<a href=showthread.php?s=&postid=15067608#post15067608 target=_blank>Originally posted</a> by HighlandReefer
I find it surprising that the coral only use the few selected dinoflagellates as symbioants. I would speculate that there may be some bacteria that have a symbiotic relationship with the coral also. I have seen no research to back this up though. 
I guess my question would be better stated as, Why do the coral pick dinoflagellates as opposed to other available algae and bacteria?
The symbiotic relationship between zooxanthellae and corals has developed
over hundreds of millions of years. In the case of zooxanthellae, they provide nutrition to their coral hosts via photosynthesis and the host provides a nice place for them to live.
Mutualistic relationships do exist between virtually all animals and beneficial bacteria and I suspect that they do exist between bacteria and corals, too.
The most obvious example of a mutualistic (meaning symbiotic) relationship between bacteria and animals can be seen in the digestive tract. Humans, for example, have hundreds of different species of bacteria that live in the GI tract (approximately three pounds of bacteria to be exact). We provide them a place to live and they provide their services in digesting food. We have evolved to be omnivores and our GI bacteria are capable of handling a wide array of food items. The same cannot be said for all animals. Just look at panda bears for example. Or look at all the different genera of reef fishes, many of which are quite specialized in their diet.
If we want to look for a mutualistic relationship between bacteria and corals, I suggest we first look at the mutualistic relationship between humans and the hundreds of different species of bacteria that live on the surface of our skin. These bacteria help us resist pathogens by denying them space for colonization. Might not the same be true for corals?
By the way, using an antibiotic bath soap makes no sense whatsoever. Unless you are getting ready to perform surgery, you should use regular soap that does not have any antibiotic properties. The bacteria that live on the surface of the skin (and hair) are good guys. They help us fight off the bad bacteria that would do us harm. However, even the good guys can cause infection if they get in the wrong place. That why you have to use an antibiotic spray or cream on open cuts. That's also why you never, ever want the bacteria that inhabit the GI tract to get loose inside your body.
It's all a matter of location. Inside the GI tract, the digestive bacteria are good guys doing the job they have evolved to do over millennia. Outside the GI tract they can cause big trouble. The same goes for the bacteria on the surface of our bodies. They're fine as long as you don't cut yourself but they can and will cause infection if they get beneath the skin.
Might not the same hold true for corals? Maybe most of the bacteria that live in coral slime are good guys that help their coral hosts fight off pathogens? Maybe it's only when this defensive system breaks down that pathogens are able to overwhelm the coral? We know a lot about human flora but little about the same bacterial mutualism in other animals, especially marine animals.
