[Coral-List] So you think you understand coral bleaching?

[Douglas Fenner]

Scott,
    Thank you for this interesting perspective.  Of course learning more
about the mechanisms of the relationship between zooxanthellae and the host
coral animal, and the onset of bleaching is a good thing.  I don't think
anyone would argue with that.
    I note that in one of your review papers, that you write that "...the
onset of the bleaching syndrome and setting of upper thermal bleaching
limits are emergent attributes of the coral symbiosis...which is known to
depend not just on temperature, but also external (seawater) nutrient
availability and zooxanthellae genotype."  I have no quarrel with that.  We
could also add light intensity, which is well documented to
exacerbate bleaching (Coelo et al, 2017), and reduced water circulation
(Nakamura, 2001, 2003).  There may be other additional factors as well.
There are, of course, differences between the temperature threshold for
bleaching of the coral symbiosis with zooxanthellae, and the threshold for
mortality for a bleached coral with 1% or less of it's original
zooxanthellae still in it.  Those are two different things, and I would
argue that coral mortality is far more damaging to coral reef ecosystems
than coral bleaching.  Corals can, if the temperatures don't get too high,
recover from bleaching and be back to normal in most ways in a year or two
and life can go on.  Damage to the ecosystem is far greater when large
numbers of corals die.
     You want people to "go for the gold" of a "cure."  The use of the word
"cure" sounds like what people use when they talk about disease, but as far
as I can tell, you're not saying that bleaching is caused by an infectious
agent (though some may be, there is literature documenting that for one
species of coral in the Mediterranean).
      There is a difference between a "cure" and "treating the symptoms."
With the common cold, we can treat symptoms, but not the cause of the cold
(viruses).  So what is the ultimate cause of bleaching?  Nutrients?  Bad
zooxanthellae genotypes?  Do nutrients by themselves cause bleaching and
colony death if the water is not unusually hot?  I haven't heard of that,
correct me if I'm wrong.  The world has several thousand atolls that exist
in oligotrophic tropical warm seas, with flourishing corals, which can, and
have been, killed by high water temperatures, not by nutrients.  In 1998
and 2014-17, about 90% of the corals in the Chagos archipelago were killed
by hot water-induced bleaching, though coral populations recovered
considerably in between.  Only one of those atolls has any people on it,
there are only tiny islands, and some of the atolls have no islands at all,
NO terrestrial nutrient inputs, and corals were bleached and killed there
just like they were near to high islands and continents elsewhere when
temperatures got to high.  Are the genotypes of zooxanthellae that corals
naturally have causing bleaching outbreaks even when the water is not
unusually hot?  Haven't heard of that one, and it would have some
explaining to do to account for why bleaching occurs in mass events in
particular locations instead of continually everywhere.
      The article you refer to by Santos et al has an interesting sentence
in the abstract, the second sentence: "Coral reefs have been devastated by
warm water temperatures at a rapid pace and currently, there is no
effective method to predict a mass bleaching event."  Apparently the NOAA
Coral Reef Watch project is a total waste of money, all those maps of where
the water is hot, hotspots, degree heating weeks, all that stuff is a total
waste of time and money, according to that sentence.  That sentence flies
in the face of an awful lot of empirical evidence, it appears to me.
Perhaps they meant something else, if they did, they needed to state it in
a way that it couldn't be misinterpreted.  Because taken literally, it is
dead wrong, and a mountain of evidence shows that.
     Why am I picking on that?  Because your "go for the gold" is implying
that the "cure" for bleaching is in the mechanisms in the coral leading up
to bleaching, not in hot water and global warming.  What is the main
ultimate cause (not proximal cause) of mass coral bleaching mortality?
(that's the thing we need to avoid more than anything else to save corals
and reefs, isn't it??  Or is it best just to treat symptoms when you can
treat the ultimate cause?  When doctors have a choice between those two,
they don't neglect the ultimate cause, because if they do, they can lose
their patient.  If we do, we could lose our patient, the coral.) Isn't the
ultimate cause global warming with temperature spike events caused by El
Nino and other ocean heatwaves??
      How about nutrients, if high nutrients were the primary ultimate
cause, SST's (sea surface temperatures) would not be good predictors of
bleaching and NOAA Coral Reef Watch would never have gotten any traction or
funding, nor would anyone pay any attention to what they put on the web.
Can it be better refined?  Surely, what can't be?  Is there more to learn?
Surely.  BUT, it is the best current predictor of bleaching.  And that is
because higher temperatures do indeed cause bleaching, it is a primary
cause, though other things can exacerbate it, like high light levels,
reduced water motion, and nutrients.  (Of course, other things than high
temperatures can cause coral bleaching, but most mass bleaching events
correlate well with high SST's.)
      You write that "the conceptual coral bleaching model adopted by
most-all coral reef scientists is WRONG; which at its crudest, explains
coral bleaching in terms of corals BOILING TO DEATH in hot water."  OK, so
high temperatures don't kill corals???  Fact is, and it is a fact not an
opinion, every living thing has a maximum temperature that it can survive
in.  That includes humans.  "Heat exhaustion" and "Heat stroke" are two
stages in the human response to limiting temperatures for humans.  Go
beyond that, and human die.  Take any other organisms, every organism, and
heat it, and when the temperature is high enough, it will die.  There are
NO exceptions.  Limits differ for every organism.  The highest limits are
for what are called "thermophyles" which are microbes that can live in
