(the currently most favored “cause” is elevated water temperature, also considered are increased level of atmospheric CO2, and possibly UV radiation...but significant uncertainty remains.)


Yes. The last section of this report did end with a rather brief dismissal of the most commonly accepted theoretical cause of mass coral bleaching, i.e. increased water temperature. However, a close look at this factor, and other proposed causes, is obviously warranted.

Scientific study of corals has recently revealed that they have been living very close to their maximum water temperature tolerance, and with the recent warming trend (1 degree C over the past 100 years) they are hitting the ceiling, suffering and/or expiring of “heatstroke” (inability to meet the metabolic demands in the warmer water). The absolute temperature at which this is occurring varies considerably around the globe - between the high 20’s and the high 30’s Centigrade - the variance is thought to be due to the genetic acclimatization to the different areas in which the corals live. Therefore those living in the hotter zones can naturally stand more heat, and this undoubtedly is true. But there are many inconsistencies in the pattern, and many unanswered questions remain.

If the coral bleaching problem is really an effect of rising water temperature, it must be the gradual long-term rise (1 degree/century), since the correlation of these events to shorter term temperature fluctuations is inconsistent.

“There are some correlations between the widespread coral bleaching in 1997-1998 and one of the strongest El Nino events of this century, but the patterns are unclear with many exceptions. The correlation exists for the east Pacific, but the bleaching in southeast Asia coincides with a strong La Nina (the complete reverse of El Nino), and the bleaching in the Indian Ocean and parts of the Caribbean do (not) correlate with either El Nino or La Nina.” (Wilkinson, 1998)

Most times - but not always - mass coral bleaching happens during the time of the annual maximum in water temperature. And years that reach higher maximums are definitely associated with greater incidence of bleaching. The correlation is quite strong, and suggests that heat stress is at least part of the picture. However there are enough irregular details to conclude that there must be at least one more underlying causative factor as well. (There are a few major inconsistencies to the “warm water causes mass bleaching” concept, and these have been noted by several writers. (Wilkinson, 1998, Pecheux, 1992, Brown, 1997))

One detail that fits poorly with the pure warm water theory is the pattern of bleaching as it occurs on individual reefs and on individual coral colonies. “Patchiness” of occurrence over reefs has been often noted, as has the pattern of individual corals bleaching most severely on their most sunlit (upper) surfaces. The selective bleaching of the upper surfaces seems to strongly suggest that light is an aggravating factor as well. However, there are inconsistencies with this pattern as well, since sometimes the lower parts of corals have been observed to bleach first...and again, the “patchy” pattern is inconsistent with constant degrees of heat and irradiance being experienced by neighbouring corals, not all of which may necessarily bleach.

Recent research into the manner in which light aggravates the bleaching problem has been reported from Australia. Ove Hoegh-Guldberg describes the toxic effects on corals of receiving an ‘overdose’ of light. Their ability to handle the incoming dose of light seems to be exceeded.

“Normally, increasing light levels will lead to an increased photosynthetic rate up until a point at which the relationship between photosynthesis and light saturates. At relatively high light levels, increasing light leads to an over-reduction of the light reactions and production of potentially harmful products such as oxygen free radicals. Oxygen free radicals, if not detoxified by several enzyme systems found in higher plants (and zooxanthellae, Hoegh-Guldberg and Jones 1999) will rapidly lead to cellular damage. In the case of higher plants, failure of the ability of the dark reactions to process photosynthetic energy results in an increased sensitivity to photoinhibition. The over-riding conclusion of the work of Jones et al. (1998) and Hoegh-Guldberg and Jones (1999) is that bleaching is due to a lowering of the sensitivity of zooxanthellae to photoinhibition. Basically, light (which is essential for the high productivity of coral reefs under normal conditions) becomes a liability under conditions of higher than normal temperatures.” (Hoegh-Guldberg 2000)

So...light is increasingly becoming a liability for corals. But it is puzzling, because the amounts of “light” corals receive has not been increasing in any significant way over the last 20 it must be happening because of the recent added thermal stress? However, mass coral bleaching incidents do not always occur during higher than normal water temperatures.

