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The world is simply too vast for any organism to find its way alone. There are too many turns,
too many challenges...inevitably, we all reach points where we need just a little bit of help.
And the microcosmos is no exception. The world’s earliest relationship builders were microbes,
who—through some combination of chance and biology—found ways to use each other.
The word for this, of course, is symbiosis, and it is not restricted to microbes. You
can watch many a nature documentary and find many examples of birds or fishes or
large mammals that form symbiotic relationships. Perhaps one feeds on the pests living on another,
and the other provides shelter. But there are many symbiotic relationships
that we can see under the microscope as well. One of the most famous examples of symbiosis
is that of Paramecium bursaria. The Paramecium bursaria looks like an ellipse with hairs
poking out of it, which means that for the most part, it looks just like any other paramecium.
But there’s one big distinction: the Paramecium bursaria is green. And it’s green thanks to a
whole host of algae called Chlorella that have taken up residence inside its body because,
well, generations ago, many generations ago, a Paramecium ate some algae. But instead of
digesting the algae, the Paramecium just didn’t. And instead, it started to consume the products
of the Chlorella’s synthesis and then it passed that algae on to the next generation.
And in return, the Chlorella gets to use the paramecium—its former predator—as
a form of protection and transportation. But scientists have separated the bursaria from
it’s algal symbionts and, it turns out, both of them can survive on their own. Because each of the
members of the relationship benefits, it’s called mutualism, but because it is optional it’s called
“facultative.” So the paramecium bursaria and its chlorella are an example of facultative mutualism.
So, this all seems pretty simple and straightforward.
Symbiosis is a biological exchange of goods and/or services, an uncomplicated
means of partnering up to help survive. But relationships are always easier in theory. In
practice, they can be much more complicated and difficult to understand. And for us,
as outsiders, it takes considerable work to decipher just what kind of relationship
microbes are forming with each other. James, our master of microscopes, found
this ciliate in some mud. And if you’re wondering what those fuzzy rod shapes are inside of it,
those are prokaryotes stuffing up the ciliate. But besides knowing that those are prokaryotes
in there, we can’t really tell much about what they’re doing or what their relationship is with
their ciliate host. Do those prokaryotes help the ciliate survive in the low-oxygen environment of
mud? Are they why we can’t see any food vacuoles in the ciliate? Are they even helping each other
at all or are they parasitizing each other? Trying to decipher all of that just by looking at
it, would be like trying to describe the complex dynamics of a family just by looking at their
portrait. So to dive deeper into what’s going on, James has isolated and frozen some of these cells
in tiny tubes, storing them for a future day when he’ll be able to study them more closely.
Now, one of the disorienting aspects of symbiosis at the microbial level
is that eukaryotic organisms are full of parts that look a bit like other organisms.
And that is not a coincidence. In some cases, things like chloroplasts and mitochondria are
the product of endosymbiosis, a prokaryote that got consumed so completely by its symbiotic
relationship inside another organism that it has become an organelle. The process of endosymbiosis
is complete when the endosymbiont has lost most of its genes or given them over to its host.
And that endosymbiotic history complicates our human desire to distinguish between an organelle
and an organism. But fortunately,
we do have molecular tools that allow us to investigate further.
This diatom, Epithemia gibba, has had at least two distinct things noted about it:
one is its unusual capacity to convert extremely stable but biologically useless diatomic nitrogen
into actually useful nitrogen compounds, we call this “nitrogen fixing”. And the other thing that
people have noted about it is the round bits that appear like beady bubbles inside of it.
When scientists looked closer, they found that the round bodies had a double membrane around them,
suggesting that they might be another organism living inside the diatom.
So they spun the diatoms and treated them with chemicals to extract these mystery parts and
study the genes inside them, ultimately finding that the diatom was occupied by cyanobacteria.
These cyanobacteria, it turns out, are responsible for the diatom’s ability to fix nitrogen. But
when it comes to the question of organelle versus organism, well this cyanobacteria is kind of both.
It doesn’t appear to perform photosynthesis in the diatom, its main function is to fix
nitrogen. But to enter into that relationship with its host, the cyanobacteria had to change
its own nitrogen-fixation schedule to match the diatom’s daily photosynthetic activities.
The extent of the cyanobacteria’s adaptation to its host and the fact that it is passed on from
generation to generation of diatom suggests that while those cyanobacteria might still
be their own selves, they may be on their way to becoming an organelle, fully endosymbiosed
into a piece of Epithelia gibba. And other mysteries remain,
even when they seem initially clear-cut. This Loxodes rostrum is a rare find for us,
but it is also quite distinctive because it bears not one, but two different types of algae in it.
And now, that might seem like yet another version of the Paramecium bursaria and
Chlorella relationship we saw earlier. We have a large ciliate occupied by algae,
combining their respective bodies and machinery to supply different forms of nutrition to each other
that neither would get on its own. There’s nothing weird about this, right? Nothing
startling, nothing extraordinarily strange. Except that Loxodes rostrum are also full of
pigments to avoid light because they don’t like light. They practically melt when they’re exposed
to light. And that’s a problem for algae because light is kind of part of their thing. Like,
it’s an important part of photosynthesis. So how do they survive together?
Why are they doing this? We don’t know. There must have been some kind of compromise.
The nature of that compromise remains unknown to us. Maybe they’re just like that couple who
seem deeply incompatible, and yet somehow, they need each other and they make it work.
At least, that’s how it looks from the outside. But in the future,
generations of Loxodes from now, maybe this relationship will look different.
And perhaps, like every other relationship, it will evolve,
helping both organisms confront new challenges that the world will inevitably throw at them.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
And thank you to Fabulous for sponsoring this episode.
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