PATHOPHYSIOLOGY OF MASS CORAL BLEACHING: A Look at the physiological
changes at the level of the individual organisms, and the distribution patterns of the
phenomenon. Are the observed changes in bleached corals consistent with what would
predictably result from their nutrient starvation? (The short answer is “yes, very much so.”)
Evidence is quite convincing that mass coral bleaching like that which is happening today
has never occurred before in geological time. It is truly something new. The phenomenon
is most obviously somehow related to the occurrence of warmer water temperatures...and
the popular understanding of this seems to be that the one (warm water) “causes” the other
(mass bleaching). This cause and effect is in fact far from established, there are some large
inconsistencies in the data that weaken the theory. This suggests that some other factor or
factors is playing a large role in the cause of coral bleaching. A look at the changes in the
corals from the point of view of “nutritional health” makes an interesting study.
EXACTLY WHAT HAPPENS IN CASES OF “MASS CORAL BLEACHING?”
First of all it needs to be noted that all cases of bleached corals cannot be lumped together.
“Bleached” simply means “white,” and corals appear white if they lose their symbiotic
algae for any reason, also if they die, since the white skeleton remains. Therefore corals,
like all animals, can die (in their case “bleach”) from many diverse causes. The
death/bleaching of nearshore corals on reefs that have been subjected to excessive nutrient and sediment
loading, or chemical pollutants from human sources...is quite a different picture from the
death/bleaching of corals that has occurred in recent years in reefs located in “pristine”
unpolluted waters. The “clean water bleaching” presents more of a puzzle, as all the
evidence points to its arising from a different cause.
Scientific investigation into mass coral bleaching events has noted the following points:
- The reason for the white appearance? Bleached corals lose their zooxanthellae, the
photosynthesizing algae that inhabit their tissue, and provide them with food-energy and
color. Besides a marked decrease in the number of zooxanthellae, it has been noted that the
individual algae in bleached corals contain less chlorophyll per unit. Another point; it has
been noted that the expelled zooxanthellae are viable (alive) in the water.
- Comment at this point: Starvation in animals results in a loss of body tissue and low
fat stores, and this is observed to occur with basically normal body chemistry parameters
such as C:N ratio. (The diagnosis of food-starvation is based purely on low body
weight/length, there is no chemical test for it.) Impaired reproduction and weakened
immunity are also well known side effects of undernutrition. Compare this information
with the following excerpt from Martin Pecheux’s comprehensive literature review on coral
bleaching:
“Tissue of bleached corals shows general atrophy and necrosis. There is 30-50% less tissue
per surface, with a normal C:N ratio (Szmant and Gassman, 1990)...one “healthy”-looking
Pocillopora was in early stage of necrosis (loss of architecture, basophilic tinge in
mesoglea), from which it was concluded that the problem is on the animal side, with maybe
thereafter nutrient-starvation of zooxanthellae (Glynn et. al., 1985). Gonads are reduced
and reproduction is generally impaired...Bleached corals had half normal lipid levels
(Glynn et. al., 1985). Phenoloxidase, a biomarker of immune capability, was found to have
lower activity in bleached and semi-bleached M. annularis (Scith, 1992). Secondary
parasites were observed in a few cases (fungus, bacteria). There is no transmission of
bleaching following iso-allo-and xenografts...During bleaching there is no visible
calcification.” (Taken from Pecheux, 1992)
There is good reason to suspect that the low lipid reserves and lowered immunity features
precede the bleaching event rather than result from it, as one might suspect. Widespread
outbreaks of infectious diseases and parasitic problems among (not bleached) corals have
been another cause of serious concern to reef scientists, also starting only in recent years.
And, as mentioned earlier, slowed growth of (not bleached) corals has actually been
occurring in some “pristine” areas for decades. Therefore it is reasonable to suspect that
some underlying factor has caused these changes and thereby weakened coral communities
as a whole, and predisposed many corals now to “bleaching” under temperature stress.
Food-deprivation is certainly consistent with the pattern of presenting symptoms
(slowed growth, low lipid reserves, lowered immunity and reproduction, ultimate death).
