- Welcome everyone to the second day
of our 42nd Darwin Festival.
We're just letting people populate.
I guess today, Dr. Hanny Rivera will be introduced
in a moment by my colleague, Dr. Thea Popolizio,
our colleague in the Biology Department.
We just hold on a few more moments
just to let people move into the virtual room with us.
All right, Dr. Popolizio, I'll hand it over to you.
- Hello, everyone, and welcome to Darwin Festival.
Many of you might be actually happy today
to be joining us from your couch on the snowy afternoon.
I'm still thrilled to be introducing Dr. Hanny Rivera
for today's second lecture.
Dr. Rivera is a molecular ecologist
and evolutionary biologist.
She studies population genetics
and gene expression and reef building corals,
and is particularly interested in understanding
how corals respond to environmental change,
what makes some corals more resilient
to environmental stressors,
and how Earth's reef ecosystems may fare in the near future.
Dr. Rivera completed a bachelor's degree
in biology at Harvard College
and a PhD in biological oceanography
at the joint program of MIT
and Woods Hole Oceanographic Institution.
She is currently a postdoctoral scholar
and associate lecturer at Boston University
in the Sarah Davies Marine Population Genomics Lab.
And in addition to teaching and research,
Dr. Rivera is active in scientific outreach
and has led and co-founded a number of events
and organizations that support
environmental sustainability and science communication.
I am lucky to have crossed paths with Dr. Rivera years ago
when we were both students living and conducting
field research in the small island community of Bermuda.
So it's with great pleasure
that I introduce her to you today,
and I hope you'll join me in spirit
from behind your screen in giving her a warm welcome
to Salem State and to Darwin Festival.
- Thank you so much for that introduction, Thea.
Certainly wishing we were back in Bermuda right now.
Okay, I'm gonna go ahead and start this.
So thank you, everyone, for being here this afternoon.
They told me the number of participants before we started,
and now I'm a little overwhelmed.
So thank you for all the varied interest in this work.
Hopefully, today, you will leave
with a better understanding of corals,
why we care about them,
and some of the threats and conditions that they're facing.
I've tried to keep this talk fairly big picture,
but if you have any questions about specific techniques
or data analysis or anything like that,
please feel free to ask those questions in the Q and A.
So first, I'm gonna be talking to you
about why corals matter to us.
They're actually really important for humans.
The kind of magic of coral is somewhat grounded
in this amazing symbiosis that they have
with the single cell algae that live inside their tissue.
So I'll talk to you a little bit
about how that symbiosis works and why it's so efficient.
And then I'll move on to some not so wonderful news,
but very important things to know
about how climate change is impacting reef health.
And then I'll share a little bit of my research
on trying to understand which reefs
might fare best in the coming decades,
where we might find resilient reefs
that are actually able of tolerating
some climate change stressors and what that can mean
for the future of reefs more generally.
So first, I'd like to talk about the many services
that coral reefs provide,
and these really range from economic services
such as coastal protection to tourism generation, right?
They're beautiful ecosystems to explore
and they drive a lot of tourism
to many countries and coastlines,
and they also provide a good number of fish
that we actually eat.
There is also a suite
of biological benefits that they offer.
Coral reefs are often touted
as being the rainforest of the ocean.
And this is because they host a plethora of species.
So there's thousands of species
that live on coral reef environments.
And there's even more species that even if they don't
spend their entire life on a coral reef,
spend some portion of their life in a coral reef habitat,
largely as juveniles.
In addition, they also offer several societal benefits,
including drug discovery and pharmaceutical capabilities.
So for instance, there are several cancer and HIV medicines
that have been developed from organisms
that live on coral reef ecosystems.
And they also just offer us a really wonderful environment
to explore and learn and gain an appreciation for nature.
The total value of all of these services
is estimated to be about $11 trillion annually.
And this is under just kind of baseline conditions.
When there are things like mega hurricanes,
the value of the coastal protection that they offer
increases substantially, for instance.
And they're also critical for millions of people.
So this is a map that I generated for a paper
I recently wrote about coral reef ecosystem services,
and it shows coral reefs here in red as red dots.
You can see that they're mostly concentrated in the tropics,
'cause they like warm waters,
though as you'll learn later, not too warm.
But there are hundreds of thousands and millions of people
that live in cities that are near coral reefs
and they use these reefs to fish for food
that they need every day to generate tourism
and economics for their countries,
as well as coastal protection against storms.
So coral reefs are really important across the globe.
You can notice in particular that the Indo-Pacific region
has a very high density of both coral reefs
and people that live on or near reefs.
In the United States, most of our coral reef environments
are in the either Hawaiian archipelago
or in the Florida Keys,
as well as a number of US territories
that are in the Central Pacific,
one of which I'll be talking about a little later today.
Now, corals are really interesting
because they form these massive structures.
Corals are calcifying organisms.
So they literally make rock that forms reefs,
and these reefs are so massive they can be seen from space.
On the left is a satellite image from the Florida Keys
that shows the Florida Reef Tract,
which is one of the biggest barrier reefs in the planet.
And this whole section that you see here
is essentially that shelf that has been formed,
or that barrier that has been formed by the reefs
that have grown to create the Florida Keys
over the last several millennia.
On the right hand side is just a short segment
of the Great Barrier Reef in Australia,
which is the world's largest barrier reef system.
And these kind of neon blue smatterings
that you see here are all reefs
that are creating that barrier system.
Now, you might imagine that these reef environments
almost look like seawalls.
And in fact, they actually offer
a lot of coastal protection against storms
by acting as a seawall, essentially.
They take a huge amount of wave energy
whenever there is a storm or say a hurricane,
as is often the case in Florida or Australia,
in which case, they're typhoons.
