Cyanobacteria have a deceiving sense of the
understated about them.
Compared to some of the complex organisms
we've featured on Journey to the Microcosmos,
these blue-green bacteria seem almost basic
in their morphology and habits.
They glide, divide, and photosynthesize.
Oh, and they get eaten a bunch too.
But you do not need to be complicated to be
important.
Ecosystems are built on primary producers
like cyanobacteria.
The energy they translate from the sun into
chemical stores gets passed up the food chain,
sustaining life of all sizes.
And long ago, some eukaryotes gained their
own photosynthetic abilities by absorbing
cyanobacteria, an endosymbiotic event that
set the stage for the evolution of plants.
And plants are a pretty big deal.
Cyanobacteria, however, are more than just
a prologue for the story of complex lifeforms.
You might say they are the creative forces
behind that tale.
Billions of years ago, these tiny, seemingly
innocuous organisms brought something new
to the planet, an innovation that would create
massive changes in their environment and set
the stage for eons of remarkable life to come.
This is the story of the Great Oxidation Event,
and how in the process of destroying the world
as it existed, cyanobacteria created the Earth
as we know it now.
The bulk of this tale takes place about 2.5
billion years ago, at the end of the Archean
eon that had started about 1.5 billion years
before.
This vast stretch of time included huge milestones
for our planet, including the early days of
the microcosmos.
But the Archean Earth looked very different
from ours.
The planet’s crust was stabilizing, and
the oceans were filled with dissolved iron.
Most importantly though, the world was anoxic,
meaning it had almost no oxygen.
And because life reflects the environment
it fills, the earliest creatures on our planet
were anaerobic microorganisms, built to withstand
and thrive in an oxygen-less environment.
Some of these organisms may have been phototrophic,
but their photosynthetic machinery relied
on iron and sulfide in place of water, similar
to bacteria like thiospirillum that thrive
in anoxic conditions today.
But for all the dramatic changes in Earth
and in life that took place during the Archean
eon, the real transformation was only just
beginning.
We don’t know quite when cyanobacteria emerged, just that it’s some time before 2.5 billion years ago.
And when they did, cyanobacteria brought something
new to the photosynthetic mix: they used light
to split water into its chemical components:
hydrogen, which could then be used to make
other energy-storing compounds, and oxygen,
which was released into the environment as
a waste product.
This simple chemical reaction is the source
of the Great Oxidation Event, which you might
also hear called the Great Oxygenation Event:
an accumulation of oxygen in a previously
anoxic world.
That reaction would go on to become one of
the most important ones to life, the source
of the very oxygen you are breathing now.
And remarkably, cyanobacteria were not just
the first organisms to evolve oxygenic photosynthesis;
as far as we know, on our planet anyway, they
are the only organisms that do so.
Other photosynthetic bacteria don’t produce
oxygen.
And the eukaryotic organisms that gained the
capacity for photosynthesis did so by co-opting
the talents and machinery of endosymbiotic
cyanobacteria.
And as wonderful as that is now, back at the
end of the Archean eon, oxygenic photosynthesis
didn’t just make cyanobacteria unique, it
made them catastrophic.
Oxygen is great--if you are evolved to tolerate
it and to use it.
But for the anaerobic microorganisms specifically
evolved for a world without oxygen, this newfangled
molecule was toxic.
The species that survived the Great Oxidation Event likely did so by finding oxygen-less sanctuaries.
And meanwhile, cyanobacteria continued pumping
out oxygen.
Yes, they are tiny, and the amount of gas
they produce is even tinier.
But with hundreds of millions of years and many many organisms, all of those reactions
added up, affecting not only the lives of
their fellow microbes, but also the chemistry
and climate of the entire planet.
For one, cyanobacteria might be responsible
for setting off a series of ice ages.
As they consumed carbon dioxide and produced
oxygen that reacted with methane, they cut
down on the planet’s stock of greenhouse
gases and dropped temperatures to glacial levels.