water just below boiling, you can find them in hot springs in places like
Yellowstone National Park in the US.  Heat them a degree or two higher, and
they die (I'm not up on the study of them around deep sea vents, there may
be some that can survive even higher temperatures there).  But the high
temperature thresholds for most organisms are vastly lower than that, and
for most the limit is not a lot higher than what they normally experience,
which has been documented for corals as well.  Actually, there are many
statements, particularly in the popular literature, that corals which die
from bleaching die when they have been bleached for a long time, because
they have lost their major source of energy, the zooxanthellae, so they in
essence starve to death.  My memory tells me there are papers documenting
loss of energy reserves in corals that have been bleached for long times.
Further, there are papers that document that corals can't sexually
reproduce for at least a year after bleaching, surely because they don't
have the energy reserves.  All makes sense.  However, as Hughes et al
(2018) have pointed out, in the recent bleaching events on the GBR (Great
Barrier Reef in Australia), corals died so fast after bleaching that they
surely didn't die of starvation.  “These widespread losses were not due to
the attrition of corals that slowly starved because they failed to regain
their symbionts. Rather, temperature-sensitive species of corals began to
die almost immediately in locations that were exposed to heat stress”
(Hughes et al. 2018). So what did they die of???  High levels of
nutrients?  Not likely, why would they only die when the water is hot?
They died because they were too hot.  The zooxanthellae and the coral
animal cells are two different organisms.  They both have maximum
temperatures they can survive, and those high temperature thresholds don't
have to be the same.  Further, you can kill a coral with hot water without
it bleaching.  Try dropping a living coral into boiling water.  Will it
bleach first??  Or just die?  Doesn't much matter whether the coral has
color when you drop it in, or is already bleached and has no zoox left.  It
will be scalded and die very quickly indeed.  If a human touches a red hot
hot plate, the skin cells will be killed extremely fast, we call that a
"burn" and humans that have large burn areas are in serious danger of death.
      The fact that when temperatures are high enough, bleaching can happen
quickly is captured in the "degree heating weeks" threshold, that works
better than just a single temperature threshold, for predicting mass coral
bleaching.  The higher the temperature, the faster mortality occurs, as
illustrated by the finger on the red hot plate, but also fits with the
"degree heating week."  Further, the fact that when high temperatures cause
corals to bleach, if the temperature recedes the corals can survive and
regain their zoox, but if the temperature goes higher the coral dies, means
that the coral animal, with few zoox left, is sensitive to additional rises
in temperature, which if they happen kills the coral, if they don't happen
the coral doesn't die.  So the mortality of a coral that has lost 99% of
it's zoox depends heavily on temperature, too high and it dies, not too
high and it doesn't die.  Only a bit more than 2 C above average summer
high temperatures (no where near boiling).
      I contend that the evidence indicates that coral mortality following
bleaching can be from high temperatures (but likely can also be from very
extended bleaching due to loss of energy supplies, though I'm not sure the
evidence for that is ironclad.  I don't pretend to be an expert on the
subject).  I wrote my arguments in my paper "Coral reef myths and
misconceptions.  Reef Encounter 34: 30-37."  See Myth #5: "Bleached corals
die of starvation."
http://coralreefs.org/wp-content/uploads/2020/02/REEF_ENCOUNTER_Dec_2019_lo-res_3.pdf
       So I think your statement that the common view that corals die from
boiling fails on two accounts, first that you haven't demonstrated that
that is the common view, and second that the question is not whether it is
literally BOILING that kills them, but whether a slight increase of a
degree or two above the mortality threshold for the animal is what kills
it, and the evidence does support that at least at times that is what kills
them.
      In summary, I have no quarrel with the details of your hypotheses
about bleaching mechanisms, or the opportunities to discover more about
bleaching mechanisms, nor the possibility that might lead to some way of
producing more temperature-tolerant corals.  More research on those things
is a good thing.  I caution that it is a huge leap from producing a
temperature-tolerant coral in an aquarium to making all the world's corals
temperature tolerant.  I don't think anyone knows how to do that, I haven't
read the people who are selecting for temperature tolerance to produce
"super corals" claim to be able to do that, I haven't heard of anyone
selecting temperature tolerant corals in reef restoration claim to be able
to do that.  You want us to "go for the gold" but that is surely the "gold"
for such efforts.  Maybe someone will come up with a way, one of those
bright young people, surely we should be thinking about how to do that.
But we don't have a way to do that yet and it doesn't look like an easy
problem to solve, and that's not the thing you're urging people to think
about.
     BUT, you are essentially playing down the need to reduce and stop
global warming, even though it is the main ultimate cause of mass coral
bleaching and death.  We need to do that not only to save corals and reefs,
but also to save terrestrial environments from things like fires
(Australia, California, Amazon being examples), and from the world becoming
so hot that 10's if not 100's of thousands if not more, people die in heat
waves, and parts of the planet become uninhabitable by humans.  We don't
lack for reasons to stop global warming, and corals are part of that but
only part of the reasons.  There is NO conflict between doing that and
researching the mechanisms that cause coral bleaching.  Also, there is NO
conflict between the need to reduce global warming, and the need to reduce
local threats like nutrients, sediment, overfishing, disease, etc etc.
      Cheers,  Doug