But they're really important for that aspect alone.
So important in fact, that recently,
the Department of Defense is trying to fund projects
to develop reefs artificially that might mimic coral,
the protection that corals already offer.
Okay, so now I've hopefully convinced you
that corals are really important to us,
that we should care about them.
Now, I'd like to tell you a little bit about what they are.
So corals are animals,
but they're a little bit more than that.
So here in this picture, you're seeing a coral colony.
That's this entire kind of lettuce-looking
brown structure that you see here.
And this coral colony is actually for and by
an amalgamation of these individual polyps
that you see in this closeup here.
And corals are related to jellyfish.
So you might notice that this polyp
kind of looks like a jellyfish
that's been flopped on its back
and has its little tentacles in the air.
And that's essentially what polyps are.
Now, the coral polyp essentially
grows this colony by reproducing itself asexually.
So it just clones itself from the initial polyp
that settles as a baby coral
and then grows over time to form
these really amazing structures.
And part of the reason that it's able to grow
and generate these reef environments is due to the symbiosis
or this very beneficial relationship
that it has with multiple partners.
One these partners is a single cell algae
called Symbiodiniaceae is the genus,
they're dinoflagellates.
They're a type of phytoplankton.
And these algae live inside the tissue of the corals.
In addition, they also host, similar to humans,
which have a very healthy, say for instance, gut microbiome,
corals have all sorts of bacteria that live
both in and on its tissue that also help stabilize
both the relationship with the symbiont,
as well as provide additional nutrients
and benefits for the coral itself.
Now, collectively, these three partners
are called the holobiont,
because they are kind of this meta-organism.
It's not just the animal,
the animal needs the plant, needs the bacteria
in order to actually function and survive.
Now, the relationship with the algae
is one of the more sensitive partnerships.
And so I'm gonna spend a little bit of time
talking about how that works and why it's under threat.
So for the rest of the talk, I'll just refer to the algae
as symbionts as opposed to saying Symbiodiniaceae,
which is a little bit of a mouthful.
So this relationship is really foundational to the coral
because the algae which are tiny microscopic plants
are using sunlight and they're creating sugars
through photosynthesis that they then provide to the coral.
So what the coral has managed to do over eons of evolution
is essentially create a relationship
that gives it an in-house factory.
So it's constantly being fed by these algae
that it's hosting in its tissue.
In exchange, the coral is providing nutrients
and shelter for the algae.
We know plants need fertilizer to grow
and be happy and healthy,
so that's essentially the role that the coral is providing.
It's giving the nutrients,
the correct amount of nitrogen and phosphorus
and other amino acids and things that it needs
to grow and photosynthesize properly.
Now, a lot of what we know about this relationship
comes from either metabolic studies
that have looked at exactly what sugars
are being transferred between the symbiont and the coral,
or gene expression studies of which
I recently started to get my hands into,
that look at what genes are differentially expressed,
meaning what genes are turned on or off
and thereby creating the proteins
that those genes encode for
when a coral is in symbiosis
or when it doesn't have that symbiont.
So here, I've done a study recently
with a coral called Oculina arbuscula,
which is one of the few corals that can actually
live happily with and without its symbionts.
And I've asked what genes are differentially
expressed in these two symbiotic states.
And one of the things that we see
is that symbiotic fragments of this coral
heavily upregulate metabolic processes,
which makes sense, because it's part of this exchange
of sugars and nutrients that really drive the symbiosis.
So here are just a couple of,
these are called Gene Ontology terms,
which are general categories for what those genes
are related to what types of processes.
And you can see that a lot of the genes
that are differentially regulated
are involved in the process of lipids
and amino acids and carbohydrates
such that this symbiosis can actually prosper.
Now, this symbiosis also allows
the coral to form its skeleton.
Biomineralization is an expensive energy intensive process.
And it's really thought that the evolution of symbiosis
is what gave corals the kind of energetic push
that they needed to really be able to form
and biomineralize at the scale that they do.
Now, this relationship is extremely beneficial
for both the symbiont and the coral,
but unfortunately, it's very sensitive,
and it can be easily perturbed.
One of the things that can disrupt it
very quickly is temperature.
And so many of you may have heard
of a phenomenon called bleaching.
It's been happening more and more.
And what it means is that there's a temperature increase,
and this temperature increase essentially sends
the photosynthesis mechanism of the symbiont into overdrive.
So now, the coral is left with this
kind of overheating engine inside its tissue
that it can no longer really control or regulate properly.
And so it ends up either the cells
that have the symbiont in them die,
or the coral expels the symbionts.
And in doing so, it loses its pigmentation,
'cause most of the color of the coral
comes from these symbiotic cells.
And what's left behind is a coral
that looks like you put it in bleach
and it lost all its color.
The bleached coral is still alive,
but the coral tissue is transparent.
And so what you're seeing is the skeleton
that it's made through that clear tissue.
Now, bleached corals can die very quickly,
in which case they just leave behind the skeleton
and the skeleton can be easily overgrown
by say bacterial biofilms or other big algae.
And they transform a reef into a really healthy ecosystem
that looks like the one on the left to not very healthy,
not very biodiversity ecosystem,
such as the one on the right,
which is essentially just a giant coral rubble cemetery
covered with filamentous algae that are not really
providing the same kind of ecosystem services as before.
Now, as I mentioned, the bleached corals are still alive,
but they're starving, right?
They've lost their main source of food.
And what this means is that they have very short window
during which they can actually recover
those symbionts and survive a bleaching event.
And in order for that recovery to happen,
it really needs to,
the temperatures really need to come back down.