Pecheux’s impression: “It is also worthy to mention ‘reef deterioration.’ It is of current
belief among reef scientists that in most part of the world, the health of reefs is
deteriorating. One may gain the feeling that underlying each local cases, a general
imbalance in reef functionality is occurring. It must be envisaged that a weakened state
corresponding to a discrete premiss of bleaching state is widespread, as is suggested by
histological signs of bleaching observed in healthy-looking corals (Lasker et al., 1984,
Glynn et al., 1985), or by the reduction of growth two months before bleaching in corals
transplanted to warmer area (Shinn, 1966). Nonetheless, an all-or-not, or an
increase-metabolism-till-collapse response is possible, suggested by the proximity of
optimum and lethal temperatures. Last but not least, bleaching could happen at the limit of
normal range of some factors (temperature, calm weather or irradiance) because of a
general weakening for quite other reasons.” (Pecheux, web article, 1992)
Why did the list of factors that might be at “the limit of normal range,”
not include food availability. Could that be the “quite other reason?”
- Mass coral bleaching events occur most often during times when water temperatures
are at their annual high point, and this is also highly correlated to periods of calm
winds, or “doldrums.” Although not always consistent, this one emerges as a very
strong pattern. In situations like this, the warm, still water is well known to become very
“stratified” and “oligotrophic” (nutrient depleted). Lack of mixing seems to be a key, but
extreme nutrient-depletion of the upper layer of water is a well known result when these
conditions develop. -- So, the warm, calm period is naturally a time of less nutrient
availability in the water.
But warm, calm periods are not something new...and today’s
temperature and wind conditions are far from “unprecedented” in the lifespan of these
coral species. The problem seems to be that corals are no longer as well prepared to
weather these annual warm “fasting” periods. And one plausible explanation is that it’s
because they have not accumulated sufficient body fat stores to see them through it.
Adding to the corals’ nutritional stress is the fact that their respiration and metabolic energy
demands increase with increasing temperature...so if they have not stored enough extra
energy, they just might not make it through. They might starve. (On a related note, when
looking for overall marine patterns, the large number of gray whales succumbing to
starvation are dying from the exact same problem - the inability to pack on enough
blubber to see them through the lean time.)
Do corals store energy reserves in their tissues? Most definitely they do, besides receiving
food produced by their algae-symbionts, all corals also must derive some nutrition from the
water, either in the form of inorganic nutrients or by the capture and consumption of edible
particles such as plankton. Corals are “microcarnivores,” preying on zooplankton in the
water. The availability of zooplankton prey is highly variable on coral reefs, and it appears
that when there’s lots of prey, corals eat a lot and become “fat.” They appear to take a
“feast or famine” approach to this feeding (again, like the whale):
“Apparently, zooplankton is an unpredictable source of nutrition, being more abundant at
some times than at others. Corals might adopt a feast or famine strategy; i.e. when large
quantities of zooplankton are available the corals feed heavily, storing some of the material
for less favorable periods. Corals do store large energy reserves in the form of lipids
(Patton et. al., 1977); the lipid content of symbiotic cnidarians can be greater than 30% by
dry weight (Bermann et.al., 1956).” (Gladfelter, 1983)
Zooplankton declines have been recorded in diverse areas of the world ocean in recent
decades (e.g. North Atlantic, North Pacific). Perhaps that trend has also extended to the
tropics, resulting in a lowering of food availability to corals in pristine, open ocean waters.
(It is difficult to find a reference to long term zooplankton data in the tropics, it might not
exist(?) due to the relative lack of resources that have been used to study those areas
compared to the northern temperate marine ecosystems.)
- There is a strong correlation between bleaching and exposure to light. Although not
completely consistent, most often the bleached surfaces are those receiving greater amounts
of sunshine. The light stimulates photosynthesis by the zooxanthellae...could the inability to
“perform” due to nutrient shortage inside the coral host somehow lead to the zooxanthellae
abandoning the host...or maybe it’s vice versa, maybe the light (heat) absorption causes yet
a greater temperature stress on the polyps living on the sunlit surfaces, hiking metabolic
demands higher yet...and when the algae doesn’t provide food....but seems to be wanting
to get “something for nothing”...maybe this all results in the partners abandoning their
relationship. It has been noted that the photosynthesis/respiration ratio falls and produces
and “energetic imbalance” (Jokiel and Coles, 1990). Perhaps the ratio crosses a threshold
below which the symbiosis simply no longer works. And a “win-win” scenario becomes
transformed into at “lose-lose” one? ...with the result that the relationship is abandoned, an
emergency bailout...every organism for itself?
Actually, recent research indicates that the
exact point of breakdown is in the natural defences that the symbionts have to the toxic
effects of light (Hoegh-Guldberg, 2000). (And this boils down to a protein shortage, as will
be discussed in more detail at a later point.)