Coelho, V., Fenner, D., Caruso, C., Bayles, B., Huang, Y., Birkeland, C.
2017.  Shading as a mitigation tool for coral bleaching in three common
Indo-Pacific species. Journal of Experimental Marine Biology and Ecology
497: 152-163.

Hughes TP, Kerry JT, Baird AH, Connolly SR, Dietzel A, Eakin CM, Heron SF,
Hoey AS, Hoogenboom MO, Liu G, McWilliam MJ, Pears RJ, Pratchett MS,
Skirving WJ, Stella JS, Torda G (2018) Global warming transforms coral reef
assemblages. Nature 556:492– 496

Nakamura, T., van Woesik, R. 2001. Water-flow rates and passive diffusion
partially explain differential survival of corals during the 1998 bleaching
event.  Marine Ecology Progress Series 212:301–304.

Nakamura, T., et al, 2003.  Water flow facilitates recovery from bleaching
in the coral *Stylophora pistillata.*  Marine Ecology Progress Series 256:
287-291.

On Tue, Mar 24, 2020 at 11:34 AM Scott Wooldridge via Coral-List <
coral-list at coral.aoml.noaa.gov> wrote:

> Dear Coral Listers,
>
>
>
> I think it is fine that the reef science community discusses strategies
> that can help formulate new ways of doing (and reporting) our science that
> can help minimize our carbon footprint. But can I suggest that we owe the
> global public and the coral ecosystems themselves so much more than this.
>
>
>
> Like those scientists tasked with finding a vaccine for COVID-19, we need
> to keep challenging ourselves to find the CURE for coral bleaching. This
> must consume 99.9% of the best thinking time of coral reef scientists. And
> before we resign ourselves to believing that reducing global carbon
> emissions (and by inference future sea temperatures) is the sole solution
> we can offer the global public, I believe we need to double down on our
> efforts to understanding the suite of cellular processes that initiate
> coral bleaching, especially those at the FRONT END of the bleaching
> response.
>
>
>
> By FRONT END I mean those processes that happen in the days-weeks before we
> see any visual sign of bleaching (i.e. symbiosis breakdown). They are
> subtle, not well studied, and not well understood. But when you start to
> investigate them more deeply, you quickly realize that the conceptual coral
> bleaching model adopted by most-all coral reef scientists is WRONG; which
> at its crudest, explains coral bleaching in terms of corals BOILING TO
> DEATH in hot water.
>
>
>
> The issue I specifically draw your attention to in this post, is the
> little-known FACT that the endosymbiont population increases its size and
> growth rate (as measured by cellular division rate) in the days and weeks
> of warming leading up to the visual coral bleaching response.  THEREFORE,
> immediately before the break down of the symbiosis, the endosymbionts are
> by our best measures very happy, and by adding some anthropomorphism -
> think that the symbiotic lifestyle cannot be much better.
>
>
>
> Don’t believe this?  Don’t take my word for it. It has been very well
> reported by others, most recently;
>
>
>
>
> https://www.researchgate.net/publication/331593854_Effects_of_increased_heat_on_fluorescence_and_dinoflagellate_density_in_the_captive_coral_Anthelia_sp
>
>
>
>
> And obviously, given my intent to get people thinking about the science of
> the response, I have already had my say on this issue.
>
>
>
> https://www.researchgate.net/publication/307695433_Breakdown_of_the_coral-algae_symbiosis_Towards_formalizing_a_linkage_between_warm-water_bleaching_thresholds_and_the_growth_rate_of_the_intracellular_zooxanthellae?_sg=-2o85tlB6whC-hSPB0t8oWFed2eECtwPxvsoVRgGKUmzu6k0TIDq5aSV6BwcOhlLqeiGGYr3KAwOMLqI2UkuYtNLnddwaWcl1ReDnhNh.pvBbZTSCSahzvcVu-JkqhLKf-zq1EbYJ6GquHXoR2Nfs2G8--J5_4bxKPwNXk0lfNXJmGnuvk3srT7GzoZSkmQ
