Old & Odd: Archaea, Bacteria & Protists – CrashCourse Biology #35


We’ve spent the past few
months talking about animals here on Crash Course, specifically
human animals, because… well, because humans…
we love talking about ourselves, and also because animals
are just really interesting. But it’s high time that we talked
about the rest of the living world. Because I hate to break it to ya,
but most of the alive things on Earth are single-celled organisms. And by “most of the alive things”
I mean that these organisms make up two of the three
taxonomic domains of all life, plus one of the four kingdoms. I’m talking about archaea,
bacteria and protists. With the exception of a few
protists, they’re all unicellular, and they are, by far,
the most abundant and diverse organisms on Earth. More important, they lay
claim to the world’s oldest and earliest living lineages,
dating back to the very first twinkle of life on this planet. So by understanding
these three groups, you begin to truly understand
life on earth, its origins, and how everything that came after
them, including us, came to be. What’s more, because
their heritage is so ancient, these organisms often
take weird, cool forms that don’t look like life
as we think about it, and they do amazing things. Some not only live but thrive
in environments that would kill you, me, and everything we hold dear. And others make their
living by invading organisms, including us, and causing disease. Then there are those that do the
opposite, making life possible by, fixing nitrogen from the atmosphere
and helping animals digest food. Members of these groups have
names like Sailor’s Eyeballs and Dog Vomit Slime Mold,
and they can take the shape of rods, blobs, corkscrews or coils. Kinda like the doddering,
eccentric relatives you’re forced to spend some holiday
with once a year, the archaea, bacteria and protists are
our oldest, oddest relatives. And it’s about time
you got to know them. There’s no denying it:
Every multicellular organism on this planet, whether it be
a mushroom or a vampire bat, evolved from a
single celled organism. And while some of these single
celled organisms evolved to populate the world as rhinos and strangler
figs, others found happiness in the unicellular lifestyle, and they haven’t changed much
in the past few billion years. Today, nearly all unicellular
organisms are either archaea, bacteria or protists. Protists, you’ll recall,
are eukaryotic organisms that make up the kingdom
Protista under the domain Eukarya. Bacteria and Archaea, meanwhile,
are their own prokaryotic domains. And I hope you haven’t
forgotten this, the big difference between prokaryotes and eukaryotes
is that eukaryotic organisms, including you and the plants, and
fungi and animals that you know, have cells with a nucleus that
hold their genetic information, while prokaryotic cells don’t have a nucleus or any
organelles to speak of. These two groups do have some
important things in common, like having plasma membranes
that are filled with cytoplasm, and ribosomes that contain
RNA and synthesize proteins. And they both have DNA that
carries the instructions for operating the cell. But eukaryotic DNA comes in
strands in the form of chromosomes, while prokaryotic DNA is
found in rings called plasmids. So, again, and this
time with feeling: Protists are mostly single-celled
eukaryotic organisms. Archaea and bacteria are
single-celled prokaryotic organisms. The word “prokaryote” actually
means before the nucleus, which is a clue that prokaryotes
are an older form of life. And we literally cannot find
anything older than Archaea. The first Archaea fossils
date back 3.5 billion years ago I’m talking just a billion
years after the Earth formed and was still bombarded
by comets and meteors, not to mention fried
by UV radiation. But in the midst of all that,
archaea were just chillaxing. Earth’s climate has calmed
down since then, so today archaea are found in some of the
world’s most extreme environments: In underwater hydrothermal vents,
oil wells, volcanic hot springs, even acidic mine drainage. Archaea were probably
the earliest living things, and their adaptability is
probably what allowed them to take root in Earth’s early,
kind of grody environment. One key group of the
archaea are the methanogens. These guys prefer more
moderate environments, like mud, swamps and your intestines,
but they derive their energy from hydrogen gas and carbon
dioxide, which is pretty cool and they emit methane