Now, oftentimes when there are bleaching events
that trigger coral bleaching,
they're often usually heating events
that last several weeks, sometimes months.
And these are particularly dangerous,
because they can then kill off
a really large percentage of corals.
And that's exactly what's been happening more recently.
So bleaching events are getting worse.
They're killing more corals
and they're becoming more widespread and more frequent.
Here, on the right, I'm showing a graph
that depicts the global ocean heat content,
so how much heat is being trapped in the ocean.
You can see that that number has been increasing
since about the 1970s or so, and it's going up quite a lot.
And now, what this has led to
is what have been called global mass bleaching events,
which means that the waters in the ocean
have gotten warm enough at such a wide scale
that we see bleaching happening
at reefs all across the planet.
So there's bleaching in reefs in the Red Sea,
there's bleaching in the Caribbean,
there's bleaching in the Central Pacific,
there's bleaching in the Great Barrier Reef,
there's bleaching in the Azores, everywhere.
And now, the first mass bleaching event
that coral scientists really recorded
was in the early 1980s.
And since then, there's been several other,
even larger events.
And you can notice here at the end
that both in 2010 and 2015,
that's a very short time lag.
And so they're becoming more and more frequent.
And the amount of time
between bleaching events is getting shorter.
And actually since 2015,
we've seen a couple of other really big bleaching years.
Not at the scale of 2015, that's still quite worrisome.
Now, another threat that corals are facing today
is that their skeletons are made of calcium carbonate.
Now, this mineral is really sensitive to pH.
And because we're emitting CO2 into the atmosphere
and that CO2 is mixing in with ocean water,
that drives a chemical reaction that actually is decreasing
the overall pH of the ocean's waters.
And this decline in pH makes it
much harder for corals to grow.
It makes it such that biomineralization process,
which was already very energy intensive,
becomes even costlier, right?
You have to essentially drive the formation
of calcium carbonate with fewer resources
and against a much stronger gradient.
It also makes it such that coral that for instance
might've died and has a skeleton that it left behind,
that skeleton is gonna dissolve
much more quickly also in lower pH.
So here, I'm showing you pictures of a coral that was grown.
So this is a baby coral during its initial sediment,
when it forms its first polyp.
It was grown in low pH or higher pH.
And you can see that as you lower the pH,
there's basically no skeleton there.
It becomes really difficult for that coral
to actually form the restructure
that is the basis for this entire ecosystem.
So I've mentioned two main threats that corals are facing,
acidification and temperature.
Both of these are directly linked
to the increase of greenhouse gases
and in particular of carbon dioxide in our atmosphere.
There are also other threats that corals are facing.
So for instance, corals can also get sick.
There are the variety of bacteria and viruses
that can infect coral and cause tissue damage.
These often interact negatively with temperature.
So for instance, there are pathogens
that become more virulent at higher temperatures
and therefore can infect corals
more easily and spread more quickly.
There are also runoffs.
So if, for instance, say there's a coral reef
near an agricultural area or a golf resort,
all that fertilizer gets into the water
and it disrupts the nutrient cycling in the reef
and it can disrupt the symbiotic relationship as well
and lead to coral bleaching.
In addition, there is overfishing.
That algae that grows on the dead coral skeleton,
it's always trying to grow on the reefs.
But when the coral is alive,
it can kind of maintain its growth
in such a way that it out-competes the algae.
But one of the things that facilitates that competition
is having herbivorous fish that eat the algae away
and keep it from growing over the coral.
When reefs are over-fished,
then those herbivorous fish are gone,
and now the algae actually have an edge
and they can overgrow the coral as well.
So this is all a little bit depressing, and I'm sorry,
but it's kind of the truth.
So I do wanna share it with you.
There are lots of threats that corals are facing.
Now, I don't wanna leave you on a completely bad note.
So I do wanna talk a little bit
about some of the hope, right?
So we have all these threats.
They are not really in the best prognosis,
but can they actually make it?
And I would like to say that yes, they can.
But we humans need to act quite fast
to address a number of these issues.
And in addition, we also need to act quite fast
to better understand symbiosis and better understand
what are the differences between the corals
that don't seem to be as effective
versus those that are really suffering
from some of these stressors.
Okay.
So now, I'd like to turn and share
a couple of stories from the work that I've done
throughout my research career so far.
And a lot of this has been focused on resilient reefs.
So what do I mean by resilient?
This can mean a couple of things.
So for instance, it might be a reef
that's not really expected to get as hot,
and that might be because maybe
it's a more northern latitude reef,
and so the waters are actually much cooler right now.
So by the time temperatures increase
one, two, three degrees,
it'll still be in a relatively normal range for corals.
So for instance, in Japan,
we're seeing that that's often the case
and there's even range expansions into areas
that were previously cooler that have now warmed.
And we're seeing corals starting to grow within those areas.
There are also coral species
that are more tolerant to stress.
There's about 800 or so coral species.
And we're starting to learn with molecular techniques
that there's probably way more than that.
And each of those species
has slightly different thermal threshold.
So sometimes, you might see a picture
from a bleached reef where you have
one coral next to another and one coral is perfectly fine
and the other one is very, very visibly bleached.
And so trying to understand what those differences are
can offer some hope and also knowing
that those differences exist is a good sign
for the potential corals to adapt
to the future climate change.
Then there are also reefs that are already quite hot
or that already have low pH or sometimes both,
but somehow they happen to have
really healthy coral communities.
I'm gonna talk to you
about one of those in just a minute.
And then there are other reefs that might be affected
and perhaps they bleach widely,
but somehow recover very quickly.
And I'll also talk to you about what are those in a minute.
So first, I'd like to talk to you
about areas where reefs already
in really kind of unideal conditions,
but have very healthy environments.
