So what is a holobiont?
Lynn Margulis coined the term in 1993 to better describe how macro-species (like a human or a panda) universally live in symbiosis with other (usually) micro-species. She claimed that all individuals who participate in a particular symbiosis are bionts and the entire organism that is comprised of these bionts is a holobiont.
By that definition, everything is a holobiont because every macro-species lives in symbiosis with some other species.
You’re a holobiont. That panda over there, wearing pants, is a holobiont.
But if you looked up holobiont, you’d find a lot about corals and not much else. That’s because scientists who study corals were the first to realize that the concept of the holobiont helps us understand the complex and apparently Darwinianly (is that a word?) aberrant organism that lives on the cusp between sea and land. They understood that thinking about a coral in isolation didn’t actually explain what happened evolutionarily to that species over time (it doesn’t really work for any species, human or panda, but it is more apparently in corals).
Before we started to think about holobionts (as they related to corals then expanding to other species), it was hard to see how other macro-organisms lived in symbiosis with other organisms. The micro-organisms on a panda or human aren’t visibly doing things for us. And Germ Theory has taught us that microbes cause disease. The only good microbe is a dead microbe. We assumed that human and panda macro-organisms responded to environmental selective pressure in genetic isolation. It was human genes that changed. Not a group of genes from many species. A sterile panda is a healthy panda. Or so we thought.
For a long time, we were able to ignore our fellow bionts unless they gave us trouble as unnecessary nuisances we couldn’t get rid of completely.
But corals are different. We couldn’t ignore the coral bionts.
See, corals aren’t one “animal” that responds to selective pressure, but a amalgamation of many “animals” (I am using animal loosely here. Most of what makes up coral is not animal at all. Species is the better word, but I want you to think of individual and specific organisms. And since Darwin talked about animals, it’s easier to use that term when discussing why corals don’t necessarily fit into Darwinian theory). All of which rely on each other for survival as a whole.
So, ecosystem (while a perfectly good descriptor) isn’t an accurate way to describe coral. In an ecosystem, when one animal drops out, there is a period of ecological dis- ease, but the entire ecosystem doesn’t necessarily fall completely apart. In the case of corals, when one component was adversely affected by pollution or a heating ocean or pH level change, the entire coral system died.
It was the equivalent of killing off all the red blood cells in your body. Sure you look the same, but you are going to die pretty quickly. You need those cells in an intricate way that can’t allow for compensation over time by another cell type.
The coral holobiont (and any holobiont for that matter) is different in the sense it isn’t comprise of all coral cells but many different species/animals which are distinct genetically from the actual coral yet functionally are completely intertwined.
Symbiosis is another good word. But symbiosis in our cultural thought implies that one organism is stronger or can survive without the other.
Think about holobiont like this: wholes who act as an essential part of a larger whole and can’t survive out of the larger whole.
It’s an ecosystem. It’s symbiosis. It’s a holobiont.