as their waste product. Methanogens, methane generators. We know that waste as swamp gas,
and also, other kinds of gas. The other groups are extremophiles,
which not only tolerate but prefer really
wicked surroundings. The most famous of
these are the thermophiles, which live in temperatures
that would melt your face off. I mean, serious: Pyrolobus
fumarii, a species of archaea discovered in the late
1990’s in a hydrothermal vent, live at temperatures
around 113 degrees celsius. Not fahrenheit, celsius! Significantly above the
boiling point of water! Most organisms can’t
take heat like that, because it causes their DNA to
unwind and their proteins to denature or permanently change shape. But thermophiles have evolved
adaptations that keep them stable at these screamin’ hot temperatures. There are also halophiles, or salt
lovers, which live in places like the Dead Sea or
the Great Salt Lake, and probably Daniel Tosh’s mouth. Most halophiles breathe
oxygen and are heterotrophic, but there are some bizarro outliers,
like species that use sunlight to make energy,
but not like plants do they have light-harvesting
pigments in their membranes that react with light and enable
the cell to make ATP for energy. I know, it’s crazy! But despite their
alien-sounding ways of life, archaea really aren’t
all that different from bacteria, which are also prokaryotes. In fact, archaea and bacteria
were classified together for much of the 20th century. It was only when scientists realized
that they had some important genetic differences, like, in the
sequence of their ribosomal DNA and the makeup of their RNA, that they were separated
into two domains. Bacteria are nearly as
ancient as archaea. Fossils show that they were
widespread about 1.5 billion years ago, but there’s evidence
that they’ve been around for more than 3 billion years. Today, they make up the vast
majority of prokaryotes on Earth, and they’re super slick when
it comes to adapting quickly. Many bacteria are parasitic. Think strep throat,
your staph infection, anything you’ve ever
taken an antibiotic for. But bacteria can
fend off antibiotics, and the ninjas in
your immune system, by garbling up their DNA from
one generation to another. They can randomly turn
genes on and off, creating unique genetic combination
as its population multiplies, keeping its host’s immune system,
and drug-makers, on their toes. Like archaea, bacteria don’t
reproduce sexually, but bacteria have devised a way
to pass their genetic material to their buddies, a little trick
called horizontal gene transfer. For example: you’ve heard
of antibiotic resistance, right? Well, horizontal gene transfer
is one reason for it. A strain of bacteria that has
genetic resistance to an antibiotic can pass some of its DNA,
and that drug resistance, to another strain, which
is why we’re always in kind of an arms race with
the bacteria of the world. And of course bacteria
are incredibly diverse, with too many phyla to name,
more than two dozen. But one way of classifying them
is by their different kinds of cell membranes, which react
differently to a staining technique scientists use called Gram staining. Gram positive bacteria
have thick cell membranes, and they’re a huge group
that includes species that live individually like
staphlococcus and streptococcus, as well as some colonial
bacteria that are responsible for diseases like
leprosy and tuberculosis. There are lots of groups of
Gram-negative bacteria too, which have thinner membranes. The biggest group
here are Proteobacteria, named after Proteus because
they take so many forms. These include bacteria that
make our lives possible by converting nitrogen in the
atmosphere into compounds available to plants, as well as
others that cause stuff like food poisoning and
Legionnaire’s disease. Cyanobacteria, meanwhile,
are the only prokaryotes that use photosynthesis
to make their food, and they’re some of the most
important members of aquatic food webs, providing microscopic
forage for all kinds of freshwater and marine ecosystems. Spirochetes are the
corkscrew-shaped bacteria that you’ve no doubt heard of most are harmless, but a
couple of parasitic species are the culprits behind illnesses
like Lyme disease and syphilis. And speaking of sexually
transmitted diseases, the last major group of bacteria
worth mentioning are Chlamydias, which are strictly parasitic and
live only in animal cells. They’re scumbags, obviously, and
are the leading infectious cause of blindness in the world,