In fact, mass coral bleaching events have been recorded during conditions of normal light and normal temperature.

The coincidence of the bleaching events with annual temperature maximums seems to be quite a consistent pattern, but the LIGHT and TEMPERATURE combination is still insufficient to explain the cause, and the reason for the recent onset of this problem. There must be at least one more factor that has contributed to the new weakness of the corals.

Regarding the “double-edged sword” of sunlight, this has always been a fact of life for organisms on earth, and plants, animals and corals have evolved defensive strategies to protect themselves from the harmful effects. The protective mechanism used by the corals, the enzyme system used to detoxify the oxygen free radicals - that is what seems to be failing them now, for some reason their defense is becoming increasingly disabled. Along with their protective immune systems, which appear to have also been failing in recent years (outbreaks of “new” infectious coral diseases), their defense against extreme (although within normal range) levels of light and temperature also seems to be increasingly inadequate. This suggests an underlying, chronic weakness, increasingly affecting the overall health of the corals.

“Brown (1997) has already made the important link between photo-protective measures adopted by zooxanthellae and coral bleaching. Brown (1997) points out that photo-protective measures are likely to play an important part in the way that corals and their zooxanthellae may be able to limit the effect of bleaching stress arising from increased temperatures and irradiance in the field.” (Hoegh-Guldberg 2000) Hoegh-Guldberg also predicts that “any stress (chemical or physical) that blocks the energy flow to the dark reactions will lead to photoinhibitory stresses at lower light levels.”

“Photoinhibitory stresses at lower light levels.”

That would appear to be a fairly accurate description of the syndrome. So...does normal water temperature maximums, alone, have the potential to “block the energy flow to the dark reactions?” It would seem impossible since it has never occurred before. It is the enzymes that facilitate the detoxification of light, or “dark reaction.” A shortage of the necessary enzyme is one obvious and plausible cause...and malnutrition/protein deficit is one way that this deficiency can develop. Simple food starvation would predictably have the observed effect.

Pecheux notes: “Interaction of nutrients was studied in Ulva, where nitrogen starvation reduced adaptive capacity to photoinhibition, as it slows down the biosynthesis of Rubisco and probably D1 protein (Henley et al. 1991); as reef organisms live in oligotrophic waters, they might be more susceptible to photoinhibition.” (Pecheux 1992)

The problem is that the susceptibility to photoinhibition is clearly increasing; does this reflect an increase in the “oligotrophic” quality of the water? Maybe water that was once “low” in nutrients has recently become “extremely low” in the point that corals are actually feeling the stress of “nitrogen starvation?”

Regarding the “Rubisco” enzyme, it is one in which the corals’ zooxanthellae partners normally invest their protein rather heavily. From Pecheux’s analysis:

“...photorespiration, favorized by high O2/low CO2, has a protective role against bleaching. Photorespiration and its links to photoinhibition, will be examined now. Carbon dioxide fixation is the subject of many reviews...CO2 is fixed by the Rubisco enzyme (Ribulose 1,5-biphosphate carboxylase/oxygenase), which is slow and relatively inefficient. This enzyme can constitute up to one half of the protein (and nitrogen) content in plants. When oxygen is present at the active site of the Rubisco, there is a “parasitic” reaction, by which the sugar substrate is oxidated instead of being reducted. This oxygenase activity, termed photorespiration, represents 20-40% of the carbon fixation, or carboxylase activity, and is the major limitation to photosynthesis. Photorespiration is proportional to the O2/CO2 ratio. It increases with temperature, due to both the enzyme characteristics and to differential solubility of O2 and CO2. The oxygenase/carboxylase ratio increases by 1.5 for each 10 C increase...In a side effect, H2O2 and ammoniac are released...Photosynthetic rate is better correlated with Rubisco activity than with chlorophyll content.” (Pecheux 1992)

Therefore, increasing temperature does not “block the energy flow to the dark reaction.” Rather, it tends to accelerate it.

