Okay. By now y'all have heard the news
about that cool new suspected biosignature they found on Mars, right?
Just in case. Here's a quick recap.
For the last year, scientists at NASA have been analyzing some pretty incredible
data generated by the perseverance rover down in Jezero Crater on Mars.
And they recently published
a bombshell paper in nature that pretty compellingly describes well.
And analysis of these little spots they found on some rocks on Mars.
Now, I'll explain more in a second.
But in short, after all
the exhaustive analysis we can do from like 100 million miles away,
it really does look like the only plausible explanation
for these spots is that they were made by living things like bacteria.
And while this paper is very exciting for folks like me, we've honestly
had so many quote unquote ""evidence of life on Mars"" moments that have ended up
getting debunked or turned out to be inconclusive over the years.
And this may very well be one of those, but it's going to take a while
to pick apart and verify the findings of this paper, and speculating
on whether or not this finding is legit is miles above my paygrade.
And so instead of adding to the pile of videos
that have rightfully been gushing about this paper,
I want to go one layer deeper and answer a question.
Basically, no one has addressed in this PR hurricane.
What kind of living system leaves behind little rock spots like this?
Exploring this question gives us incredible insight
into life on the early Earth, how the Earth itself might have shaped
the initial spark that produced life in the first place,
and what we should be searching for as we try to answer the question
of whether or not we're alone in the universe.
And to do that explanation,
when we get to hang out
with one of the wildest living things that has ever clung to our planet.
That's right, folks, today we're going to meet Geobacter Sulfurreducens
This near ubiquitous bacteria gets poisoned by oxygen.
So it burrows deep into places like swamps and breathes by plugging
into metal deposits using biological wires.
No joke. We're talking about electro bacteria.
Today it's time to explore metal, reducing respiration and all the incredible
hacks bacteria have to do if you can't breathe using oxygen.
Let's get into it.
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But anyway, let's briefly
go over the findings of this NASA paper before we get too deep into the biochem.
Plus, I really love this space stuff. Indulge me (please).
I'll make the context fun, I promise.
For background,
we've got this SUV sized robot that NASA landed on Mars back in 2021.
The Perseverance rover
ol' Percy was hurled at this place on Mars, the Jezero Crater.
As it really looks like it.
There might have been liquid water here once.
In the near five years since landing, Percy has basically confirmed
there used to be liquid water on this part of Mars.
But the findings for this paper will be far more significant
for the legacy of perseverance if they end up being true.
Let's dig into those now on Mars, to the west of the crater floor
where perseverance landed, just past an ancient river delta,
there's this little rocky area called Neretva Vallis.
where Perseverance took these pictures
and did analysis of some wind worn rock back in 2024.
This spot was probably like a muddy river
or lakebed before and had some organic matter in the mix.
Organic here just means stuff
made out of carbon, not necessarily living stuff, and mixed in with that.
Evidence for ancient organic bits were these little spots that all the fuss is about.
Zooming in real far to the evidence, you've got a central core
of one kind of rock here and darker rocks surrounding it.
This appears to be something called a 'reaction front',
where there's some sort of chemical change
happening on the edges of this spot here.
This material in the middle is gregite,
And the darker rock is probably vivianite
vivianite is the key focus for us.
It's a mix of iron and phosphate that only forms in really extreme circumstances
or when bacteria are using iron as a material for respiration
in the absence of oxygen.
Now, most of this paper goes on to discuss all the exhaustive tests and checks
the NASA team did to make sure
they completely ruled out a non-biological explanation, which is great science--
--honestly, if you have the stomach for geology papers,
you should check that out in the description.
There's also a lot of great videos that go into what else
this combination of vivianite & gregite could be,
and you should definitely go watch those if you haven't already.
And I mean, heck, you're probably only here because you did.
I don't have to tell you that.
But until we can either send geologists to Mars
or find a way to make the Mars sample return mission more feasible,
there just isn't a lot of conclusive analysis we can add to this paper
using this data.
