Iron against bleaching ?
JSprung at compuserve.com
Wed Dec 29 01:17:51 EST 1999
Some food for thought-
Following Martin PECHEUX's suggestion to experiment with iron fertilizing
around bleached reefs I am compelled to share some ideas I have that I hope
will be taken in the spirit I intend them- just ideas, a hypothesis, no ill
intent, and a hope to open everyone's eyes to the POSSIBILITY that many of
us (myself included) up to now have been preaching an erroneous scenario
concerning mass bleaching on coral reefs. I'll get to that in a moment.
First, regarding the iron fertilization suggestion. I agree with Jim
Maragos that the likely result of elevated availability of iron would be a
bloom of algae (with undesirable consequences!). Iron fertilization in
closed system aquariums often (but not always) produces that effect,
explosively at times, the availability of phosphate and CO2 also being
important in the equation.
The suggestion by Martin PECHEUX to fertilize with iron was based on the
notion that bleached corals would benefit if there was a way to stimulate
the growth of their lost symbionts. Here's where the possible error about
mass bleaching comes in.
In general the explanation for recent mass coral bleaching and mortality
events (at least as they are reported in the newspapers, dive magazines,
environmental publications, etc.) is based on a story along the following
1. High water temperatures STRESS corals.
2. The stressed corals expel their zooxanthellae for some unknown reason
(quite reasonable suggestions here usually offer that the symbiotic
relationship is possible only within a certain temp. range)
3. The "bleached" corals are still alive, but in a weakened state because
they have lost their symbionts that provide an important source of food to
4. If the corals "remain stressed" and don't regain their zooxanthellae,
they starve and eventually die (or are weakened and subject to attack by
disease and smothering mats of algae).
5. Other factors affect the recovery, such as pollution, eutrophication,
What I am going to propose is the POSSIBILITY that this scenario is mostly
wrong, based on what work done by (Toren et al., 1998) implies.
An article in a recent issue of Reef Encounter, Newsletter of the
International Society for Reef Studies describes how bleaching in a species
of Oculina is caused by a species of bacteria, Vibrio shiloi. The authors
give an analogy to help explain the distinction between the causative
environmental conditions and real "cause" of the disease (ie. the
microorganism). In their example, the flu is prevalent in the winter, when
weather conditions favor its activity. The cause of the flu is the
microorganism, however, not winter.
In their study they showed that the way Vibrio shiloi affects the
Mediterranean species Oculina patagonica is mediated by temperature. At
temperatures from 16 - 20 degrees Celsius the disease does not occur, even
when large numbers of the pathogen are applied to the coral. At 25 - 30
degrees, even a small quantity of V. shiloi will cause the disease.
Further, they showed that at this increased temperature, when antibiotics
were used to block the Vibrio, the disease did not occur.
In a separate study, (Toren et al., 1998) it was found that V. shiloi
adheres to the surface of O. patagonica via a chemical receptor. The
bacteria's counterpart adhesin that recognizes this receptor is not
produced at lower temperatures. The elevated seawater temperature
therefore is what causes the bacteria to become virulent. It effects a
change in the bacteria, not the coral- the coral is not stressed.
Although the bacteria that affect tropical corals are likely to be
different from V. shiloi, their basic behavior may be similar. Elevated
seawater temperatures on coral reefs are known to be associated with coral
bleaching. The article in Reef Encounter clearly suggests the possibility
that the elevated sea surface temperatures associated with mass bleaching
and mortality of corals on tropical reefs may indicate a similar process
happening there, caused by bacteria, not temperature "stress" on corals.
If that is the case, then the scenario could be:
1. At elevated temperatures various strains of bacteria adhere to corals
and become virulent.
2. The bacteria cause the corals to expel zooxanthellae (reason not known).
3. The bacteria destroy coral tissue (ie. the corals don't starve, they are
Based on observations in aquaria I tend to believe the latter scenario,
which I am proposing. Furthermore, I believe that there are other
environmental factors that may cause various bacteria to become virulent
and affect corals, temperature notwithstanding. Of course there may be as
many pathogenic bacteria as there are families of corals, and different
bacteria may become virulent for different reasons, not always with fatal
results. In aquaria I have seen disease affect only members of one species,
genus or family, leaving other corals unharmed. Other times I've seen
rapid death move through an aquarium, affecting all corals as well as other
invertebrates and even fishes! I have seen diseases that cause impaired
health, reduced growth, bleaching, and sensitivity to light. In many, but
not all cases, treatment with antibiotics or antibacterial chemicals
(iodine for example) reverses the symptoms. Such observations are anecdotal
and surely need closer study. The beauty of aquariums of course is the ease
with which we can control them.
Some additional remarks-
Of course I know that it can be demonstrated that changes in light
intensity and spectrum effect changes in density of pigment and
zooxanthellae. I also know that a variety of factors can cause corals to
expel zooxanthellae, not just bacteria or disease. Please don't misconstrue
what I suggest here.
Regarding stress and reduced resistance to disease, I do not doubt that a
variety of diseases may affect corals more strongly when the corals are in
an environment that is not ideal. However, it is important to accept that
at least sometimes a rapidly fatal disease may occur suddenly in perfectly
healthy corals when the disease causing organism is "switched on" by an
environmental stimulus that does not stress the coral directly.
Algae mats grow on exposed coral skeletons, smother, and usually kill
corals, but sometimes they offer a life-saving shade to live coral tissue
on lower branches during mass bleaching events.
Corals on reef flats are routinely exposed to temperatures far above those
associated with mass coral bleaching. What gives these corals the ability
to tolerate this? Is it just duration of exposure? Intense UV affecting
Changes in their surface chemistry? Production of antibacterial substances?
I have for years been bothered by the blurriness in reports of coral
bleaching with respect to the distinction between white-but-alive and
white-without-tissue. At one time I wanted to suggest that new terminology
needs to be adopted to clarify the distinction. The term "bleaching" is
truly a poor choice since the curio trade uses bleach to clean coral
skeletons, so the public perception is that "bleached" corals are dead. In
general, scientists referring to bleaching really refer to corals that have
expelled their zoox's but are still alive. Taking into consideration the
possibility (of mass bleaching events caused by disease) proposed here,
then the distinction between the two conditions ("bleached" vs. dead) in
the case of a mass bleaching event may just be a matter of time only, as
the cause is the same. Bleaching caused by shading or other factors is of
course something different. Do we need more terminology?
Finally, if some mass bleaching events are caused by bacteria, can we do
something about it? That gets back to the original intention of Martin
PECHEUX, which is in principal very worthwhile- even if idealistic- to come
up with a way of reducing the loss of corals by intervening when a mass
bleaching event ocurs. Treatment with antibiotics is of course out of the
question, but what about iodine or other substances that hinder bacteria?
Alternatively, is it possible to immunize corals? Is it possible to
chemically block the adhesion of bacteria to the coral?
I hope that this post stimulates active exchange of ideas in a positive
direction to further understanding of the mechanisms behind coral bleaching
and disease. I don't mind if I am wrong. I hope others feel the same way.
Rosenberg, E. and Y. Loya. 1999. Vibrio Shiloi is the Etiological
(Causative) Agent of Oculina Patagonica Bleaching: General Implications.
Toren A., Laundau L., Kushmaro A, Loya Y, and E. Rosenberg (1998) Effect of
Temperature on the adhesion of Vibrio AK-1 to Oculina patagonica and coral
bleaching. Appl. Environ. Microbiol. 64: 1379 - 1384.
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