Hello! Bea here, and welcome back to Biology! We're continuing our journey through biological
diversity today with a look at the least understood domain and kingdom, archaea.
The word archaea comes from the Greek word meaning ancient or primitive. It is thought
that these organisms are directly descended from some of the oldest life on Earth,
and yet, they represent the newest addition to the taxonomic classification system, as they weren't
discovered until late in the 1900s. Just like bacteria, the archaea domain contains one kingdom,
which is also called archaea. Compared to the other two domains, scientists know relatively
little about these creatures, as they tend to live in places that are difficult to study and
are known for being difficult to grow in the lab. So, there still exists an air of mystery
around these microscopic beings. We'll take a look today at what we do know about them.
Before we get started, let's look at our goals for this lesson. By the end,
you'll be able to compare and contrast archaea and bacteria cells and give examples of archaea,
both in extreme and familiar environments.
Let's take a quick minute to review how we ended up with the archaea domain.
Hundreds of years ago, scientists initially grouped living things into two groups,
plants and animals. Eventually, it became obvious that there were microscopic organisms that didn't
fit into either of these kingdoms, so a third group was made for the tiniest of creatures.
However, by the mid-1900s it was becoming apparent to most scientists that simply
putting all microscopic organisms into one group made little sense, as there were vast differences
in the cellular structure and genetic makeup of these organisms. After all, a yeast cell and a
bacterial cell have very little in common other than the need to use a microscope to see them.
So, bacteria were separated out as one kingdom and the other microscopic
organisms were largely placed into the protists and fungi kingdoms,
giving rise to the five-kingdom system we saw in the introduction to this unit. But,
in the 1970s, biologist Carl Woese was studying the RNA of a particular type of bacteria at the
University of Illinois and concluded that they were fundamentally and vastly different from any
bacteria he had ever seen before. So different, that he said they needed their own kingdom.
It was quickly decided that even giving them their own kingdom was not enough.
This represented such a large shift in what we knew about living things that an entirely
new level was added to the taxonomic system, domains, and these unusual organisms found by
Woese were given their own domain, completely separate from bacteria. What was so different
about these microorganisms that justified such an overhaul of the whole taxonomic system?
We'll look at that as we compare them to the other prokaryotic domain, bacteria.
From a simple list of characteristics, bacteria and archaea would appear very similar. They are
both single-celled prokaryotes, they both reproduce asexually by binary fission,
their DNA is circular instead of linear, and they can both be either autotrophs or heterotrophs.
So what makes them different enough to warrant being in entirely different domains? It's going to
come down to the specific biochemistry that makes up the cells. One of the main differences can be
found in the cell membrane. Remember from our cells unit that the cell membrane is a critical
structure for the cell that controls which substances can enter and leave. We learned that
the cell membrane is a lipid bilayer consisting of fatty acid molecules with phosphate heads. Sound
familiar? Well, some archaea have a different structure and instead use only a monolayer as
the membrane. It's thought by some scientists that this structure makes them more resilient in harsh
environments, which they are often found living in. Further, the fatty acids that make up the
membrane are ether lipids instead of the ester lipids that make up the membranes of bacteria.
You'll learn more about what those words mean when you take chemistry, but, for now, just know that
it's like having one house made out of brick and one house made out of wood; fundamentally
different materials forming the structure of the houses, right? It's the same idea here.
Remember that prokaryotic cells also have a cell wall outside the cell membrane. A major difference
is found here as well. We learned in the previous lesson that bacteria cell walls contain
peptidoglycan, which causes them to turn different colors in the process of gram staining depending
on whether they have a thick or thin layer of peptidoglycan. Archaea do not have peptidoglycan
at all in their cell walls, so gram staining cannot be used to classify or separate them.
With no peptidoglycan to absorb the dye, they would simply all show up as negative.
A final difference appears in the process of DNA replication.
We'll have to think back several units to remember what this process involved - the
unzipping of the strands of DNA so that complementary base pairs could
join each strand and make two identical strands from the original one strand.