It's an exciting result,
and it hopefully will add to the motivation
we have to go through the ludicrous engineering challenge
of bringing some Mars rocks back for human scientists to poke at.
But for now, this dataset is really just a compelling question mark.
And therefore, let's focus on questions we can actually answer, especially
since this is a biochemistry channel and not like a space channel.
So it's more important for us to pivot
and look at what produces Vivianite here on Earth.
How on earth is Vivianite a biosignature?
What living process leaves behind an iron mineral crystal?
That sounds baffling to me.
Vivianite gets produced here on Earth in lots of different ways, but
a lot of times it's the leftovers of bacterial metabolism,
specifically, a class of cells called dissimilatory metal-reducing
bacteria, which I know is a mouthful.
Basically, metal reducers, quote unquote """"breathe"""" by grabbing electrons
from their food source
and shoving them onto metals, specifically a form of iron called iron (III) (Fe III) ,
which, to oversimplify, is a form of iron that's missing three of the electrons.
It would prefer to have. (In it's outer shell).
This gives iron (III) or ferric, as it's sometimes called, a strong
positive charge that can pull on negatively charged electrons really well,
which makes it a great candidate for respiration without oxygen.
But of course, that begs the question how exactly do you breathe metal?
To answer that, let's first meet a metal breather that's really well understood.
Now there are lots of different, dissimilatory metal-reducing bacteria that can leave behind
gregite and vivianite in the combination that perseverance observed on Mars.
Groups like Shewanella immediately come to mind.
But we're going to zero in on one of the best
researched members of this incredible little group.
GEOBACTERSULFURREDUCENS (!) You're going to find this guy
breathing rocks in basically any watery environment
that is suitably protected from the pernicious influence of oxygen.
We're talking like peat bogs, the deep ocean, certain pockets of the soil, etc.
and just on the surface, Geobacter here.
Looks so weird, right?
this hairy look to go back to he's got going on
is critical to their metabolism, which is the main point of this video.
That and the incredible molecular work it takes to respire using rocks.
But wait,
let's start off with some background real quick compared to Geobacter Sulfurreducens,
your metabolism is honestly easy to understand.
Your cells are going to break down your food
into fundamental molecules which your mitochondria are going to """""burn."""" (oversimplification)
In order to strip electrons off them.
Congratulations on suffering through the world's
most oversimplified explanation of the Krebs cycle, by the way.
Well, that or the citric acid cycle.
If you're one of the cool kids,
those electrons are then shoved onto carriers like NADH and FADH,
which are then pulled along the electron
transport chain and the inner membrane of your mitochondria.
You siphon a little energy from this electron flow to turn
the turbine of ATP synthase, thereby powering every single thing your cells do.
But to keep this basic electron flow moving, it's
going to need a powerful vacuum to pull electrons down the chain.
that's why we need the oxygen in the air we breathe.
O2 has one of the strongest quote unquote, "pulls" for electrons in nature.
Oxygen is incredibly electronegative.
So it provides an incredible sink for electrons to flow into.
As long as you're breathing,
the electrons in your mitochondrial membrane will flow.
And the interesting part here is that oxygen
hasn't always been a big part of Earth's atmosphere.
O2's electronegativity, makes it incredibly reactive,
and we have only established and maintained an oxygen rich atmosphere
thanks to the advent of photosynthesis
and the constant work of cyanobacteria in the ocean.
And like trees too, I guess. Which is one of many reasons why
Geobjacter is an important model organism.
Not only are these little critters a critical window for understanding
how life could have evolved in the pre-photosynthesis version of Earth,
but their metabolism means that they actually end up
growing in super weird spots, like literally on electrodes.
So there's mountains of research using these guys to develop tech
called microbial fuel cells and all sorts of other applications.
But I'm going to get into that later because I'm getting distracted.
So let's spend a couple of minutes in the deep biochem exploring.