And so one of those examples
is in the island archipelago of Palau,
which is in the Western Pacific.
It's a little bit too small to see it here
zoomed out at the scale,
but it's just east of the Philippines
and north of Papua New Guinea.
Now, what's really unique about Palau is that A,
it has a very extensive reef environment.
So this is an outline of those reefs,
all the things that you see here
in this sort of dark blue grayish color
are the outer barrier reefs.
So it has this very nicely formed
sort of atoll barrier reef structure.
And then in addition, all of these islands
here on the bottom have this like very intricate
network of bays and little channels and inlets
where there are reefs and coral reefs in there.
Now, one of the really fascinating things
is that the environmental conditions on the outer reefs
are what we would expect for the open ocean.
It has still relatively basic pHs,
has slightly cooler temperatures,
but as you move into these rock island environments,
what we see is a pretty substantial
increase in both temperature,
sometimes as much as one or two degrees Celsius,
which is plenty of a temperature difference
to cause bleaching in many coral species.
And in addition to the increase in temperature,
we see a decrease in pH.
Now I'll remind you that pH
is measured on a logarithmic scale,
so this difference is actually quite severe.
Now, this picture here was taken in the rock islands,
and as you can see, the coral there look beautiful,
they're colorful, they're happy.
It's actually really densely populated with corals.
And so in Palau, these rock island environments
are actually healthier by some of the metrics
that we often use to measure coral reef health
and some of the outer reef environments,
which in theory, have better environmental conditions.
So here, on the top left,
I'm showing you a graph that measures coral cover,
which is essentially if you were to take a square meter
and plop it down on the reef bottom,
how much of that square meter is covered in coral,
coral that's alive.
And on the x-axis here is aragonite saturation,
which is a measure meant that's very related to pH,
so the lower the aragonite saturation, the lower the pH.
And as you go into these low pH rock island environments,
what we actually see is an increase in coral cover.
So there's actually more coral
living in these rock island environments per square area
than in some of these outer reef habitats.
Likewise, when there are bleaching events in Palau,
corals that live on the outer reefs,
which is shown here in blue,
have a higher percentage of bleaching.
So in the sites that are on those outer barrier regions,
sometimes we see as much as 60% bleaching,
whereas the rock island environments
have much lower bleaching overall.
So this has presented a really interesting kind of paradox
where these environments that one would think
should be really stressful to the corals
seems to have bred very tolerant corals.
And so this is actually one of the questions
that I set out to answer in my PhD,
and it's something that I'm still working on
several years later.
So one of the things that I've done on this front
is to use genetic analysis to look
at how these populations differ from each other.
And I've focused in on a coral called Porites,
which grows in this massive form here.
They can grow as big as like a small car.
And their growth form is actually very helpful
for a reason that I'll mention in just a minute.
But across several years, I've been lucky enough
to go to Palau and sample several hundred of these colonies
and extract DNA from them and then look
at what genetic differences might be going on
between the ones that live in these hot environments
and the ones that are found outside.
So I've sampled several hundred of these.
And then to give you the kind of biggest picture,
what I'll show you next is a breakdown
of genetic kind of subpopulations.
So the different colors in the chart that I'm about to show
represent populations that have been somewhat
genetically distinct from each other
for some amount of time.
Now, what we see is that on the cooler high pH reefs,
so these are those outer reef environments
that are shown here in this square on the left,
there's a predominance of a particular population
that I've colored here in dark blue.
Whereas when we move into the hotter lower pH reefs
inside those rock island networks,
we see almost a complete shift
in the population structure of these environments.
We see that the reefs are now overtaken
mostly by this light blue population.
And in some other cases, this red population
that's actually almost completely absent
from the outer reef environments.
And so this is an initial kind of finding that suggests
that there's some genetic differentiation
that's occurring across this environmental gradient.
Now, another question that I asked was,
does this genetic differentiation map to differences
that we might find in thermal tolerance?
Now, one of the neat things about Porites
is that because it grows in this massive morphology,
we have a way of essentially
looking back into its past growth history.
And we do that by taking a coral core.
So you can dive down with a drill
and drill into the coral colony.
And I promise you that this does not kill the coral.
You can like patch the little hole that you made
and then it continues growing through the rest of it.
The live part of the coral is actually
only in the upper like centimeter or two.
So the rest of it is just dead, not dead skeleton,
but previous skeleton that it has deposited.
There's certainly other critters that live in there now.
And so you end up with the skeletal biopsy, if you will,
the cylinder that then you can scan
and see how the coral has grown throughout time.
Now, similar to tree rings,
corals lay down their skeleton in a particular way,
so you can count back and figure out, in 2010,
for instance, when there was a warming event in Palau,
what was happening.
And so what we found when we looked
at a number of different coral cores across the Pacific
is that when there are events of high temperature,
and therefore the coral is bleaching,
it leaves a scar in the skeleton.
It leaves a signature behind
that we can measure and quantify.
And that's something that I'm showing you here in the right,
this bright white band is a time
when the coral was actually bleached.
And the reason that it's white
is that it's a very densely packed
piece of aragonite skeleton.
And what we think is happening
is that because the coral is bleached
and it's not really in a very energetically favorable state,
it's not growing upwards.
So the little aragonite that it's depositing
is just being compacted in that row
and it leaves behind this really bright band.
Now, what that allows us to do is look back in time
and count how many of these corals had stress bands
and then relate that to the genetic signatures
that we're seeing across these populations.
So I'll just show you one very small
data point from this analysis,
where we looked at stress band
of individual colonies in 1998.
So we looked at that core
back to when it would have corresponded to 1998
and figured out which ones had stress bands in their cores
and then related that to their genetic population.