- On affected reefs, all corals are not equal. The faster-growing coral species seem to be
consistently more vulnerable to the mass bleaching events. This observation would be
expected if the underlying cause was a generalized food shortage. Those with the highest
metabolic demands, and need for food supplies, will run short first (i.e. starve).
- Even among the vulnerable species, all corals are not equal. One hard to explain part of
the widespread pattern has been the frequent patchiness of the bleaching. On a given reef,
some specimens of a given coral species will bleach while others in the vicinity retain their
color. This is hard to reconcile with the pure “increased water temperature” cause - still
water over a given reef, with a consistent depth, exposed to the same air temperature, wind
and solar radiation - it seems that the temperature stress experienced by neighbouring
corals should be quite constant.
Could food availability on reefs be “patchy” to this extent, and might that be the element
that determines which individual coral bleaches and which one does not? It is plausible;
since the distribution of “resident zooplankton” on reefs is known to be unpredictably
“patchy.” (Bak, 1983) Beyond zooplankton, the distribution of reef fish tends to be
“patchy” as well, with a little school of fish often lingering in a favorite area, they are typically described as "sedentary" -- who knows
what determines their choice of locale, but might the nutrients excreted by these living fish
(such as ammonia, which is immediately available to the coral/algae symbionts) be the thing
that tips the balance and sustains one individual coral colony through the famine, while a
not-favored-by-the-fish neighbouring coral starves? The patchy occurrence of nutrient
sources (zooplankton and fish) on coral reefs, could therefore possibly translate into the
patchy occurrence of bleaching survivors...Food is "patchy"...bleaching is
"patchy"......hmmmm, could it be?...just how important to coral health is the presence of
those living reef fish? (It appears that the supportive role they play goes well beyond
algae-munching.)
- The behavior of the polyps has been observed in bleached corals. They often recover,
and it seems that they can generally survive on their own for approximately a few months
before dying. They are appearing white, they are clearly stressed, but they are not dead yet.
What do they do? Pecheux’s comments: “Polyps response is variable, from normal
behavior, to extention without prey capture, to total retraction (Williams and
Bunkley-Williams, 1989, Land in Holling, 1988) probably depending on the intensity of
the perturbation. Abnormal expansion of polyps (Faure et al., 1984), of continuous
extension night and day of normally night species (Glynn, 1989, Williams and
Bunkley-Williams, 1989) were noticed.” Although far from conclusive, parts of the
behavior described could easily be interpreted as increased efforts to obtain food. The
increase in “extension” behavior, might that be a display of increased hunger as well as
increased respiratory demand?
- Bleaching is not contagious. This much has been experimentally proven, so the key
causative factor is most unlikely to turn out to be a mystery pathogen.
- Recovery of corals from bleaching episodes is slow, the effects linger long after
the temperature maximum has passed. Scientists in Florida noted that: “Although recovery
of zooxanthellae density and coral pigmentation to normal levels may occur in less than one
year, regrowth of tissue biomass and energy stores lost during the period of low synmbiont
densities may take significantly longer.” (Fitt et al., 1993) This suggests perhaps a chronic
weakness, only visibly exacerbated when the water is warmer, but always there to some
degree...the sluggish rebounding of “tissue biomass and energy stores” seems to strongly
suggest the possibility of suboptimal nutrient availability in the corals’ environment.
WHAT DOES DELIBERATE FOOD STARVATION IN CORALS LOOK LIKE?
HAVE EXPERIMENTS BEEN DONE?
The sequence of physiological events in known food-starvation of corals appears to be the
same as those observed in wild corals affected by mass bleaching events. Simply put:
“Under starvation, corals expel algae and die.”
(www.uc.edu/geology/courses/coralreef/notes.htm )
That sums it up, but smaller details are also the same, for instance the fact that the expelled
zooxanthellae are still alive.