How Geobacter Sulfurreducens, quote unquote breathe using metals.
Finally getting to the good stuff--
Geobacter metabolism. And we're going to peel this back layer by layer.
At the simplest level, we can chart Geobacter's respiration like this:
We need a source of electrons to consume.
And then a sufficiently attractive place to put those electrons.
And so Fe(III) (Iron 3) is going to be our electron sink, the positively
charged vacuum that pulls electrons across this chain.
The electron source for Geobacter is remarkable.
Geobacter Sulfurreducens is going to use the exact same
Krebs or citric acid cycle you do to break electrons off food.
But what's the fuel here If you can't use oxygen?
Turns out: Geobacter loves powering
its metabolism with vinegar.
Straight up acetate ions eventually get processed into acetyl-CoA,
and that runs through the Krebs
cycle, eventually throwing electrons onto carriers like NAD, nADPh,
and even exotic compounds like reduced Ferredoxin, which we haven't seen
since my photosynthesis series. Nice.
And you know what? Let's tackle some annoying
chemistry lingo before it gets out of hand.
I'm throwing the word reduced around on the open internet a little too loosely.
Here's the gist:
So much in biochem comes down to where electrons are flowing.
For the most part,
we describe electron flow using the terms "oxidation" and "reduction."
When something gets oxidized, it loses electrons,
and when it's reduced, it's gaining electrons.
Your core metabolism is oxidizing that salad bowl
you just ate and reducing the oxygen you breathe.
I'm years into writing about biochem, and I still have to remind myself
about which word is which, and I use the classic mnemonic here to help me remember:
O I L R I G Oxidation Is Loss Reduction is Gain
Chant that to yourself a few times.
Turn it into a meditative mantra.
See what new heights of clarity you can achieve.
But anyway, the interesting part here is level two.
How are we moving
electrons from food metabolism to this big old block of iron compounds?
At this moment, it's important to remember that
most prokaryotes, like Geobacter here are double layered.
You've got the internal cytoplasm surrounded by a barrier membrane.
And then that is also covered by another membrane.
You might remember some species like mycobacteria
take this to a really intense level with their super thick outer Mycomembrane.
But for the most part, this double membrane allows for a place
where the power stroke of metabolism can happen.
This space between Geobacter's inner and outer membrane is called the Periplasm.
Down here, beneath the inner membrane Geobacter Sulfurreducens
will "burn" acetate and generate those electron carriers,
which will then gradually shuttle them up to this inner Periplasmic membrane.
And here we've got some familiar faces. Geobacter Sulfurreducens
Honest to God uses the same complex I
you do to shuttle electrons along the cytoplasm.
But that's where the similarities end.
The real workhorse for Geobacter here is electron transport chain.
It's going to be this guy ImcH, which grabs electrons from carriers
and also helps shuttle protons into the Periplasm
so that ATP synthase can use proton flow as its energy source.
The wild thing about critters like Geobacter here is-- depending on the situation,
this guy can hot swap different electron transporters into its periplasmic membrane. Carriers
like CbcL or CbcBA are better suited for different electrochemical situations.
Keep that idea in your back pocket for a second.
We might come back to it later. But, here's where things get really wild.
This might be hard for critters like you and me to wrap our heads around,
because Geobacter respiratory model is basically backwards compared to ours.
You and I keep things simple by having lungs and red blood cells that ferry
oxygen from the outside of our body to every corner of every tissue.
We have the good sense to bring our electrons acceptors
to the factories that needed them.
Geobacter has to send its electrons.
Geobacter is going to have to do a lot more work to keep the electron flow going.
Sothe Periplasmic space here in Geobacter Sulfurreducens
is chock full of these wild heavy duty electron carriers.
a whole alphabet gang of PPC proteins.
These little globs are stuffed
to the gills with heme groups to help ferry electrons.
You've got electron carriers just like this inside your mitochondria.
But as a proud eukaryote, your situation is a little more sleek and refined.