Now, what you might notice
is that that dark blue population,
which is shown here in this first graph, first bar,
the one that's most common in those outer reef environments
has the highest proportion of stress bands.
So those corals are highly susceptible
to the warming that occurred in 1998.
Now, the corals that mostly live
inside those rock island environments,
which by the way, still get hotter still
in 1998, when there's a bleaching event,
actually showed a very low percentage of stress bands.
So even though they're in warmer environments overall,
these environments get hotter still
when there's a bleaching, when there's a warming event,
most of them are not bleaching.
And so what this hints at is that Palau
is likely home to corals that have a genetic basis
for higher thermal tolerance.
And so a lot of the followup work that I've done since
has been trying to get at this question
and try to understand the nuances
in the system a little bit more clearly.
So if you have questions about that,
please feel free to ask me in the Q and A.
I just didn't wanna throw too much data at you.
All right, so now I'd like to take you
to a different system.
Now, this is Jarvis Island.
It's a US territory that you might not have ever heard of,
but it's actually located in the center of the Pacific,
pretty much on the equator, about as far away
from any continental mass as you can get.
And this color scheme that you're seeing on the map here
is representing the temperature anomaly.
So how much hotter than normal the waters were in 2015
when there was that massive global bleaching event.
Now, you'll notice that Jarvis
is in pretty much the hottest spot of this map.
Now, under normal circumstances,
Jarvis is a really healthy,
really beautiful coral ecosystem.
It's even been rated
one of the healthiest coral environments
in the world on multiple occasions, sorry.
But it's located in a region that sees frequent warming,
due to what are known
as El Nino-Southern Oscillation events,
which are climate oscillations that cause warming
particularly in the Central Pacific region,
but they also have effects worldwide.
Now, in 2014 to 2016 when we visited
this site multiple times,
we saw almost 100% bleaching.
So pretty much every single coral
that was on that island was bleached.
And when we returned a year later in 2016,
almost 90% of them had died,
or at least we thought they had.
Some of my colleagues returned yet again in 2017,
and what they found was a really surprising recovery.
So this is a colony, again, that same massive Porites.
In 2016, you can see that it looks like
there's absolutely no live coral tissue
anywhere on this piece of rock.
It's kind of all been overgrown by this red fuzzy algae.
And this is 2016.
You'll notice there that my colleagues
are taking a core of it.
Now, in 2017, when they returned,
this looks like a pretty healthy, happy colony.
Most of this tissue, and this is not a small colony,
here's a scuba tank for scale, has recovered.
Most of this tissue is alive and healthy.
Here's a closeup.
This is all happy coral tissue.
And if you're not convinced that this is the same colony,
I'll guide you through this kind of very unique pattern here
that we see in the skeleton, and we see it here too.
So this particular colony was able
to get really impacted by this bleaching event,
presumably almost entirely die,
yet, within a year, it had recovered
most of its tissue and most of its symbionts.
Now, when we've cored colonies from this area,
what we find is really interesting.
So here's a long core from another colony at Jarvis.
And you'll notice that it's,
in this case, it's going back to about 1980,
and you'll notice that there were multiple
stress bands in this individual colony.
So every time that there's a really big bleaching event
or a very, very hot temperature anomaly,
this colony is bleaching, right?
It does so during the first
massive bleaching event in '82, '83,
it does so again a few years later,
and then again, and then again, and then again,
and then again, and then again.
But it's able to continue growing and surviving.
This is again the same individual colony.
It has not died during these times.
And so we see this pattern,
actually multiple of the cores from this island.
And in one case where we have the longest core,
we actually see stress bands
that date back to as old as 1920.
So it seems like in this region,
bleaching has been occurring way before scientists
even knew that bleaching was a thing.
And that the colonies in this area
are capable of bouncing back from it very quickly,
even if they are affected.
So today, I've shown you two examples
of what are called coral reef refugia,
which are places where corals might have a chance
to really live a little bit longer
than we project for many areas.
Japan, as I alluded to briefly,
might be another example of that
where we're seeing range expansion
of corals into cooler waters.
So all of these examples provide some hope for the future
that corals might be able to persist,
even in light of a lot of these
very heavy stressors that they're experiencing.
So what other things are being done?
And how else can we look towards the future
of coral reef environments?
So one thing that's happening
and increasing throughout the world
are large-scale restoration and captive breeding efforts.
A lot of this work is currently focused
in Florida and in the Caribbean
where coral disease is a very big stressor as well.
So people have started focusing on trying to breed
disease resilient corals and outplant corals
of species that have suffered
really big losses in the Caribbean, such as acroporas.
There's also a lot of research that's trying to evolve
more thermally tolerant symbionts in the lab.
So one of the things that I kind of glossed over
is that there's several different species of symbionts.
There's almost hundreds of different
species of symbionts that associate with corals.
And each of those symbiont species
has slightly different thermal sensitivities.
Some of them can tolerate much higher temperatures
before they break down and lead to bleaching.
And so there's a lot of work
that's trying to understand why that's the case,
as well as evolve in the lab,
symbionts that are perhaps
a little bit more thermally tolerant,
and then try to use those to reinfect corals
with more thermally tolerant symbionts.
There are also assisted evolution
efforts of corals themselves,
both in the field and in the lab.
So taking corals that are either,
you either subject them to heat stress in the lab
and then take the ones that are most strong against that
and then try to breed those with each other.
Or in the field, you take survivors from bleaching events
and then try to use your selection pressure that way.
Then there's a lot of work that's being done
and that I'm quite interested in
in trying to really understand
what genes are controlling thermal tolerance
and how we can use that information to identify
corals and species and locations that might have corals
with higher thermal tolerance than others.
