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Let’s start today by agreeing on one thing: this is weird. We’re looking at
a dark purplish screen with little bits of white floating towards the top, as if they’re bubbles
rising to the top of a glass of grape soda. But that’s not the weird part.
The weird part happens when those bubbles suddenly move in concert away from the frothy edge,
dragged towards one corner of the screen and then towards the other. The effect is cosmic,
like we’re watching a million stars get flung towards each corner of the sky.
And it won’t make it any less weird to know that
we’re not looking at bubbles or stars. We’re looking at bacteria.
A field of them shaped by an invisible force they are innately built to respond to.
Every swing towards one direction and then another is visual proof of this unseen thing.
So what is it? What is the force that’s driving them to move this way?
Technically, it would not be wrong to say that the answer is James, our master of microscopes.
When James first saw these bacteria, all he knew is that they came from a sample taken from a
Portuguese beach. And on each slide, the bacteria were swimming in a stark line. And usually,
it’s not that strange to find bacteria swimming in a line, especially when you realize that those
lines fall along particular chemical gradients that correspond with the bacteria’s own needs.
But what was weird to James is that the bacteria seemed to be congregating entirely on one side
of the slide. Now at first, he thought that there might be some kind of food source over there
that was drawing the bacteria towards that edge. So when he disposed of the slide,
he moved on, not expecting to see it happen again. But the next day, James took a drop of water
from his sample and added it to the slide. And there it was, another line. Just like he had
seen the day before, but the line was sticking to one side of the slide, almost as if they were being
pulled by some invisible string in that direction. And that gave James an idea. He took out his phone
and opened up his compass app. Then he placed the phone on the microscope stage to see what
direction the bacteria were swimming in. And he found that the bacteria were all swimming north.
So if we’ve seen this line of bacteria that’s all swimming north, what’s with the other things we’ve
been watching, where the bacteria look like they’re being dragged in other directions?
Well, as soon as James realized that his bacteria were swimming north, he decided to go and grab
some magnets to see if he could trick them into thinking some other direction is north instead.
So he held the north end of the magnet close to the slide, and the bacteria began to migrate,
drawn in the direction of the magnet. And then, he rotated the magnet so
that the south end pointed closer, driving the bacteria to swim away.
What James had found—what we are seeing in action—are magnetotactic bacteria,
or bacteria whose movements are controlled by the earth’s magnetic field.
Magnetotactic bacteria were first described in 1963 by an Italian scientist named Salvatore
Bellini, who found them living in freshwater. And when he saw them move with the same singular
northward focus that ours have been exhibiting, he considered the idea that they might have
some kind of internal compass within them. But while Bellini wrote up his work, it was only
circulated as manuscripts to some of the universities in Italy instead of being published
in a journal for more scientists to find. That meant that the existence of these bacteria wasn’t widely
known when a decade later, a graduate student named Richard P. Blakemore collected some more bacteria
from a pond in Massachusetts and witnessed that northward microscopic migration for himself.
Blakemore coined the term “magnetotaxis” to describe what they were doing. And as he dove
deeper into the bacteria to understand how they moved this way, he realized that they did in fact
have their own internal compass. Or rather, compass-es.
Within these bacteria are a set of organelles called magnetosomes. The core of the magnetosome
is a magnetic crystal—either iron oxide or iron sulfide—that’s made through a series
of processes that involve the cell taking in iron from its surroundings and mineralizing it
within them. The crystal is then wrapped in a membrane to create a single magnetosome.
But as we said, magnetotactic bacteria don’t just have one magnetosome. They have a set of them,
usually around 10 to 20, arranged in a chain to create a dipole that can respond to the magnetic
field that crosses the world around the bacteria. Magnetotactic bacteria are found all around the
world, but their preference is to live in areas with low levels of oxygen,
like deeper down towards the sediment and away from the surface. And this preference
has potentially important consequences for the direction that magnetotactic bacteria travel in.
All of the magnetotactic bacteria discoveries we’ve described so far, including the bacteria
that we’ve been watching, travel north. However, all of those bacteria share something else in
common besides their propensity towards the North Pole: they all live in the northern hemisphere.
Meanwhile, scientists have found that magnetotactic bacteria from
the Southern Hemisphere will actually travel towards the South Pole.
This is probably due to the incline of our planet’s geomagnetic field lines, which
mean that as a bacterium travels towards its specified pole in its preferred hemisphere,
it also swims downwards in the water towards the sediment it prefers.
If a magnetotactic bacterium from the Southern Hemisphere was forced to navigate the magnetic
field of the northern hemisphere instead, it would find itself swimming upwards,
reaching waters higher in oxygen and far less comfortable for the bacterium.
At least, this is the hypothesis for why this apparent polarity within the distribution of
bacteria exists. But as always, there are exceptions—southern-seeking bacteria found
in the northern hemisphere, and other strange behaviors that scientists are still deciphering.
There are many other mysteries to magnetotactic bacteria, like what advantages magnetism
even gives them—whether it’s to find the right habitat in the water, or to hunt down particular
nutrients, or a combination of different factors. And the bacteria probably don’t know why they
move this way either. It is simply baked into them, a crystal that draws them
into invisible tracks. They are mysterious and enigmatic. You might even call them, magnetic.
Thanks for coming on this journey with us as we explore the unseen world that surrounds us.
And thank you again to MagellanTV for sponsoring this episode of Journey to the Microcosmos.
There are a lot of unseen worlds that surround us, and while we tend to focus on the ones we
can fit under the microscope, the James Webb Space Telescope is out there studying the atmospheres of
exoplanets, and searching for the building blocks of life elsewhere in the universe. And you can learn
even more about that by watching Planet Hunting With The James Webb Space Telescope on MagellanTV.
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All the peoples names you see on the screen right now, they are the people who support us
on Patreon. They give us a little money, so that we can continue making this show, because look,
we understand that we are extremely excited about magnetotaxis, but many people aren’t, and
that’s okay. It’s a big diverse world. But with tools like Patreon, and great people like these,
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