Using this fella, a nice, uncomplicated cytochrome c with a single heme group.
And yes, quick aside, this is the same heme group
your red blood cells rely on to ferry oxygen. Geobacter's
PPC carriers look pretty crowded with heme. And, if you think this is a lot,
then the last few minutes of this video are going to blow your mind.
BUCKLE UP, HEME FANS.
The PPC pool here will carry electrons up to the outer membrane of Geobacter
where there are, depending on the situation, tons
of different potential electron carriers to finish the journey here.
Geobacter is going to have to put a ton of work
to actually bring these electrons home.
And this is the most incredible thing about Geobacter
in order to connect to its electron sink,
Geobacter produces
one of the most sci-fi-ass biological structures known to science.
Behold: BACTERIAL NANOWIRES.
You all know how electricity works, right?
A wire is a piece of metal that connects an electron source to an electron sink.
Technically, like electrons move
through the metal in power, like a lamp or whatever.
We figured out how to pass electricity through wires around about 300 years ago.
but critters like Geobacter
have potentially been working through
that part of the tech tree for billions of years.
Maybe.
You know, that feels like an oversimplification I'll get torched for.
Anyway. Look at these things.
Nanowires are delicate
little protein packages encasing a little chain of heme groups.
Specifically, a single nanowire subunit
is basically a jumbo sized cytochrome c stuffed with six heme groups.
Based on the information we have so far, Geobacter
has a pretty simple philosophy.
When in doubt, just throw more heme at the problem.
You know...blah blah, something something....
"You can buy anything as long as you're paying the iron price." ANYWAY--
Electrons are pushed onto the nanowire by the PPC gang,
and then as long as the other end
of the nanowires in contact with a place with positively charged iron three ions,
that charge difference will be enough to pull electrons
along this cute little relay race across each of these heme groups.
It is amazing how conductive these little nanowires are.
All Geobacter has to do is produce a bunch of nanowire subunits
like OmcS here, and hurl them outside the cell
where the subunits will self-organize like magnetic Legos into these wire shapes
thanks to the magic of protein polymerization.
This way, Geobacter can constantly produce more nanowires
and probe deeper and deeper into its, you know, air tank equivalent.
But again, how does that work?
And so this is the part where Geobacter breathes metal, or rather **respires** metal (ions).
Each Fe(III) (iron 3) ion
that Geobacter shoves an electron onto gets converted into Fe(II) (Iron 2).
Or rather, it gets reduced to Fe(II) (Iron 2). And wow, you know, neat.
That's actually a great way of illustrating why
we call this process "reduction."
We're *REDUCING* the charge,
making it *more negative.* Neat! and this is why nanowires make sense.
Eventually this whole pocket of usable iron is going to get reduced to Fe(II) (Iron 2)
which has a less positive charge and therefore doesn't pull
on electrons as well.
Geobacter needs to be constantly building new nanowire
networks to keep probing different parts of their local environment.
They are constantly depleting their air tank equivalent and need to keep hunting
for more electron acceptors.
But before we get too far in the weeds, let's pull back and just review
at a high level how Geobacter pulls off its respiration.
So Geobacter metabolizes its food, preferably vinegar or the acetate
ion, and then shuttles electrons to its inner membrane.
Those electrons travel across
a bunch of carrier proteins that use the electron flow to pull protons
into this Periplasmic pocket, thereby generating usable energy for the bacteria.
Heme-stuffed electron shuttles like the PPC gang to transfer electrons
from these carriers up to the outer membrane, passing them on to nanowire
networks.
Eventually, those electrons arrive at this lovely little Fe(III) (iron 3) deposit.
Those Fe(III)(Iron 3) ions are more than delighted to suck up those electrons,
and that little pull powers the entire electron relay race.
With no iron.
Up here, nothing is pulling the electrons from nanowires,
which then can accept electrons from the PPC gang,
which then can pull electrons from the inner membrane.
No electron pull, no ATP power.
