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Bacteria and Corals

By Eric Borneman. Posted to ReefKeepers emailing list, Wednesday 13th January 1999.

The reef building corals have long been known to harbor various strains and species of symbiotic algae known as zooxanthellae within their tissue. These algae are known to be responsible for provided much on the energy needed to the coral animal for its metabolic needs. But there may be new symbionts on the block - bacteria!

Corals have also long been known to harbor many different species of bacteria in their gastric cavity and on their surface, where their carbohydrate rich mucus is exploited as a medium for microbial growth. Other invertebrates, such as sponges, may harbor photosynthetic bacteria within their tissue, much as the corals harbor algae. It has also been known that many of these bacteria not only provide a substantial nutrient source for the corals, but also provide many antibacterial substances against competing strains of bacteria. Originally, the gorgonians and sponges were described to produce antimicrobial substances against bacteria, and then the soft corals were found to also produce a number of such compounds. In 1997, Esther Koh analyzed over 100 species of stony corals, and found them all to produce antimicrobial compounds, as well. Thus, corals were found to not only produce and be endowed with their own microbial defenses, but the strains which inhabited their surface were found to be producing their own defenses.

Perhaps most importantly, research has shown that these various antibiotic substances act with a fair degree of specificity. They are not typically active against gram-negative bacteria which, not coincidentally, are often the ones found on the coral surface. Furthermore, some degree of specificity was found whereby the bacteria most susceptible to these compounds were species not typically associated with the corals. There is quite a bit of specificity and variance between and within individual cases. It has also been found that the various microbes are capable of transforming various organic material, perhaps even the mucus itself, into different nutritional and protective substances which the coral needs for its own use; amino acids, vitamins, and secondary metabolites. Finally, corals are capable of modifying the mucus secretory cells to change the amount and composition of their mucus, perhaps allowing for the selective growth of one type of bacteria over another.

Late last year (1998), several papers were published which provided even more interesting findings regarding the various microbes that live ubiquitously with corals. First, the coral Paraerythrodium fulvum fulvum was found to produce antibiotic compounds which showed a high degree of specificity toward microbes with which it was not ordinarily associated. This further confirmed some of the earlier works, but provided very clear evidence of coral's tolerance of normally associated bacteria, and suggested that a "normal flora" is present and perhaps even symbiotic.

Corals are also known to utilize dissolved organic matter for a substantial amount of their nutritional needs. In another work, it was found that the bacteria living on the surface, in the gastric cavity, and within the tissues of the coral Galaxea fascicularis were highly efficient at the uptake of dissolved organic matter. They not only took up DOM from the water, but also that released by the coral (predominantly carbon), and quickly transformed it into their own nutritional biomass. This highly nutritious microbial mass can then be available for the coral. This type of recycling is precisely the same type of integral nutritional conservation which has been thought to be the sole domain of the coral/algae symbiosis. As work continues, it will be quite interesting to see the degree and exact nature of the new coral/bacterial associations. As far as I know, this is the first time these works have been brought to the attention of the aquarium hobby.

Killer article. Might be a good source to start looking into RTN(I know we all hate this acronym). Immune systems of the corals.

Yeah...coral immunity - what a concept ;-)

Go to and see article on such by me -n-Jonathan Lowrie....If you are interested in refs on coral and invert immunity, I probably, humbly, have one of the largest collections of that literature available...

Is it possible through shipping we are killing these bacterium. Once in our systems that particular bacteria may not be available.

Yes, and no. There is a lot of work to be done in this area, but if you look at the works which have been done, there are definitely some bacteria genera which are typical - Vibrios and Psuedomonas are very common colonizers, and Ritchie, et. al., showed a trend from Psuedomonas to Vibrios when coral tissue becomes necrotic - Vibrios exploit that material better...Ducklow and Mitchell showed Vibrios (mostly V. alginolyticus and V. parahaemolyticus) to compose over 50% of the flora on healthy corals...others have found similar least some species are "swarmers" and use their flagella to actively stay around coral mucus rather than being sort of "drifters"...

Obviously, the ability of the coral to change its mucus content and composition will influence those types that colonize to some well as the nutritional status of the coral and, hence, the composition of the mucus produced. Shipping plays its role in that mucus is not well flushed and becomes thick...bacteria in logarhythmic growth can quickly become problematic in that they affect coral respiration by their numbers....if anything, shipping doesn't eliminate the bacteria but causes a dangerous amplification of their numbers. Over time, though, we may see the extermination of one species by substrate dominance and without access to potential recruits, yes, we might be eliminating some of these "normal" flora. However, I don't think anyone has (or could) actually get into a repertoire of tanks to analyze the average numbers and types of bacteria available or present. Marine bacteriology is pretty cloaked in mystery yet...Most of the work has been towards those species which show a potential for human pathogenicity and identification of species is very complex and many species are unnamed/unreported. The studies of coral mucus bacteria usually identify only by genera or, sometimes, even less. That is one of the hurdles we are trying to overcome in our RTN bacteria work...Rob Toonen, who has our RTN cultures in perma-storage, has talked to a lot of people, asking for help, and they mostly laugh at the implications...."Impossible," they intimate. We do have a protocol now, and you can do some work via selective platings and antibiotic sensitivity assays, as Jonathan and I did originally, but apparently amplification of DNA using restriction length fragment polymorphism is required for good results and that costs $$$. AH for an aquarium industry grant! LOL


Created by liquid
Last modified 2006-11-24 18:40