as well as that eponymous infection of the urethra that makes
me kind of want to, cross my legs,
just thinking about it. So, Archaea have managed
to make a nice, multi-billion-year living
by surviving in weird, out of the way places, and
bacteria have developed ways to pass their DNA
without sexual reproduction. But you know who’s a hot
freakin’ mess? Protists. Evolutionarily, they’re the
youngest of the three, having evolved from bacteria
around 1.7 billion years ago, and in a lot of ways
they’re more sophisticated. For starters, they’re eukaryotic,
but also, some are multicellular, and a few kinds can
even reproduce sexually. But their domain is a big crap
circus, because some protists seem to be more closely
related to plants or animals or fungi than other protists. So scientists tend
to talk about them based on what else they resemble. There are Protozoa,
which are kinda animal-like, Algae, which are kinda plant-like, and fungus-like ones, including
the tastefully-named Slime Molds. The one thing all
of these have in common is they need to live somewhere wet: in a bog, or in your body,
or in a snow bank, wherever. Protozoa are actually really cool
because they’re like tiny animals. Like us, they’re heterotrophs, so they have to eat other
stuff in order to live. And because they need to eat,
they’ve got mouthparts, or at least mouth-part sorts
of things, and they can move around by using all kinds
of really cool structures. Some have flagella, the whip-like
tails, to propel them through the water, or cilia,
little hair-like structures that work like oars,
and some move around with a kind of blobby
amoeba-like motion. I say amoeba-like
because the protozoans that move this way are amoebas. And speaking of amoebas, some
protozoans are parasitic. You’ve probably heard of amoebic
dysentery: that’s caused by amoebas. Malaria is caused
by this little guy, a protozoan called
Plasmodium vivax. While African Sleeping Sickness
is caused by Trypanosoma brucei, this guy here. Moving on to the plant-like
protists, which are algae. All algae photosynthesize
like plants, even though they’re not plants, because they use different
kinds of chlorophyll molecules. Some are unicellular,
like tiny diatoms, which have a hard
shell made of of silica. But the amazing thing
about single-celled algae is that they can get
really honkin’ huge. For example, ladies and
gentlemen, cast your gaze upon the Sailor’s Eyeball,
thought to be the biggest single-celled
organism on the planet. Also known as “bubble algae,”
it lives on the sea floor in tropical oceans and can
grow up to 5 centimeters across. How is that thing one cell? Anyway, you already know
multicellular types of algae, aka seaweed. They’re closely
related to land plants, as you can tell by looking at them, and they’re generally grouped in
to red, green and brown varieties, although these all have their
unicellular forms as well. The green algae are
probably what gave rise to land plants about
475 million years ago. They’re the most
abundant and diverse, and they have chloroplasts
very much like land plants, so they can only
live in shallow water because they need
a lot of sunlight. Red algae is able to
live at greater depths and has an extra pigment
in it called phycoerythrin, which gives its chlorophyll
a boost in deeper waters. And brown algae is what most of the
seaweed you see in the ocean is. Kelp is an example. They’re the largest and most
complex of the multicellular algae. Finally, we have our
fungus-like protists, which include the
delightful slime molds. They absorb nutrients
from their environment and produce fruiting
bodies like fungi, but even though they
look like piles of barf, they can actually move
around like an amoeba and eat bacteria by phagocytosis. Slime molds can be
pretty easy to spot because they’re
often brightly-colored, like this charming species
which, in all seriousness, is known as Dog Vomit Slime Mold. You heard me.
These organisms are so freakin’ screwed up that scientists
couldn’t think of a better name for it than Dog Vomit Slime Mold. Like I said. They’re old.
They’re odd. Get used to it. Thanks for watching this episode
of Crash Course Biology. If you want to catch up on
anything you’re a little fuzzy on: table of contents over there! Thanks, of course,
to all the people that helped put this
episode together. And if you have any
questions for us, please Facebook, Twitter,
or the comments below. Goodbye.

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