Hi marine biology students.
In this video we're going to talk about seaweeds
and see why it is they aren't exactly plants.
[Intro Music]
There are many ways that seaweeds are like
plants in that they are multicellular, they
are photosynthetic, they are important primary
producers.
But, the reason why seaweeds aren't plants
is that they lack the vascular system of plants.
They lack the structures of plants.
The plants that we know have to transport
liquids internally and seaweeds just don't
need to do that.
Plants we know have different sides to their
leaves.
The top half is different from the bottom
half and that's just not true for seaweeds.
And the roots, while seaweeds have something
that look like roots, they function very very
differently.
So seaweeds, they are their own group.
They are their own thing.
And we call them marine algae or macroalgae,
and this is to distinguish it from the phytoplankton
or the microalgae we were talking about in
the previous video.
As I had said, they are eukaryotic, they are
multicellular
and when we look at the structures of a seaweed,
there are two main sections.
There's the holdfast, which connects the seaweed
to the seafloor in that area, and then the
thallus, which is the part that is extended
up above the holdfast.
Now the thallus itself is made out of blades
and stipes.
So when we look at these different structures,
the thallus is the main body of the seaweed,
and then we have the holdfast, which again
sort of look like roots, but behave very differently
than roots.
The whole purpose or goal of the holdfast
is to simply hold the algae in place.
It's not bringing anything up from the rocks
below, it's not transporting liquid or moisture,
things that roots of terrestrial plants would
do.
So it appears like a root but it doesn't function
like a root.
Then, when we look at the structures of the
thallus, the leaf-like structures are called
blades.
As I had mentioned, unlike earth plants, the
blades of a seaweed are not going to be different
from one side to the other.
You aren't gonna have one side that's the
top and one side that that is the bottom and
part of the reason for that is that these
blades on these algae are just going to be
swept back and forth by the seawater and so
it's not known which side is going to be facing
the Sun to receive most of the solar input
for photosynthesis.
And then, the stipe, the stipe is going to
be the body or the connecting portion of that
algae.
Now, the blades, the Stipe, in fact even the
holdfast, they all have chloroplasts and they
all have photosynthetic pigment and so really
every surface of a seaweed is able to photosynthesize.
When we talk about reproduction for seaweeds,
it turns out that there are a few different
options.
There are some seaweeds which alternate from
haploid to diploid generations, back and forth
again, there are some seaweeds which they
don't alternate.
The primary seaweed you see is either in the
haploid or the diploid generation.
Meiosis and sexual reproduction still occurs,
but the main body of the algae is either going
to be haploid or diploid depending on the
species that you're talking about.
And then, there are those seaweeds which will
alternate, or some aspects of the seaweeds
life it will be a diploid organism, some will
be a haploid organism, and if you look at
the red algae for instance, they in fact have
three generations.
They have their sporophyte diploid generation,
that gives rise to a gametophyte generation,
and that in turn gives rise to a Carpospore,
which ends up becoming a sporophyte, so they
have three distinct phases to their life cycle.
So, sexual reproduction in many seaweeds involved
complex life cycles, often consisting of different
generations.
So let's talk about the seaweeds.
It turns out that there are three major groups
of seaweeds.
We have our green seaweeds or green algae,
brown seaweeds or brown algae, and red seaweeds
or red algae.
So, the green seaweeds, there are about 7,000
species.
Again, mostly marine, but there are some green
algae found in rivers and lakes and aquatic
environments on land.
In fact, sometimes even your Brita pitcher
or the aquarium in your house.
They can range from microscopic to macroscopic
and when we look at their photosynthetic pigments,
we see they have chlorophylls a and b as well
as carotenoids and this is the same for true
plants.
These are the main photosynthetic pigments
we'll find in most terrestrial plants.
As green seaweeds perform photosynthesis and
accumulate extra photosynthetic product, they
end up storing that excess as starch and this
is also very similar to land plants.
Things like carrots and potatoes they store
their excess photosynthetic product as starch.
These green seaweeds even have cellulose in
their cell walls.
These green algae in a lot of ways they are
very similar to plants, however they do not
have the same structures.
They don't have roots and shoots like plants
do.