And then there's also a lot of work going
into identifying other resilient reefs.
So I've shown you two or three examples of resilient reefs.
There are a few others, but there's a lot of effort
in trying to find more of these examples
and trying to protect them so that the few reefs
that we have that are stronger than most of them
don't also get bombarded by additional stressors
such as runoff or coastal development
or other things of that sort.
Okay, now that being said, refugia,
even though they're very important
and they offer us some help, they're not invincible.
So Jarvis Island, yes, it can recover quite quickly,
but it still had 90% coral mortality during that year.
So it's gonna take a long time for that reef
to really regain the same kind of coral cover
that it had prior to that event.
And if these bleaching events are getting more severe
and they're coming in shorter and shorter intervals,
they're becoming more frequent,
it's gonna be way harder for Jarvis
to continue to keep up that level of resiliency.
Likewise, in Japan, because these environments
are still not like highly suitable for corals,
when the temperatures go back down to being slightly cooler,
then that also impacts the corals.
They bleach because the temperature's too cold
and then they die, and so it's still not,
it's not a safe haven, let's say.
And in Palau, even though the rock islands
do have more thermally tolerant corals,
they're not immune to bleaching, right?
We still see some bleaching in these environments.
And as the temperature gets hotter and hotter,
we might start hitting their higher thermal limits.
So while refugia are important
and while a lot of the efforts that I just mentioned
are incredibly important and we need to do them,
we also need to understand that the root cause
of all of this is really rising greenhouse gas emissions.
They're the main cause of rising ocean temperatures
and of rising air temperatures.
And in order to actually fix the problem,
These are all solutions, these are all like medicines
that help alleviate the symptoms,
but you need to actually treat the cause of the issue,
which is rising greenhouse gas emissions
in order to curtail some of the changes
that are projected to happen across the planet.
And this here on the right is just a graphic that shows
how much of the heat that we kind of produce
through greenhouse gas emission
has been absorbed by the ocean.
And what you might notice is that the ocean
has done the bulk of the work
in taking up all of the heat that we've produced,
which is part of the reason why corals
are being some of the kind of heralds of the future
in regard to their climate change impacts.
They're being impacted first
because the oceans are warming so quickly.
All right, and all of this work is not just me.
There's a lot of people involved.
So I like to take a few moments
just to acknowledge all of my advisors and mentors,
a lot of the co-authors and collaborators
on the work that I presented today,
as well as the field crews in the boats
that I've worked on the Republic of Kiribati and Palau,
both of which are amazing countries to visit
and have incredible people.
And then of course, all of the funding
that went into making some of this research possible.
So I'll just give a quick summary
of what I shared with you today.
First, it's that coral reefs
provide invaluable ecosystem services.
Corals need stable ocean conditions to live.
They're kind of a little bit of a Goldilocks organism.
They need to be at like just the right temperature
at just the right pH with just the right light
in order to be really happy.
And a lot of the things that humans are doing
are changing all of those parameters,
so they're really struggling to keep up.
High temperatures in particular trigger bleaching
and can kill corals on a massive worldwide scale.
Lower pH is making it harder
for corals to grow across the planet.
And scientists are desperately trying to understand
why and how some corals or some reefs
are stronger than others.
But at the end of the day, really,
we need collective climate action
in order to advance both ocean
and human health and prosperity.
So with that, I will take questions.
- Dr. Rivera, this is Nelson Scottgale.
Thank you so much for a wonderful talk.
- Thank you. - We really appreciate it.
And we do have many questions here.
And so the first one we got was,
can bleaching also happen
due to a change in the pH of the water?
- Yeah, so sometimes you can see
bleaching induced by pH changes.
Usually the pH change would need
to be very extreme for that to happen.
But bleaching overall is kind of a breakdown
in the symbiosis of the coral.
And there's a lot of things that make that symbiosis stable.
So temperature is the main one
that has the kind of fastest effect.
One of the other ones that can also lead to bleaching
quite quickly is changes in nitrogen availability.
So the coral is actually trying to
kind of keep the symbionts in check, right?
It needs to do this delicate dance
where it wants to have enough symbionts in its tissues
to provide it with the nutrition that it needs,
but it can't really like over-fertilize them
or they grow so fast that it kind of hurts the coral.
And so for instance, in the case of agricultural runoff,
which is very nitrogen replete,
that can often trigger coral bleaching as well.
Because now, the coral can no longer like control
how much fertilizer it's giving the coral.
It's getting all this extra fertilization
and the symbionts start growing and overtake things
and then the coral freaks out
and throws them all out and bleaches.
With pH, it's a little bit more nuanced.
So for instance, with pH, there's often increasing CO2,
which can like simulate photosynthesis,
and so the symbiont is happy for a little bit,
but then if it like, again,
kind of gets over-fertilized and then grows too much, then,
again, the bleaching can kind of occur because of that.
But it usually needs to be like very low pH, very high pCO2
for that to actually trigger bleaching on its own.
- Thanks for a wonderful presentation, Dr. Rivera.
This is Ryan Fisher
with the next question from our audience.
And it's sort of a two-part question.
What determines which corals have symbiosis,
and is it by chance?
- So that's a very good question.
So corals are part of Cnidaria,
which is like a really speciose phylum.
As I mentioned,
that includes jellyfish and includes anemone.
And then within like the coral category,
there are hard corals,
which are the ones that I've been talking about today,
known as scleractinians are the ones that form reef,
they like precipitate calcium carbonate.
And then there's soft corals,
which don't really form like as hard of a structure.
Both hard and soft corals can have symbiosis.
Most scleractinian corals
of the reforming type have symbiont.