As you might imagine, these early ice ages
were not particularly hospitable for life,
including for cyanobacteria.
Coupled with the anaerobic microbes that died
due to all of that oxygen in the atmosphere,
the Great Oxidation Event is responsible for
an extinction so ancient that we don't have
the tools or physical evidence to even fully
describe it.
But life still managed to endure.
And as it did, the oxygen produced by cyanobacteria
would go on to form our planet's ozone layer,
shielding the survivors from dangerous ultraviolet
wavelengths of light.
Moreover, the availability of oxygen in the
water and air would go on to make aerobic
metabolism possible, providing organisms more
energy than anaerobic metabolism ever could
and thus the capacity for more complex life.
This intertwined story of destruction and
evolution is built on evidence scientists
have gathered from geological and biological
sources.
Some parts are etched in rocks, like the banded
iron formations formed by oxygen reacting
with iron in the ocean.
And other parts are encoded in the genes of
modern day cyanobacteria, whose sequences
and machinery elucidate what their ancient
counterparts may have looked like.
But characterizing the lives of microbes that
existed billions and billions of years ago
is really, really hard, especially because
we haven’t uncovered many Archean fossils.
One recent paper begins with the following
confession: “Writing about early microbial
evolution is a daunting task, so it was possibly
unwise to agree to do it.”
And as you dig through the research on the subject,
that honest evaluation rings true.
There are many gaps in our understanding of
the Great Oxidation Event, gaps built on unanswered
fundamental questions like, “how did photosynthesis
even evolve?”
We alluded to one of the most important gaps
earlier.
It’s the question of when cyanobacteria
first appeared, and that question is key to
understanding how they caused the Great Oxidation
Event.
If that event began shortly after the evolution
of cyanobacteria and oxygenic photosynthesis,
then it all seems relatively straightforward:
the cyanobacteria produced oxygen, the oxygen
accumulated, and catastrophe ensued, but some
evidence suggests that cyanobacteria emerged
well before the accumulation of oxygen on
Earth, and if that’s the case, then we have
to ask, “What took so long for the Great
Oxidation Event to begin?”
Maybe there was a geological delay of some
sort.
Or maybe oxygenic photosynthesis wasn't the
only cyanobacterial innovation: maybe after
their first appearances on the planet, they
developed some other trait that bumped their
oxygen production up even more.
One group of scientists investigated that
very possibility and came to a possible explanation
that we don’t often associate with bacteria:
multicellularity.
Some modern cyanobacteria species form filaments
of individual cells, a few of which even take
on specialized functions like nitrogen fixation.
These scientists traced the origins of that
trait to before the Great Oxidation Event.
So maybe multicellularity allowed cyanobacteria
to not only better contend with the challenges
of their environment, but to produce more
oxygen than before.
And perhaps this was the added trait needed
to, say, allow a new kind of bacteria to take
over an entire world...a claim over our planet
that they have, in many ways, never given up.
Of course, that idea is scientific conjecture
built on the available evidence, which means
it is still subject to debate.
But the seemingly simple question underlying
it, the question of when cyanobacteria evolved,
reflects what’s so important about the Great
Oxidation Event in general: that questions
of biology and geology and climate are so
intertwined that to understand the history
of this planet, we have to piece together
the history of the microcosmos.
And if something as small as cyanobacteria
can set off a catastrophe of extinction and
ice, well, then what will the planet look
like when we’re through with it?
Thank you for coming on this journey with
us as we explore the unseen world that surrounds us.
The Stentor coeruleus pins have been restocked.
There’s a link in the description so you
can check that out.
Thanks to everybody who’s sporting one of
their favorite microorganisms on their backpack
or jacket now.
And thank you, also, to all of these people
who make this show possible.
So, if you like what we do, you can thank
them for that.
What a wonderful group of people, that allow
us to explore these big weird mysteries of
life on this planet.
If you want to see more from our Master of
Microscopes James, you can check out Jam and
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