My trip to Australia is part of a larger project to identify the ‘good’ microbes associated with corals. It may seem odd at first to talk about ‘good’ bacteria and viruses. They get us sick and spoil our food, and we’re constantly trying to get rid of them with antibacterial hand soap. Since scientists first began describing them, we have largely focused on these ‘bad’ microbes, for perfectly valid reasons. It’s (relatively) easy to make an association between a disease and the odd but plentiful bacterium that’s teeming in a sick person. It’s also natural to study how it works in order to stop it. On the other hand, the bacteria that just seem to hang out in healthy people don’t seem very interesting. As a result, when we talk about microbes, we tend to forget about the vast majority of them, which don’t cause disease. And if they don’t harm us, then why does it even matter if we forget about them?
The answer is that many microbes dismissed as loiterers are in fact hard-working members of the community known as the holobiont. This term for the combination of host and all associated microbes recognizes that the physiology and evolution of all members of such a community are linked together in a highly complex and stable way. We’ve only recently begun to appreciate just how important the microbiome can be. In various plants and animals, they produce essential nutrients, help digest food, compete with pathogens, interact with the immune and nervous systems, and even cue developmental processes. Without microbes, we ‘macrobes‘ would have to be extremely different. In hindsight, this shouldn’t be too surprising. There is practically no surface on Earth that isn’t naturally covered in microbial life, and larger organisms evolved in this context. If a given ‘function’ required for animal life is already performed by microbes, there is no reason to re-evolve the mechanisms to do it yourself.
How does this relate to corals? Well, changes in ambient water temperature, nutrient levels, or fish communities can lead to shifts in the functions being performed by the microbial community, and we think this may be part of the reason we’re seeing strange diseases popping up around the world. The best example that we already know of is the phenomenon known as coral bleaching. As I mentioned in a previous post, corals form a close partnership with single-celled algae called Symbiodinium. This partnership, or symbiosis (hence the algae’s name), has existed for so long that most corals simply cannot live without their algae. However, when sea temperatures rise, runoff pollutes the water, or the corals become otherwise stressed, the algae is often expelled from the coral tissue. Because Symbiodinium is actually the source of most of the coral’s colors, the coral tissue is left white, or bleached, following this expulsion. In this bleached state, the coral becomes even more stressed, and eventually dies if the symbiosis is not re-established.
Coral bleaching is an interesting case of sickness caused by the lack of a microbe, and we believe there are other similar cases waiting to be discovered. The large cell and population size of Symbiodinium, plus its conspicuous coloration, makes its absence more noticeable in unhealthy reefs. But the microbes we’re looking for are much more subtle. For one thing, they’re hiding in a crowd. No environmental factor will change the fact that hundreds to thousands of bacterial species are associated with coral at any given moment. And although many of them are somewhat transient, that doesn’t mean they aren’t performing an important function. Often, bacteria that are almost completely unrelated can fill the same role in a community. If two bacteria can each provide the same nutrient or chase off the same pathogens, lacking only one of them won’t affect coral health. So we need to use some tricky analyses to tease this all apart. I’ll go into the details some other time for those brave readers!
The first stage of this research, though, begins with sample collections. And even though it’s tough, stressful work, I think I’m the man for the job!