They would have a holdfast and a thallus,
in fact there are some calcareous green algae
in which calcium carbonate ends up being deposited
within their thallus.
In the picture that we see here is of a type
of green algae known as dead man's fingers
and it has a rigid structure within it, mostly
those calcium carbonate salts being deposited
as a part of its structure.
Next we'll talk about some brown seaweeds
or brown algae.
This is the category that kelps and many other
types of seaweeds fall into.
There are about 1,500 species total and most
of them are marine.
Some are microscopic, but most are larger,
and in fact, some can be up to 100 meters
long.
That's 300 feet!
These brown seaweeds are typically going to
be found in shallow cold waters.
Unlike the green algae, they contain chlorophyll
a and c and also a pigment known as fucoxanthin.
It is a brown photosynthetic pigment.
Now recall, photosynthetic pigments, they
absorb certain wavelengths of light and reflect
others and it's the reflected light that is
the light that we see.
A specific type of brown seaweed are the kelps.
Kelps are found in temperate and polar locations
in the oceans.
They are the largest of the seaweeds.
In some species, like giant kelp, each individual
can be hundreds of feet in length.
Kelp forests are among the most productive
and diverse marine communities.
There's high biodiversity and even though
you might think it otherwise, something like
a coral reef is actually very nutrient poor.
There aren't a lot of primary producers there,
but in the kelp forests, these kelp are performing
photosynthesis and capturing a lot of energy
that can be used by everything from prokaryotic
cells to plants and animals of various types
because the kelp are performing photosynthesis.
Now, most seaweeds are usually attached to
the bottom, but there are some forms of seaweeds
that can actually float.
They're pelagic.
They can achieve all of the steps of their
life while floating in water.
They can reproduce and grow and divide.
One type of seaweed like this is the pelagic
Sargassum.
So, Sargassum 
is a brown seaweed that forms massive floating
mats in the Sargasso Sea, which is the section
of the Atlantic Ocean north of the West Indies.
This pelagic Sargassum provides shelter and
structure in the open ocean environment, allowing
animal communities 
to thrive in places that they are normally
absent.
You often don't find large communities of
small fish out in the open ocean.
There just isn't shelter and protection for
them there nor the resources they need, yet
these Sargassum weeds can end up forming these
very diverse dense biological communities
out in the open ocean.
The last type of seaweed that we're going
to talk about are the red algae or the red
seaweeds.
There are around 4000 species, almost all
marine.
They contain chlorophyll a and then a photosynthetic
pigment known as phycobilins and other red
photosynthetic pigments as well.
Most species are found in warm or cold shallow
waters, but some can be found in relatively
deep waters.
If you recall, one of the properties of light
is that red light does not travel as deeply
into the water column as do the blues and
the purples and even green lights.
So you might say, “Well, why are the red
seaweeds deeper in the water if red light
doesn't reach there?”
Well, recall the light that you see is the
light that is being reflected, not the light
that's being absorbed.
So these red seaweeds, they specialize in
absorbing wavelengths of light other than
red to power their photosynthesis.
And that allows these to be better adapted
to those deeper, lower light areas.
In this picture, we see a coralline red algae
and within its thallus, it accumulates calcium
carbonate.
So, seaweeds are important primary producers
in the marine environment, but as humans,
we also find seaweeds to be very important
and useful and we get a variety of materials
and substances from them.
One example is Algin.
Algin is obtained from kelps and it's used
as an emulsifier in many food items and in
the making of many products.
Carrageenan is another compound from seaweeds.
Carrageenan is used as a thickening agent
in dairy products such as yogurts and milkshakes.
Agar is another product that comes from red
seaweeds and it can be used for biological
purposes as well as a thickener for food,
fillers in cosmetics and pharmaceuticals,
and even to protect canned meat.
Some sea weeds are also harvested directly
for consumption purposes, whether it's for
inclusion in ramen or sushi or a variety of
other meals and dishes, seaweeds can be consumed
directly by humans.
So that takes us to the end of our discussion
of multicellular algae and seaweeds.
Before our next video I would like for you
to think about “What sort of plants can
grow entirely submerged in seawater?”
We'll find out in our next video.
