enteric and associated with enteric diseases of humans and animals; hence occurrence of white pox disease is also indicator of marine pollution by industrial and domestic sewage.
White plague disease: Coral plague is characterized by sharp line between diseased coral tissue and healthy tissue. Rapid progression of lesion or line indicates fatality of disease and associated mortality. Aurantimonas coralicida and Thalassomonas loyana are causative bacteria of white plague.
White band disease: Disease is caused by many species of vibrios (especially, Vibrio carchariae) and rickettsias in corals of Caribbean islands. Characteristic feature of disease is that it starts at the base and progresses towards the tip of coral colony.
Coral diseases caused by bacteria are responsible for diminishing the population of important coral species and irreversible disturbances of coral ecosystem. Being epidemic, they are infecting other healthy coral communities with great progression and resulting in greater economic and ecological loss.
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