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

Scott Wooldridge swooldri23 at gmail.com
Sun Mar 29 07:00:59 UTC 2020


Hi Dennis,



Thanks for your query.



I am a strong advocate for intracellular limitation of CO2 within the dark
reactions of symbiont photosynthesis as a key candidate for triggering
thermal coral bleaching (symbiosis breakdown). This limitation has the
potential to trigger a build-up / blockage of electrons (so-called over
reduction) of the photosynthetic light reactions; leading to excessive
production of reactive oxygen species (ROS) that have been implicated in
host cellular damage and symbiont release.



The diurnal dynamics of the processes leading to CO2 limitation are not
easy to explain succinctly, since it needs consideration of both the supply
and demand-side of the intracellular CO2 story.



If you are interested, I have done my best to describe daily scenarios for
how the process may progress:



https://www.researchgate.net/publication/240506685_A_new_conceptual_model_for_the_warm-water_breakdown_of_the_coral-algae_endosymbiosis



IF CO2 limitation is the triggering process as I suggest, then a
pie-in-the-sky engineering solution would be to provide a burst of
readily-diffusable dissolved CO2 during the midday-peak of photosynthesis.
The assumption here being that cellular diffusion kinetics from seawater to
symbiont is fast enough to offset periods of excessive CO2 demand. There is
no practical approach that I am aware by which this could be achieved in
the field at large scales – but is certainly testable in the lab.



Interestingly though, when electricity is passed through metal rods in the
ocean upon which corals are grown (e.g. the well-known Biorock artificial
reef system) it is reported that the electrolysis of seawater produces heat
at the electrode surface. The initial thermal decomposition of the seawater
at the electrode interface results in the removal of carbonic acid (H2CO3)
and the release of carbon dioxide (CO2). Now the thermodynamics of this
chemistry is above my pay grade, but what I take from my limited
understanding is that when the electricity switch in ON, then CO2 is being
made available to the microenvironment of the coral.  If true, then it
would seem possible that the the operation of the Biorock system (or
similar) could be managed to help promote bleaching resistance corals.



Care is needed here though. You would ONLY want enhanced CO2 during periods
of high photosynthetic demand. High CO2 permanently, would permit the
opportunity for an elevated (excess) zooxanthellae population to persist
that would be characterised by high division rates: based on the
presumption, that is common for many fringing systems, that nutrient
resources are sufficient to meet cellular cytokinesis requirements.
Interestingly enough, one of the key physiological changes reported for the
Biorock system, is that the corals have elevated symbiont population sizes
and cellular division rates. See:



https://www.researchgate.net/publication/279755881_Increased_zooxanthellae_numbers_and_Mitotic_Index_in_electrically_stimulated_corals



To my way of thinking, this should make the Biorock system very SUSCEPTIBLE
to thermal bleaching, a prediction for which I have no knowledge and have
never looked into. But could it be, that a managed automated system that
turned the electricity ON during periods of peak irradiance/photosynthesis
and OFF at other times, could help corals resist thermal bleaching? Again,
it is something that someone may consider testing.



The much more obvious way of reducing CO2 limitation is to consider the
demand size of the story. Manage our water quality to ensure symbiont
population sizes are not excessive. As I have drawn attention too before,
it is my belief that the physics of the situation will mean that there are
very strict optimal population sizes that cannot be exceeded without
causing CO2 limitation.  Obviously, irradiance levels, and skeletal
morphology, water flow interact in defining this optimal population size.
Which gets to another possible engineering solution – that of shading. Many
examples already exist in the literature where shading has been shown to
lower bleaching risk – which is also consistent with the CO2 limitation
bleaching model.



Anyway, I am sure people are sick of my ideas. I freely put them out there
for others to criticize and scrutinize. My hope is that it gets people into
a solution focused mindset. Ultimately though, the fundamental science of
the breakdown process needs to guide our solutions thinking. Hence, my
stated urgency in the initial post, to stay focused and continue investing
in fundamental research at the coral-symbiont scale, esp the FRONT END
story of the bleaching process.





For your consideration,



Scott Wooldridge
>Date: Wed, 25 Mar 2020 11:12:14 -0400
>From: Dennis Hubbard <dennis.hubbard at oberlin.edu>
>To: Scott Wooldridge <swooldri23 at gmail.com>
>Cc: Coral -List <coral-list at coral.aoml.noaa.gov>
>Subject: Re: [Coral-List] So you think you understand coral bleaching?
>Message-ID:
 >       <CAFjCZNZaS8C7_LfT2Y74hzmoU1hct-EHCt2Amnv7ap48CHx+PQ at mail.gmail.com
>
>Content-Type: text/plain; charset="UTF-8"
>
>Thanks Scott. This is a VERY interesting finding. I have often wondered
>whether the pathway involved the corals expelling the endosymbionts of the
>latter "bailing out" as a defense (or other) mechanism. Might you have any
>ideas on what the specific pathways and linkages look like and how we might
>"short-circuit" them - if I understand your point?
>
>Best,

Dennis


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