- You're probably familiar with the idea
that corals host symbiotic dinoflagellates,
which contribute to their overall health
by converting sunlight into food for the coral
but where do they come from?
Are corals just sort of born with them
when they settle out of the water
and start encrusting on the rocks early in their lives?
Hi, guys and girls.
I'm Reefman and today we're going to explore
the wonderful world of symbiodinium
and how that relates to the color of our coral.
So there are more than 40 different species
of symbiodinium that are found
throughout the world's tropical oceans
in Cnidarian, everything from jellyfish to Acropora.
Even zoanthids have them.
These are tiny little single-celled dinoflagellates
that embed themselves in the tissue of their hosting coral,
where they use photosynthesis to convert sunlight
into energy for themselves
and also for the coral that hosts them.
In turn, corals use a variety of fluorescent proteins,
link above, that serve to protect the dinoflagellates
and optimize the amount of wavelength,
the amount and the wavelength of light
that reaches them to maximize the photosynthesis.
When a coral bleaches, it expels all
of its dinoflagellates into the water
so we know that there's gotta be a mix
of a whole bunch of different species
of symbiodinium that are just floating around
in tropical reef waters.
We also know that coral is able to gather symbionts
from the water that surrounds the coral.
In fact, more than 70% of our corals
gather their dinoflagellates in this way
and some corals are actually even born with them
from their parents but most don't get them that way.
So we know that young coral can gather dinoflagellates
from the water that's symbiodinium
and we suspect that adult corals can as well
because they can recover from a bleaching event
where they've expelled all their dinoflagellates
into the water.
Symbiodinium can take two different forms.
One is where they're able to move around.
They have a flagella and they're able
to swim around in the water and the other,
where they're just not motile.
They can't move.
So when they're inside a coral's tissue,
they're always in their non-motile forms.
They don't need to move but after being expelled,
they quickly change their form and they're able
to swim around in the ocean within a few hours.
So the ocean is a huge place.
How do these tiny little single-celled organism
actually find a coral?
How do they choose which coral they want to be in?
This is an exciting set of questions
which is actively being studied by scientists today
and we just don't have the answers to all of them.
One hypothesis is that corals emit marker chemicals
that attract symbiodinium from the water.
Another is that they're attracted
to the same fluorescent proteins that are
in the tissues of the coral,
which would protect them
if they weren't swimming around in the ocean.
A paper about this was just published
earlier this year, in 2019.
It's somewhat tellingly titled,
"Green Fluorescence From Cnidarian Hosts
Attracts Symbiotic Algae."
There's a link in the description below.
I would encourage you to check it out.
The authors wanted to find out
if symbionts would swim towards a coral
with green fluorescent proteins.
Given that sunlight itself contains light
in the same wavelengths,
would symbiodinium be able to tell the difference?
Would they gather around the green coral
or just kind of disperse into the water?
To test this, they added symbiodinium
to a tank and then turned on a green fluorescent light.
They then took half the tank water closest to the light
and the half that was furthest away
and they looked at the concentration
of symbiodinium in each sample.
The closer to the light the sample was,
the more symbiodinium was in the water.
So they do swim towards a green fluorescent light.
Next, they used a live coral,
a green Echinophyllia aspera,
which glows bright green under blue actinic light.
They put a piece of live coral in a tank
along with some dead coral of the same type.
Would the symbiodinium be attracted
to the live coral that's glowing green?
As you can see in this video, yes.
The symbiodinium cells were found
in significantly higher concentration
around the live coral after shining a blue actinic light
to excite the fluorescent proteins.
They tested the same with red and with green light,
as well as with no light at all,
but they didn't find the same clustering effect
around the coral when the green proteins
were not fluorescing.
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Now you might be wondering,
What about the chemical attractants
that the coral might be emitting?
I mentioned those earlier in the video.
Well, to avoid those entirely in this experiment,
the team covered both the live and the dead coral
in a resin plastic case
so that they could only get the light out
and no chemical markers could pass through.
They repeated the same test using painted discs.
They painted a disc with green fluorescent protein.
They found the same result.
Then they also ran the same test
in the ocean using cell traps,
one that was just white
and one painted with green fluorescent protein.
The painted trap had much more symbiodinium in it
at the end of the test than the plain white one.
So, symbiodinium does preferentially
seek out green fluorescent protein.
So the colors the coral glow serve
at least two purposes now,
beyond simply looking cool.
They protect the symbiodinium from harmful light
and, it turns out, they play a role
in attracting the symbiodinium
to the coral in the first place.
Maybe we could use this in our own tanks
to help weaker coral get stronger.
Could we maybe increase the actinic light
that we use to trigger the symbiodinium
that's in our tank water to cluster around coral
and give the coral a greater chance
to gather more so that they can get more energy
and be healthier? Maybe.
You might think that other fluorescent proteins,
remember there are four different colors,
there's cyan, green, yellow, and red fluorescent proteins,
would these also attract different species of symbionts?
But it doesn't seem to be the case.
Previous studies actually used a prism
to split the light into its wavelengths
and then they shined it on water
and they looked at where the symbiodinium gathered.
It was almost exclusively in the green wavelengths.
So could this be why so many of our corals are green
under actinic light?
Thanks for watching.
I hope you found this as interesting as I did.
It's a really awesome paper.
I would really recommend checking it out.
It's in the link below.
It's a really easy read.
It has way more detail in the study
and also in the results than I was able
to share here but that's all.
I'll see you next week.
Hope you enjoy. Bye.
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