Coral Mucus and Bacteria: A Symbiosis?
Anyone who has ever scraped or cut their skin on a living coral can attest to the malignant nature of what should otherwise be a minor abrasion or cut. These scrapes don’t heal very quickly, and can become infected very easily. The reason is that the mucus coating secreted by the coral harbors a dense population of bacteria that apparently gains protection and nutrition from said mucous. The relationship between mucous bacteria and coral is only now beginning to be unraveled.
That there were at least 16 presentations and posters that focused specifically on coral mucus at the ICRS, shows the level of interest this topic has been receiving within the marine biological world.
Several of the research projects concluded that the bacterial populations within the coral mucus are in fact mostly unique and independent of the bacterial populations that are found in nearby environments (surface sediments, biofilms, water column, etc). This indicates that the coral mucus/ bacteria relationship is more complex and specific than previously thought.
An important research topic on coral mucus bacteria revolves around their relationship to coral immune health and disease prevention. It is speculated that the mucus bacteria are somehow capable of thwarting coral infections (other bacteria, protozoans, etc), by maintaining a balanced population within the mucus (perhaps a similar theory of using ‘probiotics’ as preventative measure?). Whatever the mechanism, it appears that the coral mucus and resident bacteria population acts as a protective barrier against pathogenic invaders.
Another important role of coral mucus is to act as a medium for nutrient transport. I assume that this perhaps relates to the mucus’ ability to help adhere to and coat food particles, thereby aiding and enabling digestion. This is a conjecture on my part, but it seems like a logical process.
A research poster entitled “A Quantitative Approach Linking Coral Mucus And Their Symbiotic Zooxanthellae in Response To Environmental Change” found that 45% of the daily fixed carbon (i.e. the food produced from photosynthesis), was incorporated into coral mucus in Montastrea annularis. This demonstrates the vital importance that coral polyps place on mucus production.
In the poster mentioned above, the researchers determined that as water temperatures were increased by 1.5 degrees C, mucocyte density (specialized cells that produce mucus) increased, while zooxanthellae density decreased. They draw the conclusion that increasing temperatures cause M. annularis to rely more upon heterotrophy (eating), than upon autotrophy (zooxanthellate photosynthesis). Bleached corals were found to have lower densities of mucocytes, but the remaining mucocytes were greatly enlarged, indicative of highly increased mucus production per mucocyte.
A thought that popped into my head while reading the results of this paper, in combination with the other information I picked up in several lectures on the topic, is that perhaps the coral actually digests the bacteria that live in the mucus layer, thereby adding an additional symbiotic food source (Zooxanthellae being the other “food” producer). It seems possible that by providing a suitable medium for bacterial growth, the coral is able to culture its own “bacteria garden” that is consumed at a rate that is balanced with mucus production and the bacterial growth within it.
I hope that future research continues to look into this area, as it is possible that there is still a piece of the coral nutrition puzzle that is still waiting to be unraveled.