But it seems that the “energy flow to the dark reaction” is accomplished at a cost, and some fixed nitrogen ends up being released as a “side effect.” This author does not claim to understand all of the biochemical intricacies involved, but this much is clear: A simple nitrogen shortage (food starvation) could clearly translate into a shortage of the necessary enzymes to cope with exposure to intense light (a thing that corals have successfully done in the tropics for millenia).

Both increasing temperature and increasing irradiation normally stimulate increased activity of the Rubisco enzyme. Nitrogen deficiency disables this essential protective system used by corals, and lowers the temp/light threshold to a point where the system fails. Corals today appear to be existing perilously close to the brink -- very little leeway remains to tolerate unusually warm (even normally warm) water spells...and that "brink" is starvation...

If, as is suspected, this whole picture is a reflection of systemic degradation/depletion, an insidious, unsuspected long-term side-effect of fishing....does any evidence indicate that corals were stronger in the past? The theory would predict that they were.

Martin Pecheux’s review on coral literature was very thorough. His investigation into corals’ tolerance of extremely warm temperatures revealed:

“Gardiner (1903, in Orr and Moorhouse, 1932) said that he measured 56 C in a reef flat in Minikoi, Laccadive, noting no damage to the corals. Although incredible, it is not quite easy to reject the observation of the scrupulous Cambridge naturalist. Motoda (1940) measured in a reef flat of Palao, 1936, up to 39 C, with more than 38 C during 1 hour and more than 30 C during 4 hours without damage or bleaching of g..a., a particularly resistant species. It bleached or died however in Okinawa, 1986, where temperature reached 40.3 C (Tsychiya et al., 1987). Potts and Swart (1984) measured up to 36 C in the lagoon and 37 C in the inner reef flat of Heron Island during their 1974-76 survey, but ‘saw no signs of extensive mortality attributable to thermal extremes.’” (Pecheux 1992)

Recent bleaching events have been recorded at temperatures well below these. Heat has long been known to occasionally kill corals in excessively heated reef flats, they will die of “heatstroke” in that case, although other organisms such as molluscs have been noted to die first in those cases. That pattern - molluscs dying first - is not repeated in the “mass bleaching” events.

Interesting, that the record of an “incredible” ability of corals to withstand heat is a century old, and also that records from 60 years ago suggest higher thermal tolerances in the past. Is it possible that the extreme lowering of the thermal tolerances of corals mirrors the lowering of the sheer quantitiy of sea life that exists (and endlessly recycles nitrogen) in their ecosystems? “Incredible?”...yes, “incredible,” the 35 million 200 lb sea turtles that once inhabited the Caribbean...

Warm water alone, if hot enough, as it occasionally is on heated reef flats, may induce death of corals by “heatstroke” but not death by starvation.

“Heatstroke” is a relatively short-term condition with quite a sudden onset. Previously healthy organisms pass a thermal threshold that exceeds the capacity of the organism and its chemistry to adjust and compensate. Rather than "running out of fuel," organisms suffering "heatstroke" experience some toxic derangement of their normal metabolism. Serious illness and possible death result -- but cooling restores previous health fairly quickly to the survivors of “heatstroke.” Mass coral bleaching does not resemble heatstroke as much as a chronic illness or disability -- since recovery times can be very long, and signs of weakness have been noted in corals prior to the onset of the bleaching events.

“Heatstroke” will predictably kill fat and thin individuals alike; it is hard to guess which would be more naturally susceptible. But all will die if it gets hot enough. Fat and thin individuals, however, will obviously have widely different susceptibilities to death from starvation. Therefore, the suggested coral-feeding experiment should demonstrate the relationship, if any, between the nutritional health of tropical corals and their tolerance to heat and light.