Regarding experimental starvation: “The first response to starvation in filtered sea water is
expulsion of zooxanthellae, which can be rather fast, as soon as 7 days or can take up to as
long as 5 months” (Pecheux, 1992)
Coral bleaching has been experimentally induced by warming corals in laboratories. The
temperature increases used to trigger the condition are generally several degrees higher than
those recorded in the field. Regarding the physiological effects of the warming on the
corals, here is part of Pecheux’s commentary: “Death of various species of corals were
observed at 40 C, and death or bleaching at 36 C within 1/2 to 2 hours by Yonge and
Nicholls (1931a). ...Expulsed zooxanthellae appeared healthy. They suggest that
starvation of CO2, nitrogen or phosphate is a likely cause of the bleaching.” (Pecheux,
1992)
So, the manner in which heat kills is likely via the induction of starvation of CO2, nitrogen
or phosphate? (Pecheux makes an complex argument for CO2...but it’s quite a stretch. It’s
just difficult to imagine any organisms suddenly dying from “CO2 starvation” in the
present “CO2 enhanced” planetary environment. Pecheux believes that the bleaching
results from “reduced exchanges,” and he concludes that “They would involve rather the
O2/CO2 couple, as it is hard to believe that nutrients are involved in mass bleaching.” Hard
to believe? HARD TO BELIEVE?! ...that seems to be reason enough to dismiss the
possibility that the fatal “reduced exchanges” involve nutrients. But Pecheux is determined
to find a connection to the recent rise in atmospheric CO2. Unfortunately that conclusion is
also “hard to believe” since as recently as 60 million years ago, atmospheric CO2 levels
were almost 10 times what they are today, and the coral reefs nicely survived that time... But Pecheux deserves a lot of credit, his extensive review of the coral bleaching literature is very valuable.)
So...in conclusion, if not starved for CO2, then the heated corals are most likely
succumbing to starvation of either nitrogen or phosphate. That’s pretty simple, pretty basic,
in short it’s just “food.” It seems that the problem boils down to insufficient food reserves
to sustain corals through what is normally the leanest feeding time of the year, the period of
warmest water. And the worrisome thing is that it’s being observed in “pristine” waters, in
reefs that are well flushed by “clean open ocean water.” Why are these organisms now -
apparently for the first time in their long existence on the planet - why are they now not
getting enough to eat? Has the removal of reef fishes depleted the local nutrient pools, or
rather has a similar effect taken place on a global scale, with the ultimate effect of
all-fishing being nutrient depletion of the open seas? Have recent climate changes so
reduced the ocean currents that the normal bottom-to-top mixing is much reduced, or is
there now less falling to the bottom to be raised by “upwelling” currents...and could this
possibly be an insidious end result of all-fishing? Very interesting, very disturbing, the
starvation of the coral reefs represents an important piece in the overall marine decline
puzzle. Since they are not heavily dependent on the ocean mixing/upwelling dynamic (in
fact the best ones develop where this pattern does not exist), thriving in warm, oligotrophic
water....their sudden starvation implies that “oligotrophic” may have been taken to a
dangerous extreme, to the point where the sustenance of corals is no longer a sure thing.
Corals succumbing in “mass bleaching events” in clean waters, are they dying of
simple food-starvation? Yikes! ...this is NOT good...but one “good” thing is that at
least the theory is testable...
In the case of the starvation of the “clean water” coral reefs, it appears that the major
aggravating factor has been the removal of the bulk of the fish and other animals that once
inhabited (and nourished) them. Therefore one would expect to see increased vulnerability to mass
bleaching in those reefs that have experienced the heaviest fishing exploitation over time,
and the healthiest, most bleach-resistant corals in some remote location that fishermen have
been physically unable to access. Briefly, this pattern does seem to hold true, although
taking a detailed inventory of all coral reefs in the world is beyond the scope of this report.
The highest risk factor for the coral reefs, however, is often noted to be “proximity to
centers of human population.” And that’s surely where most of the fishing of reef
organisms has occurred. But the whole issue gets blurred however, since that very
proximity many times results in increased nutrient loading of the coastal water, one well
known negative result of human activity. So, increased nutrients and nutrient depletion
simultaneously? ...hmmm....Yes and no?... “Eutrophication” is one way to stress and kill
nearshore corals in polluted water (although the harmful effects are much worsened when
fish are simultaneously removed (McCook, 1999); ...now, it seems that “oligotrophication” may be the comparable term for the
problem now emerging in the clean seawater. One means that nutrient levels are “too
high,” the other means that nutrient levels are “too low,” and either one can surely result in
the death of the corals.
The next section of this report takes a look at the
global mass coral bleaching pattern, with a focus on fishing as a isolated “bleaching” risk
factor. (In places where “eutrophication” is clearly NOT the problem, consideraton is give to the question
of whether or not the problem might be something like “oligotrophication” instead...although that's still probably not the right word to convey the concept, since the "low nutrient level" problem is not really captured by a word that only conjures up thoughts of "liquid" nutrients...)
