Rosenberg et al. (2007) have described coral reefs as ?the largest structures made by living creatures. Biogenic activities over millennia has lead reefs to evolve to become a complex, productive, yet fragile marine ecosystem with a biodiversity to rival that of a terrestrial rainforest (Rohwer et al. 2002, Allen and Steene, 2007). Microorganisms are critically important components of coral reef ecosystem research has provided ample evidence that corals support mutualistic symbiotic relationships with autotrophic Symbiodinium dinoflagellates, commonly referred to as zooxanthellae to continue their hermatypic (reef building) host’s ecological success. These are quite a diverse group, spanning several different clades in the Symbiodinium genus.
Zooxanthellae inhabit host tissue cells to form a symbiosome: a host membrane which encompasses the endosymbiont, separating it from host cytoplasm. Carbohydrates pass from symbiont to host through these membranes and it has been proposed that coralline amino acids stimulate this transfer. Zooxanthellae occupy the primary producer niche, fixing carbon dioxide to release energy and oxygen for use by the host and gaining carbon dioxide, nitrogenous compounds and shelter in return. This activity has culminated in reefs having very high productivity despite residing in oligotropic waters of little nutritive value.
In addition to Symbiodinium, corals support diverse and abundant bacterial, archaeal and viral communities. A complex symbiosis between the coral animal, endobiotic alga and an array of microorganisms helps in coral reef formation. Within individual coral colonies, bacteria have been isolated from the skeleton, tissues the lipid-rich surface, mucus microlayer and surrounding seawater. Naturally associated bacterial communities play beneficial and detrimental roles in the holobiont; for example, isolates have antimicrobial properties to act as antagonists against opportunistic pathogens. There is also sufficient evidence to suggest that bacteria could act as alternative sources of nutrition when nutrients are scarce and that some residents possess the ability to fix nitrogen or carbon for consumption.
Coral reefs produce a surplus quantity of carbon and are a massive primary producing ecosystem despite residing in oligotrophic ocean stretches of poor nutritive value. This carbon comes from a variety of sources, with the majority assimilated from photosynthetic products of symbiotic zooxanthellae located in the gastrodermal cell layer. Additional carbon sources include suspension feeders and bacterial aggregates on detritus particles in surrounding seawater which become trapped in the mucoid layer and ingested by mesentery filaments which transfer the matter toward the mouth. These aggregates account for up to 23% of bacterial activity on atoll reefs. There is considerable evidence to suggest that bacterial growth on the mucus surfaces enhance the probability of particle aggregation and the likelihood that mucus acts as a trap in order to create these aggregations which are used as a source of food for the coral, given that the highest densities of aggregates have been isolated from around the polyp mouths.Another element of coral nutrition that has been under recent investigation is nitrogen fixation. Nitrogen is a critical component of any organism’s makeup since by being part of the protein structure nitrogen is involved with most biochemical processes in an organism. In the ocean, nitrogen is primarily available as gaseous dinitrogen which needs to be fixed to form ammonium in order to be made available to living organisms. This procedure can only be carried out by some bacteria and archaea which possess the nitrogen fixing enzyme nitrogenase.
One bacterial group involved with nitrogen fixation are symbiotic cyanobacteria, which have been identified in large volumes in Montastrea cavernosa epithelial tissue layers to carry out photosynthetic dependent nitrogen fixation. This location makes them ?an integrated member of the coral holobiont. So they are likely to be permanent coral residents.