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Sometimes James, our master of microscopes,
likes to go through his samples with the lowest magnification objective, looking at the organisms
as they’re floating in a Petri dish so that he can see everything from a distance and get a sense
for what mysteries await him when he zooms in. And during one of his recent Petri dish scans,
he found this stunning ciliate, autofluorescing a whole crayon box’s worth of colors under our
combination of UV, blue, green, and red light. And when you switch to white light,
the ciliate is still quite colorful. Maybe not the same fluorescent rainbow it was before, but
full of varied shades of purple like a blueberry bush ripening in summer.
But the ciliate didn’t seem to be doing so well in the Petri dish.
So for James to be able to take this video with the clarity and focus it needs, he first had to
move the ciliate to a slide—a procedure that is quite normal and safe for most microbes.
But for our already dying ciliate, it proved to be too much. As soon as it hit the slide,
it burst open. And all of those little blueberries came pouring slowly out as well.
The ciliate died too fast for us to be able to identify it. There was just too
much damage, erasing all clues to its identity. But the ciliate wasn’t the only organism in the
video. Those round bits of purple were once alive as well. And what remains of their color provides
more than enough information to identify them. These are purple sulfur bacteria. And ciliates
are not the only danger they face. No, its biggest foe is something even more pedestrian than that.
You see, something you need to know about the purple sulfur bacteria
is that it’s a bit of a contrarian, specifically when it comes to photosynthesis.
Now, what do you think of when you hear the word “photosynthesis?”
You might think of trees and plants and leaves. Or since you’re here with us right
now on this journey, maybe you’re thinking of algae and cyanobacteria and euglena.
What most of us are probably imagining could boil down to the color green. Lots and lots of green.
And for good reason. Chlorophyll, the pigment that absorbs light and kicks off the reactions that
drive photosynthesis, is green. And that green has come to dominate the world that we live in.
But there’s another world too, inhabited by purple sulfur bacteria.
And these purple sulfur bacteria can also do photosynthesis. They have bacteriochlorophyll.
But they also contain carotenoids, pigments that create colors ranging from red to purple, and that
help the bacteria harvest light and get protection from the bacteria’s primary foe: oxygen.
“But wait!” you might be saying to yourself. “How can an organism be photosynthetic and so afraid of
oxygen? Doesn’t photosynthesis create oxygen?” And yes, you would be correct—most of the time.
The most common form of photosynthesis in our world is oxygenic photosynthesis. It makes oxygen.
This is what happens in all those plants and cyanobacteria, where energy from light
sets off a chain reaction involving water and carbon dioxide. The end result is a sugar for
the organism and oxygen for the atmosphere. That is the world we live in, a world where oxygen is a
necessity and readily supplied by photosynthesis. But purple sulfur bacteria, contrarian that it is,
prefers the worlds in this world that do not have oxygen. Our samples, for example,
came from the sediments at the bottom of ponds and puddles, places where oxygen does not arrive,
consumed as it has been by the organisms above. Down at the bottom of their ponds,
the purple sulfur bacteria do their own form of photosynthesis called anoxygenic photosynthesis.
And they’re not the only organisms to prefer this oxygen-less approach to making their own
food—confusingly enough, there are also green sulfur bacteria and purple non-sulfur bacteria,
both of which also do anoxygenic photosynthesis though sometimes with their own twists.
For the purple sulfur bacteria, the key ingredient is, well it’s sulfur. You may not think of sulfur
as an essential element to life, but that’s because we have bodies borne out of a world
full of oxygen. But in the stratified world of the purple sulfur bacteria, the void left by
a lack of oxygen instead is filled by sulfur. So where a normal photosynthetic organism would
consume water to drive their photosynthesis, they consume sulfurous chemicals like hydrogen sulfide,
which you might recognize from its rotten egg smell. And when they’re done making
their own food, the byproduct of their endeavors is not oxygen, it is sulfur.
And because they live in an environment that is constantly changing, purple sulfur bacteria are
able to store up sulfurous molecules for whatever the microbial equivalent of a rainy day is.
All of this sulfur-based photosynthesis makes purple sulfur bacteria something better than
just contrarian: they are a good neighbor, helping to consume hydrogen sulfide that might be toxic
and clearing out the waters for their upstairs neighbors that
require a more oxygenated environment to survive. Though that role, that place as a good neighbor,
is based on the ecological neighborhoods we have today, where lawns of oxygen give way to lawns
of sulfur. That neighborhood has gone through some changes though, and there was a time before
oxygen’s dominance, where purple sulfur bacteria were less of a contrarian than they are now.
In 2005, researchers uncovered photosynthetic pigments, fossilized and preserved in rocks
that are more than a billion years old, lying in a large basin in Australia. These fossils suggested
that purple sulfur bacteria may have existed in ancient oceans during a period
when oxygen was still rising in the world but had not come to take over the oceans.
The fossils suggest an ancient world that would be unrecognizable to us. In fact, we probably
wouldn’t be able to survive there. But as the world has shifted, the purple sulfur bacteria
found a way to shift with it, to make space for themselves even as new neighbors move in.
But of course, those other organisms don’t seem to be looking to thank the purple sulfur
bacteria any time soon. I mean, who needs a neighbor when you can have a meal instead?
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
We’d like you to know that we’re going to be taking a short break from uploading for the next
couple of weeks, but we will be back soon with brand new videos. If you’d like to keep up with
exactly when we’ll be back with a new episode, make sure to follow us on twitter @journeytomicro.
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Now after watching this video, we would hope that you would
feel pretty comfortable saying that you know what Purple Sulfur Bacteria are,
but let me ask you a question. Do you know what a lexicographer is?
Well, I could not hear your answer, but I’m going to go ahead and assume that some of you said no.
A lexicographer is a person who writes dictionaries.
It’s a very cool and interesting job that involves way more time and research than
you might initially think, and in the book Word by Word, author Kory Stamper
explains how there is no right or wrong when it comes to grammar usage and reveals the three
criteria that a word must meet in order to be considered for the dictionary. You can find
out more about this fascinating profession by checking out Word by Word on Blinkist.
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people who make it possible for us to continue diving into this bizarre and lovely world,
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these are the people to thank. And I also want to thank them personally, y’all are great. Thank you
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