One key difference between prokaryotic and eukaryotic cells that we mentioned at the
time was in the origins of replication. We said that prokaryotic cells have only one site where
replication begins, and it then continues all the way around the circle of DNA. In eukaryotic cells,
replication begins simultaneously in multiple locations. Well, it turns out that even though
archaea are prokaryotic, their DNA replicates in a manner much more like eukaryotic cells,
happening at several sites at once and using the same replication enzymes as are found in
eukaryotic cells. After much genetic testing, including technologies we have learned about
such as PCR and genome sequencing, it has been determined that archaea are more closely related
to organisms in the eukarya domain, including you and me than bacterial cells are. So,
the common ancestor to all eukaryotes was likely an archaea, not a bacteria. For this reason,
Carl Woese chose to redraw the tree of life like this, showing archaea and eukarya branching off
more recently than bacteria and eukarya. We are descended from these strange tiny organisms.
Archaea are some of the least studied organisms, but if you look up information on them,
one of the first common traits to show up is their ability to live in extreme environments.
We're talking about places where we would generally assume no life would possibly be found.
Thermophiles, such as sulfolobus and pyrococcus, thrive in hot springs and
geysers where the temperatures can be above the boiling point of water. Scientists think archaea
living in these conditions may be very similar to some of the first life that evolved on Earth,
as the conditions of early Earth would have been much like this.
Time out for a quick vocabulary lesson. Do you remember what the suffix "-phile" means?
Several units back, we talked about the hydrophilic heads and hydrophobic tails of the
cell membrane. "-Phile" means liking, or friendly towards, while "-phobe" means dislike, or fear.
Our hydrophilic heads liked water but our hydrophobic tails disliked
water. That's why they were on the inside of the membrane.
We just talked about thermophiles - can you guess what the two parts of that word translate to?
"Therm" means heat, and we now know that "phile" means liking, so thermophiles like heat.
Let's look at another type of philic archaea. In addition to being thermophiles, some archaea are
acidophiles as well, meaning they like living in acidic conditions that would lead to instant
death for most organisms. A species of archaea named ARMAN was discovered here at a mine in
California where the pH is below 1.5. You'll learn more about the pH scale in chemistry,
but that's about as acidic as an environment can get. Another category of archaea known as
the halophiles like very salty environments, like the Dead Sea, where the water is ten times saltier
than the ocean. Can you guess what the prefix "-halo" means? Salt. These archaea like salt.
But what about the cold? We said archaea could withstand intense heat;
is the opposite true? If you guessed, yes, you're right. Cyclophiles, such as methanococcoides,
have been found in the Ace Lake in Antarctica where the temperature rarely dips above freezing.
Closer to home, archaea can be found in almost any patch of soil. In fact,
around ten percent of the microorganisms in a scoop of soil will likely be archaea. They are
decomposers, just like many bacteria, and help recycle nutrients in the ecosystem.
They can also be found right inside of you and me. Millions of archaea cells known as methanogens
live in the guts of animals and produce the methane gas that they release. And yep, I do mean
that kind of gas. Does it stink in here? Other than the occasional unpleasant smell, archaea are
not generally thought to be dangerous to humans or other animals, and there are no currently known
pathogens in this domain. However, their role in human health has not been adequately studied and
some scientists have found correlation between the presence of archaea species and conditions such as
gum disease and digestive complications. But not enough is known to be able to conclude that the
archaea are the root cause. As a whole, they seem remarkably harmless when compared to bacteria.
As we went through the lesson today, we looked at how archaea compared to organisms
in the other prokaryotic domain, bacteria. We saw that, of the prokaryotic domains,
archaea are likely our closest genetic cousins. And we saw some examples of the
extreme environments that archaea can live in, including right here in our own bodies.
In the lesson PDF, you'll have a chance to further compare bacteria and archaea
and explore the archaean phyla. Next time, we'll move on to our first eukaryotic kingdom, protists.
Until then, remember that biology isn't just science, it's the way of life. See you next time.