So there's maybe I wanna say like eight or so species
of scleractinian corals that have a facultative symbiosis.
So for instance, Oculina arbuscula,
the coral that I mentioned briefly
when I was talking about gene expression
is one of those corals.
And usually, the corals that have a facultative symbiosis,
and by that, I mean that it can be
without symbionts and still be happy,
it doesn't actually die,
those are usually corals that live
a little bit further out of the tropics.
So there are species that have expanded
either for instance up the Eastern Seaboard
or down into like New Zealand
into waters that don't have as much sunlight,
and they rely instead on increased heterotrophy.
So these kinds of corals usually have like bigger polyps,
they feed a lot more,
and so they get most of the nutrition that way.
But there's very few of them
that don't have obligate symbiosis
when we're talking about reef-building corals.
- The next question is climate change
is causing many different species
to shift the ranges in which they are found.
Is there any expectation that we might see
coral spread in areas not as hot?
You sort of touched on that, but how likely is that?
- Yeah, so as I mentioned,
we've started to see a little bit of that in Japan.
So there are areas in Japan that used to be
entirely algal dominated a few decades ago
and now have started to see really kind of wonderful
coral growth over the last few decades.
There aren't a ton of those examples.
And part of the reason is that corals are,
they grow slowly and they also,
they can disperse very widely,
but they need to have like very specific conditions
in order to attach to a substrate.
So they want a very kind of curated mix
of bacterial and like algal chemical cues
that actually allow them to like swim down
and attach and be like, okay, this is where I wanna live.
And so I think some of those changes need to happen first
before corals can really start expanding into new areas.
So there needs to be a little bit
of a shift in the ecosystem,
so that those chemical cues that corals use to settle
and disperse are there for them to actually attach
and then start forming reefs as well.
- Dr. Rivera, the next question is,
how long is a coral bleached before it dies?
- That really depends on the species.
So there are certain species like acroporas,
which are often the corals
that you see depicted in like movies.
And so for instance, this coral here on the top
right there would be an acropora type coral.
Those tend to be pretty sensitive.
And that's actually one of the most speciose genus,
genera of corals.
So a lot of corals are very sensitive to bleaching.
Some of the more resistant ones,
like that Porites that I work on,
that can be bleached for, you know,
maybe a month or two and still be okay.
The acropora start dying within a few days to a few weeks.
- The next question is
climate change, I'm sorry,
temperature and pH reminds me of denaturation of proteins.
Does this relate to the types of corals
that can't come back from irreparable damage?
- Yes, so you're absolutely right.
Some of the things that happen at a cellular level
when there's an increase in temperature
is protein denaturation.
So there's been a lot of work that's been done
looking at, for instance, gene expression changes
when you subject a coral to heat stress.
And some of the most resistant corals,
the genes that kind of seem to be involved
in helping them be resistant
are things that are called heat shock proteins.
So these are proteins whose job it actually
is to keep other proteins from denaturing.
And so when you have either
a high abundance of these proteins
or very effective versions of these proteins,
you can kind of protect yourself
a little bit from that heat stress.
But nevertheless, there's like
only a limit to that kind of strategy.
But yes, it certainly does play a role,
especially at the physiological cellular level.
- Dr. Rivera, the next question is a deeper question.
Can you please explain a deeper connection
of how coral reefs came about through evolution?
- Hm.
I'm not entirely sure what the person means
by a deeper connection, but,
and I'm not like a reef paleobiologist,
so I don't know all that much about
like the very precise intricacies of reef evolution.
But part of what we think might have happened
is that the mutualism may perhaps
started off as a parasitism.
And so we had, first of all, like coral reef environments
tend to be what's called oligotrophic,
meaning that the water itself has low nutrients.
And so these symbionts want higher nutrients, right?
And so you can get higher nutrients
if you're, say, inside the stomach of a coral
that's digesting things and releasing nitrogen.
And so perhaps this might be one of the ways
in which that symbiosis first evolved
where the symbionts or the algae in this case,
before there were really symbionts,
infected the corals to try to steal some of those nutrients.
And over time, that exchange
evolved to be a mutualistic response.
And in fact, some of the work that I've been doing
at Boston University is looking at,
kind of based off on that hypothesis,
how the symbiont modulates the immunity of the coral.
And one of the things that we see
is that oftentimes when you have a coral
associated with its dominant symbiont,
there's essentially like an immunosuppressive
element to that symbiosis,
where the symbiosis causes a shutdown of genes
that have to do with immune response.
But when you have a symbiosis with a symbiont species
that's not usually found with that coral,
that shut down doesn't happen.
Which seems to suggest that perhaps
that evolution of the symbiosis
was kind of this parasitic thing
where the symbiont that was best at shutting down
the host immune response and really sneaking in there
was the one that ended up establishing
this long-term symbiosis over time.
- Another question is,
where would you recommend we start to help solve
some of these problems plaguing the coral reefs?
- Well, as I mentioned, the root cause is rising CO2,
so really trying to do everything in your power
to both limit your own personal emissions,
as well as get our governments and our climate treaties
to function and be effective mechanisms
for changing the trajectory that we're on.
It's probably the most powerful thing.
On a sort of more local scale,
you can get involved in restoration efforts.
You can also get involved in environmental advocacy groups
that seek to say, limit runoff of certain chemicals
into coastal environments, things of that nature.
- Dr. Rivera in your final summary slide,
your final bullet point is collective climate action.
And my colleague Nelson asked you a question
that was heading down those lines.
The next question is similarly phrased,
but once I read it and you've answered it,
I was wondering if our colleague,
Dr. Popolizio might rejoin us.
She has a question to ask.