>
>
>
>
> https://www.researchgate.net/publication/44644540_Is_the_coral-algae_symbiosis_really_'mutually_beneficial'_for_the_partners?_sg=j-rvYFlYazoFU-eah3oORE-zhln-XE-Rt9sIsBFdHrjKHFWO3E1jvDuSBJmwoJ97CP1_RYFtuY-gXbicnyXyRuLK-0gAgwvn2XMNqukP.Mdg_vAKy_Zgedumry2ZUKz9kDoEj6KH_K84VDQ57AebVQnp8oLXLx95adt-cwLzuRjJ2I-j5St_pcMDr-2jtEA
>
>
>
>
> All these articles ULTIMATELY explain coral bleaching in terms of a loss of
> host control of the demographic parameters of its endosymbiont population.
> This is very far from the CORALS BOILING IN HOT WATER conceptual model. And
> that is good news!! Since it provides hope, that if we can limit the
> thermo-kinetic growth potential of the endosymbionts, whilst simultaneously
> retaining their capacity to release fixed carbon, we may be well on the way
> to understanding what constitutes a BLEACHING RESISTANT coral. Much more
> testing needs to be done on this issue. For example, can this favorable
> symbiotic condition be achieved by ensuring a severe limitation of the
> seawater supply of nutrients, such that the symbiont doesn’t have access to
> the prerequisite nitrogenous material needed for cellular division, even
> during thermo-kinetic events that promote increased division? Or, do we
> need to look/find/evolve a new type/species of endosymbiont, most likely
> one with a bigger cellular diameter, that will have a slower thermo-kinetic
> division rate?
>
>
>
> There are so many questions. Still unknown. Still unconsidered. Still
> untested.
>
>
>
> My challenge, especially to all the fresh thinking young scientists out
> there, is to keep an open mind. Keep striving for the CURE. Never let
> yourself be convinced that we already know all the necessary science to
> answer this. WE DON’T and we are so very very far from it….
>
>
>
> And finally, for a bit of comic relief as well as encouragement I am
> forever drawn to the excited words of an old Australian swim coach, who
> famously stated STUFF THE SILVER, WE CAME FOR THE GOLD. For those old
> enough to remember, he orchestrated a brilliant tactical race for his
> swimmer to beat the best simmers in the world at the 1988 Olympics. The
> SILVER in coral bleaching research is to be content to quibble about the
> relative merits of ways to reduce our global carbon footprint. We need to
> refocus, go back to the beginning and keep searching for a CURE. Or in the
> words of the excited swim coach STUFF THE SILVER, GO FOR GOLD.
>
>
>
> https://www.youtube.com/watch?v=zgGr-quFf2k
>
>
>
> For your consideration,
>
>
> Scott Wooldridge
> _______________________________________________
> Coral-List mailing list
> Coral-List at coral.aoml.noaa.gov
> https://coral.aoml.noaa.gov/mailman/listinfo/coral-list



-- 
Douglas Fenner
Lynker Technologies, LLC, Contractor
NOAA Fisheries Service
Pacific Islands Regional Office
Honolulu
and:
Consultant
PO Box 7390
Pago Pago, American Samoa 96799  USA

"Already, more people die  <http://www.nws.noaa.gov/om/hazstats.shtml>from
heat-related causes in the U.S. than from all other extreme weather events."


https://www.npr.org/2018/07/09/624643780/phoenix-tries-to-reverse-its-silent-storm-of-heat-deaths


Even 50-year old climate models correctly predicted global warmng
https://www.sciencemag.org/news/2019/12/even-50-year-old-climate-models-correctly-predicted-global-warming?utm_campaign=news_weekly_2019-12-06&et_rid=17045989&et_cid=3113276

"Global warming is manifestly the foremost current threat to coral reefs,
and must be addressed by the global community if reefs as we know them will
have any chance to persist."  Williams et al, 2019, Frontiers in Marine
Science