1. Heat is clearly part of it, increasing temperature increases the metabolic/respiratory rate of corals (including “photorespiration”).

2. Light is also part of it, increasing amounts of light increases the necessity to compensate for the toxic effects.

3. Too much heat can kill by heatstroke - BUT, corals have survived more heat in the past.

4. Too much light exposure can kill - BUT the amount of sunlight exposure has not increased significantly -- in any case, any variation in recent years in likely to have been matched by changes in the earlier history of corals. (Regarding ultraviolet radiation, in has not increased significantly in the tropics; while potentially harmful, it cannot be a primary cause of mass coral bleaching...besides, it's not "patchy," the agent of death is "patchy"...)

5. In cases of mass coral bleaching the biochemistry seems to be failing at the point of the “dark reaction,” protective against the toxic effects of light, this requires an adequate supply of (protein) enzymes, Rubisco in particular.

6. Heat alone does not inactivate Rubisco activity (it actually increases it).

7. Light alone does not inactivate Rubisco activity (actually increases it also).

8. Heat plus light (in all normal ranges that have been tolerated before) cannot be predicted to inactivate Rubisco activity.

9. Lack of synthesis of Rubisco will predictably decrease Rubisco activity.

10. Nitrogen starvation predictably limits synthesis of Rubisco, and thereby also predictably lowers the corals’ resistance to “photoinhibition” and bleaching (...and the availability of nitrogen to corals is predictably "patchy.")

11. The pathophysiology of coral bleaching appears to be consistent with that of simple nitrogen starvation.

12. Old records are scarce, but they seem to hint at a significantly greater ability of corals to withstand heat in the earlier part of the last century (during a time when the ocean contained dramatically more fish than it does today)....does this reflect the enjoyment of much greater nutritional health by corals living in those times? How much fatter were the corals in 1903? Have they gradually lost their former plumpness and health, while retaining normal levels of color, in recent decades? A gradual insidious effect, one that falls into line with the myriad other ill-effects seen in sea life today. VERY FREQUENTLY, be it whales, fish, seabirds or seems that the root of the problem at sea is emerging as simple food shortage/starvation. We fear that the downturns in marine life reflect “anthropogenic impact” and we are justified in that fear -- but the really terrible question is “Might we have caused all of this - including coral bleaching - by fishing?”

13. A note on the rising concentration of CO2 in the atmosphere: this has been proposed as a cause of decreased growth in corals, but, even theoretically, it cannot be a sufficient cause for the decreasing rate of growth and calcification noted in corals today. One major contradiction to this theory is the fact that corals thrived in previous eras on earth when atmospheric CO2 and temperature were both significantly higher than they are today.

“..numerous reef-building coral species have endured three periods of global warming, from the Pliocene optimum (4.3-3.3 million years ago) through the Eemian interglacial (125 thousand years ago) and the mid-Holocene (6000-5000 years ago), when atmospheric CO2 concentrations and sea temperatures often exceeded those of today....(also) increase in sea warming of less than 2 C would result in a greatly increased diversity of corals in certain high latitude locations.” (Glynn, 1996)

“Increased diversity of corals in certain high latitude locations?” -- This suggests that some corals, those living at the edges of their lower thermal tolerances, should be enjoying increased health and prosperity, (e.g. the southern limits of the Great Barrier Reef?). Paradoxically, perhaps, corals at the higher latitudes today also appear to be at the limits of their stress tolerances. Another coral health problem, “emerging diseases,” does not appear to be limited to the warmest areas. Mostly of an infectious nature, these diseases “may be interpreted as the consequences of (1) changing coastal ocean water quality favoring the proliferation, attachment and colonization of microbes, and (2) reduced efficiency of the coral’s normal defenses.” (Hayes and Goreau, 1998)

To sum up, regarding the known causes of coral bleaching, the statement that can be made with the most “certainty” is that “considerable uncertainty remains.” (i.e. Our understanding of the problem remains "patchy.")

Next section: CONCLUSIONS

Index to coral discussion

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