And we have a little bit of time,
maybe we can also talk about what other things
we can do collectively at the sort of grassroots level
to help our coral reefs.
So the question I have is,
do they have protection laws put in place
and do they treat the reefs like they protect forest parks
to stop the continuation of bleaching?
- So the short answer to that question is, yes,
there are some reefs that are protected.
There are not as many as probably
should be protected, in my opinion.
So protected areas in the ocean are actually,
there's a pretty small overall
surface area of ocean that is protected.
I believe the number is between like less than 1%.
And a lot of the kind of levels of protection
really vary depending on the country
and the type of protection strategy that's being used.
So it could be that you have a reef area
that's quote, unquote protected.
But what that means is that maybe you're not allowed
to fish there during a certain part of the year,
or maybe you're not allowed to dock there
or use anchors there or things like that.
There's very few protected areas
that are kind of blanket protection,
like you really shouldn't go there.
You're not really allowed to be there
unless you have special permission.
And there's a few of those,
but these are usually places that are already very remote.
So for instance, a lot of the work
that I also did in the Central Pacific
is in a place called the Phoenix Island Protected Area,
which is one of the largest in the world.
And it's a really beautiful, amazing nation.
And most of the use of that region
before it was protected was for tuna fisheries.
So these are open ocean fisheries
that aren't really going near the reefs all that much.
Now that that entire area is protected,
there's no fishing allowed anywhere in that region.
And that's also helped the briefs recover,
'cause it always helps to not have humans
coming in and taking things out of the water.
But those types of really big
blanket protections are quite rare.
They're getting more common, which is good.
But that's another thing that you can do to help
is kind of encourage the formation
of marine protected areas in your local environments
and the governments that you have some say in.
- Dr. Popolizio, you're back with us.
- Yes, sure, I was curious about,
I guess my question is related to coral reproduction,
and I'm wondering, when you were talking
about the differences in the population genetics
between the rock islands and the outer reef in Palau,
if there's any evidence of gene flow
between those two regions in the reef.
- Yeah, so there's, sorry, go ahead and finish.
- If that would have an impact on the resiliency
of the corals on the outer reef at some point
if there was genetic exchange.
- Yeah, so there certainly is a genetic exchange
in the sense that, so the lineages themselves,
if I look at genetic differentiation between those groups,
so the dark blue, the light blue, the pink, and the red,
they're quite diverged.
So gene flow between those lineages is quite limited.
However, it's not that the lineages
that are more tolerant are only found in the rock islands.
So there were some of those individuals in the outer reefs.
And when we actually look at those specific individuals
and look at their stress bands,
even when they're in the outer reef environments,
they're still bleaching less,
which suggests that they're retaining
that thermal tolerance that's perhaps genetically based.
So that does lend some hope in the sense that,
if say the outer reef environments are decimated
by a mass bleaching event over time,
larvae from the rock islands can make it out
to the other reefs and help replenish that population
with more thermally tolerant individuals.
So yes, that was essentially your question, right?
- Yeah, thank you. - Yep.
- We have another question.
I know that a lot of fisheries try to restore coral reefs
by placing nursery grown corals back onto the reefs.
How do they build up coral resilience?
- I'm not entirely sure I understand the question.
Is it why, or how nurseries promote resilience?
- I'm not sure, but I think that what they're saying is,
can those nursery grown corals
sort of repopulate and provide resilience?
I guess, that sort of the way I interpret that question.
- Yeah, so they certainly can.
And so restoration efforts, they certainly can,
but they're kind of very limited
in the impact they can have.
One reason being that corals grow very slowly.
So it takes a really long time,
like a decade to grow a coral in a lab
to like kind of like this size
to then be able to put it out on a reef.
And oftentimes, if you put it out on a reef
and then it gets really warm on that reef,
that nursery coral is not gonna
necessarily be immune to that warming,
even when you might've tried to like kind of optimize
for more thermally tolerant genotypes.
And so restoration efforts are important
and they can certainly help
if you're seeding in area that has nothing,
but it's not gonna be, like we can't do it at the scale
at which it would actually be like
the sort of saving grace for corals.
And there's a lot of people putting in
very amazing efforts to make it more efficient
and make it more amenable to larger scale restoration,
but it's still not quite there.
- I'll follow up with them with another one.
Does higher biodiversity increase the odds of corals
tolerating or surviving bleaching events?
- So that's a good question.
And the short answer is yes.
The longer answer, that it's kind of complicated.
So when you have a really biodiverse reef,
that can often promote resilience
'cause you have higher genetic diversity
and you maybe have more species of corals,
some of which might be more tolerant,
some of which might be less tolerant.
And so the ones that are more tolerant can live
and they can still maintain the reef structure.
That's gonna help recruit new corals
to come back and regrow.
So there's all sorts of interplay that happen at that scale.
I don't know that I can speak to like specifically,
if like biodiversity itself
is gonna facilitate thermal tolerance.
I can't think of a mechanism
by which that's necessarily the case.
It's more through these like interactive feedbacks.
- Thank you so much, Hanny, for that fascinating talk.
I think many people can appreciate
the beauty and wonder of coral reefs
and also know that they're threatened by human activities.
But you've allowed us today to go beneath the surface,
pun intended, and get a sense of the complexity
and diversity of their responses to their environment
and how the work that scientists like you do
is so vital to our ability to protect them,
as well as the policy decisions
we make with respect to climate.
So thank you.
- Thank you so much for having me.
It was great.
And thank you, everyone, for all the amazing questions.
- Thank you as well, Dr. Rivera.
And thank you to our audience.
We'll see you tomorrow at 11:00 a.m.
for our next talk on day three.
Bye bye. - Bye.