[PALUMBI:] Corals
are small animals,
but they build structures that
you can actually see from space.
And as we zoom in we see
the reefs where I work
on a little island called Ofu in
American Samoa, the ridge there,
the edge is the coral
reef, and as we zoom
in we see what those reefs are.
Now the reefs are made
of individual coral
colonies like we see now.
That coral then is actually made
up of a whole series of small...
polyps they're called.
These are the colonial organism,
and the polyps themselves
have all the structures
that an animal needs.
It has a mouth, it has
tentacles, it has gonads,
it can live, it can
reproduce, it can grow.
They're in a colony of
genetically identical polyps.
Now the color on these
tentacles, like I said,
is not the color of the
coral itself, it's the color
of the symbiont, and
as the focus racks in
and out a little bit
here, then what we see is
that we can just see the little
globules of the symbiont.
Well, let's take a closer look.
We'll go into a tentacle
and see those.
These cells, the symbionts
are not just floating around,
they're actually
inside the coral cells.
The corals are simple, they
just have two cells layers,
an epidermis and a
gastrodermis inside.
The symbionts are
inside the gastrodermis,
and you can see it there.
Now this is a life form
called a dinoflagellate.
It has chloroplasts because
this is photosynthetic,
but it has very odd-shaped
chloroplasts
like these yellow
structures here.
We're going to zoom in to
the chloroplasts itself
because that's where the damage
happens during bleaching.
What do chloroplasts
have in them?
They have membranes
called thylakoid membranes.
Those membranes hold the
proteins called the photosystems
that then capture light energy
and turn it into
chemical energy.
It's the molecules that turn
all of the sunlight that we get
on the planet into
the food that we eat.
The rain of photons down
here hits these photosystems
and they gather them up.
Now if the temperature goes up
and if the light goes
up, then they freak out.
There's too much energy,
the photosystems break
and they no longer can function
the way they do, but the rain
of photon keeps going,
the energy is still there,
and as a consequence
that energy is now turned
into reactive oxygen molecules.
Those are damaging to cells,
so it damages the inside
of the symbiont, it damages
the inside of the coral cell
and they spit the symbiont out.
That spitting of the symbiont
out by one coral cell isn't bad,
but if the entire colony does
it, then that's coral bleaching.
What you can see here is
simulated of the spitting
out of these symbionts
and the gradual whitening
of this particular part of
this particular coral colony.
Well, when that happens
across an entire colony,
then the coral turns
from its normal tan color
into a white color.
What difference does that make?
The symbiont provides 75-80%
of the energy the coral needs
to survive, and without that
energy, it can't make a skeleton
and it can't live very long.
So as a consequence,
a lot of the corals
that bleach eventually die.