And that's the single-cell equivalent of suffocating folks.
And congratulations, you have a decent surface
level understanding of the metabolism of Geobacter Sulfurreducens--
--One of the wildest current examples of what could have been
the original metabolism on Earth, or at least
part of the original metabolic picture of ancient life on this planet.
And therefore, now we finally have enough context to return to this Mars data.
Surprise, y'all, this wasn't clickbait after all.
Better understanding
bacterial respiration helps us make better sense of these rock spots.
Now, we've barely scratched the surface of our current understanding
of metal reducing microorganisms, but we have enough to work with.
So let's just slam through this thing.
Remember, at the simplest level,
Geobacter Sulfurreducens is going to reduce Fe (III) (Iron 3) to Fe (II) (Iron 2) .
That's the key step here.
Fe(II) (Iron 2) has the perfect charge to join up with phosphate ions
which leads to the creation of crystals like vivianite
So if the hype bears out here, these little spots are the ancient remains
of a dissimmilatory metal-reducing microbe
gradually exhausting its ""air tank"" of electron acceptors.
And I promise one last tangent here.
The material in the middle of these spots: gregite,
also implies one more step: reducing sulfur ions into sulfide.
Iron isn't the only element anaerobes can breathe.
Sulfur is also a big contributor here on Earth.
We can find evidence of sulfate reducers and iron reducers in close proximity
just like this all over the place.
But I'm not about to do a whole new spiel on a completely different rock folks can breathe
Sound off in the comments if you really want me to do a part
two here covering the second half of this microbial puzzle, or better yet,
go to Patreon.com/clockworkshow and harass me there.
That will really stack the deck in your favor.
But yeah, let's go ahead and put a bow on this thing before I get too carried away.
I could gush so much more about how powerful
this little metabolic trick pulled by Geobacter SUlfurreducens is
Like, real quick:
Obviously rocks are going to be consistent.
So what's our hero going to do when there's different metals
with different electric potentials in contact with the nanowires?
Well, Geobacter will just swap out that
electron transporter for something like CbcL or CbcBA,
which is going to be better suited for a less-than-perfect electric potential.
Like game that out for a second.
Imagine if your mitochondria were fast enough to swap out your electron
transport chain, so you were better suited for breathing something besides oxygen.
Also, communities of Geobacter Sulfurreducens can use those nanowires
to move electrons between other community members,
making a whole electric network of cooperating individuals.
As the bacterial colony explores new breathable pockets of metal.
But the most important thing
here is simply respecting
how Geobacter demonstrates the core maxim of this channel:
Life Just Works.
If you can find an electron flux and keep it going, you can build a whole
biosphere off that flow.
Which is why finding Vivianite on Mars is such a big deal.
If it actually turns out to be real, And also from like a biological source.
Organisms like Geobacter probably evolved this mode of metabolism
on a pre photosynthesis version of Earth.
Our most ancient ancestors took advantage of the incredibly active geology of this
good Earth to catch natural electron flows coming out of the cracks.
But the Earth is so dynamic and in such constant motion
that we simply cannot find rocks old enough to give us concrete answers
to some of the foundational questions we have in evolutionary biology.
There are tons of great theories that fill in the gaps
enough to explain how life could have begun here.
But if similar processes allowed a completely separate tree of life
to evolve on Mars, then the Red planet might just be a critical conduit
that will help us understand our own history.
That is, if these results hold true.
The only way to know for sure is to boldly go. To get enough Martian dirt
into a human lab for us to do more robust testing.
But like, what an exciting time to be in this business, y'all.
That's a good place to leave this here, though.
I really appreciate your time. Thank you so much for being here.
And hey, thank you so much for reaching the end of this video here.
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I promise it is happening.
Otherwise I don't want to keep you too long here.
I really appreciate your time.
Thank you so much for being here and as always,
I like to leave you with peace, love and heme without end.
Everyone be well. Thank you so much.
I.