The ability to host or interact with other species is solely dependent on the complexity of the architecture of coral colonies. Architecture also creates the potential for diversity and unique habitats that will entice species. Gates and Ainsworth describe symbiomes as “individuals whose functional attributes reflect the collective performance limits set by the multispecies assemblage they host” therefore the many different coral species have many different architectural attributes and work together in different ways in different locations . Structure of the coral community differs from each reef and associated ecosystems due to the abiotic and biotic needs of each species within it. These multivariate functions come together to develop feedback loops with environ mental gradients that create patterns of vertical zonation in coral reefs .
By reviewing the different species of corals and the different environments that they grow in, the ecosystem can play a role in the colony architecture formation. The branching corral varieties are more complex architectures in comparison to the mounding and lobed species. These species also have less variability based on ecosystems and more uniform within the colony. But how much does architectural intricacy impact on the multispecies symbioses that live within the coral symbiomes ?
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"Biological Diversity: The nature and taxonomic composition of coral symbiomes".
Corals of both the massive and branching varieties are greatly affected by changes within their environments. Branching types are sensitive to changes and grow quickly while massive types can resist stress better but grow slower. Data regarding symbioses in endosymbiotic dinoflagellate communities shows that the species are very different. Acropora millepora hosts multiple Symbiodinium types from many different lineages and new individuals are added from the environment for each new generation. However, the environmentally strong, Porites lobata, is the host to only closely related individuals that are directly related as parent and offspring. The significant differences in characteristics such as diversity, position, ability to be cultured, and ways in which the host acquires Symbiodinium can be considered based on other characteristics such as integration and evolution, specialization, functional range, and physiological performance. It is considered that the complexity of the architecture of the corals that provide basis for variations in diversity of communities of Symbiodinium due to differences in quality and quantity of architecture and growth .
Another symbiotic relationship between corals and invertebrates showed that up to 102 species were associated with corals. Fifteen percent of these species were only found within the coral communities indicating specialization with the coral. The study shows that there is great intricacy when multi-species groupings within in corals. The host and symbiomes relationship is very closely intertwined and can be unique. As the two species interact and become specialized the biodiversity of the ecosystem becomes even more heightened. Colony morphology shows how frequent symbiotic associations occur and how they are influenced greatly by thresholds of the environment and symbiomes of coral. One must also consider the makeup of the guest species and their relationship with the host, as well as interactions with other species and individuals in the colony as they compete for resources. These interactions are motivating factors for stratification, compartmentalization and specializing within the community .
The symbiomes of coral, phenotype made up of many different species, works together to fill a function niche. The relationship allows the corals to have better success, fitness, and adventive that it would not have alone through production of nutrition, thermal protection, and physical defense .
Corals thrive in tropical reef waters which are generally limited in nutrients such as nitrogen and carbon. Microbes provide and cycle nutrients in all ecosystems and coral reefs as well. One study found a direct link between symbiotic microbes and the coral host. The ability of diazotrophic bacteria to improve nitrogen levels benefits Montastrea cavernosa. Colonies of coral also see functional benefits from the addition of ammonia by fish and bivalves that live within the community, but this relationship is beneficial for other reasons. Improved ammonia levels improve the growth of corals, increase tissue thickness and the density of endosymbiotic dinoflagellate density. Dinoflagellates also provide carbon to the community for respiration .
Ostreobium sp. is beneficial because it helps to protect against temperature stress and has the ability for photoacclimation. These two benefits produce blooms in times of bleaching and disease. These bacteria provide carbon to the corals when dinoflagellates are unable to provide it during bleaching or another incapacitation. Mutualism between the coral and Ostreobium sp. is either limited to times of need .
Shifts in bacterial load often occur based on situational needs. Bacteria can switch between those that produce antibiotics and those that are pathogenic based on the needs of the host and that helps produce defense. Some members of the bacterial community are more beneficial than others but in some coral/ bacterial communities the symbiosis helps to support host immunity. For example, bacteria within the mammalian gastrointestinal system help to stimulate the immune system and therefore protect the host from pathogens. While microbes within the digestive sections of corals have not yet been identified, assumptions state that they may work in similar manners as those within mammals . Considerations that help to support this assumption are the marked similarities between changes in bacteria in both the mammalian gastrointestinal system and within coral reefs based on age. One study considered the size of the coral colony size as an indicator of age and reviewed the relationship between the coral sand microbes living within it. It was found that there was differing amount of diversity of bacteria based on the size of the colony from similar habitats in intertidal areas. Small colonies considered to be juvenile included a less rich mix of bacteria than medium and larger colonies. Diversity increased as the size and therefore age of the colonies increased. Williams et al. note “These changes appear to resemble a successional process which occurs over time, like that observed in the ageing human gut”
There are other aspects of defense that occur in the coral symbiomes. An innovative coral–hydrozoan relationship takes place only with Acropora muricata of the Great Barrier Reef. The large population of hydrozoan nematocytes give improved defenses on the coral apical tip. Also, crabs associated with the corals defend against crown-of-thorns predation, the potential overgrowth of algae, and sediment removal .
Resistance to stress has been studied with endosymbiotic dinoflagellate communities. Clade D Symbiodinium has been found to have better resistance to temperature changes than clade C varieties. Corals that host the clade D bacteria have better thermal resistance but do not grow as well. The symbiosis of clade D Symbiodinium with coral might be opportunistic. The bacteria are often found to exploit weakened corals that have been degraded, stressed and unhealthy .
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- Gates, R., & Ainsorth, T. (2011). The nature and taxonomic composition of coral symbiomes as drivers of performance limits in scleractinian corals. Journal of Experimental Marine Biology and Ecology, 94-101.
- Harborne, A., Mumby, P., Zychaluk, K., Hedley, J., & Blackwell, P. (2006). Modeling the Beta Diversity of Coral Reefs. Ecolgy, 2871-2881.
- Williams, A., Brown, B., Putchin, L., & Sweet, M. (2015). Age-Related Shifts in Bacterial Diversity in a Reef Coral. PLoS One, 1-16.
