Trachyphyllia geoffroyi is
often suscpetible to bleaching in aquariums. Part of the reason may stem from the fact
that many of the bright colored (red and green) specimens are being collected in deep
water. The coral above has bleached, and is showing signs of tissue loss, probably from
starvation resulting from the loss of zooxanthellae.
BLEACHING AND TISSUE LOSS IN CORALS - WHAT'S
THE DIFFERENCE?
Many of the questions
I am asked have an "ailing" coral as the subject. Often, the problem involves
some amount of paling to the normal coloration of a coral, or a visible white area on the
coral. It is very difficult to ascertain the nature of the problem under any
circumstances, but one of the most common mistakes is the misidentification of coral
bleaching. In the next few articles, I intend to look at some of the causes, appearance,
and effects of bleaching and then, in subsequent articles, a group of problems of various
types characterized by actual tissue loss. These events can be difficult to tell apart,
may have similar or different causes and effects, and may even be related to each other.
Background
What is bleaching? Bleaching occurs in corals that maintain a
symbiosis with various types of dinoflagellates called zooxanthellae. By one common
definition, bleaching is the release, rejection, or loss of zooxanthellae from coral
tissue.
Zooxanthellae are acquired by corals in two ways: first,
they may be given a "starter culture" by the parent if the parent colony broods
its planulae. Alternately, in corals that release sperm and eggs into the water and where
fertilization takes place externally in the water column, planulae (lacking zooxanthellae)
can swallow the algae from the water column. Once swallowed, the algae are not digested
but are brought into the cell and put into a small intracellular bag called a vacuole.
Once inside the vacuole, they are "trapped" and somewhat at the mercy of the
coral polyp. The golden brown algae reproduce within the cell and form a dense, but very
thin layer within the polyp. The zooxanthellae are found mainly in the inner tissue layer
of corals called the gastroderm, although they can occasionally be found in the outer
layer (ectoderm) and are in the tentacles of day-feeding corals. Night feeding corals have
transparent tentacles that normally lack zooxanthellae.
Proud sponsor of this column
The Goniopora shown is
bleached. The tissue is transparent, indicating a dramatic loss in zooxanthellae. This
coral is probably not bleached from excess radiation or temperature, as it was
photographed in deep water where light levels are low and temperature is relatively
constant.
Once inside the polyp, zooxanthellae are provided
nutrients that are controlled, and usually limited, by their host. In return, the algae
use sunlight to photosynthesize and provide the energy rich products of photosynthesis
(photosynthate) to the coral polyp. The nutrients for the zooxanthellae are mainly the
products of coral metabolism; that is, carbon dioxide and nitrogen.
One of the advantages to living within polyp tissue is that
zooxanthellae have constant access to nitrogen in the form of coral metabolic waste
products. In contrast, the usually nitrogen deficient seawater may not be able to provide
a plentiful source of nitrogen for growth and reproduction. However, the coral can and
does control the amount of waste released intracellularly to the zooxanthellae, excreting
any excess back into the seawater. Under normal conditions, the balance is very precise
and there is very little deficiency or excess, with virtually all of the coral's metabolic
waste consumed by a precisely moderated population of zooxanthellae.
Bleaching Variation:
Conditions can occur, however, that change the balanced symbiosis
of algae and coral. Where there is chronic or acute nutrient limitation, the coral may not
produce enough waste to sustain the zooxanthellae. Conversely, the zooxanthellae will not
be able to produce enough photosynthate to sustain the coral. If the deficiency is great
enough, the zooxanthellae density will be reduced. This can happen in three ways: the
zooxanthellae will simply die and be ejected from the coral; the coral can digest the
zooxanthellae for its own energy needs (if it is a species that can digest plant material,
specifically cell wall components); or the coral can release some of the zooxanthellae
from its tissues back into the water. This is bleaching.
This Cynarina lacrymalis is
severely bleached. The tissue is clearly visible and inflated, although without the
pigmentation of zooxanthellae. The white coloration comes from the skeleton visible under
the tissue. This coral will probably need to capture prey or be fed to prevent starvation
and recovery of a full complement of zooxanthellae.
Proud sponsor of this column
Similarly, although for
different reasons, chronic or acute excess of nutrients (especially nitrogen) can also
cause bleaching. Since corals can absorb dissolved nutrients directly from seawater, they
can benefit from energy obtained in this way. However, when dissolved nitrogen is absorbed
into the tissue and cells, the zooxanthellae can also have access to the material. In this
case, there may be an excessive nutrient availability and the zooxanthellae are less
nutrient limited by the coral, and can use the nitrogen to grow and reproduce. The growth
to higher densities of zooxanthallae is not necessarily good for the coral, and the growth
can become unbalanced and unchecked. If this happens, bleaching may be required to
maintain proper densities of algae within the tissues.
Zooxanthellae also have finite life spans,
and at any time there are numbers of them that become senescent and are no longer useful
to the polyp. These zooxanthellae are ejected, and this is also a form of bleaching.
The descriptions above sound
like adaptive and productive behavior involved in maintaining a balanced symbiosis, and
they are. So bleaching, by definition, is not necessarily a detrimental occurrence as is
widely held. However, there are degrees of bleaching, and there are other factors that can
cause bleaching. These are cases where bleaching is not a normal regulatory mechanism, but
are caused by various factors that not only jeopardize the symbiosis, but the health of
each partner.
Another Definition of Bleaching:
Coral bleaching has another and more popularly conceived
definition. This definition states that a coral is considered bleached when there is a
visible lightening of the normal coloration state, translating to an approximate loss of
50% of the standing stock of zooxanthellae. Most people associate a bleached coral with
the images of stark white corals on a reef. This is considered to be severe bleaching,
with mass bleaching defined as when an entire community of corals has become partly or
totally bleached.
When I say totally bleached, this is a bit of an
overstatement. There are, to my knowledge, no cases reported where bleaching is total
except in experimental conditions (difficult to even achieve) and where some temperate
corals can exist naturally either with or without zooxanthellae. The densities of
zooxanthellae, at most, become extremely low so that their brownish coloration is no
longer visible and the coral tissue becomes largely transparent, revealing the white
skeleton below.
Proud sponsor of this column
Mechanisms and Effects of
Bleaching:
The low numbers of zooxanthellae remaining in
bleached coral tissue are also the reason that bleached corals often recover. It is
unlikely that they recover to an appreciable extent by acquisition from the water column,
but rather from the reproduction of those left in the tissue. However, if zooxanthellae
densities are extremely low, the coral cannot get much energy from the products of their
symbiotic algal’s photosynthesis. This creates an energy deficit that must be filled
by either feeding or by direct uptake of nutrients from seawater. While possible, severely
bleached corals often do not recover, and they die. Why?
Seawater is often nutrient poor, and so
direct absorption may not take place to a degree, or at a rate, that can provide enough
nutrition. Secondly, even if there are adequate prey items for the coral to capture, the
maintenance of capture mechanisms, such as nematocysts, are energetically costly. The
coral may not be able to effectively maintain these structures and, therefore, be unable
to effectively feed. Furthermore, it costs energy to swallow and digest prey items. For
aquarists, this is readily obvious in bleached corals that appear to have no interest or
ability to capture food items offered to them. In the end, bleached corals operating at an
energy deficiency must metabolize their own tissues to survive, and this is seen as
recession and tissue death. It is also called starvation.
The best solution to a severe bleaching event, beyond
removing the stressors that caused the bleaching in the first place, is to provide enough
nutrients to sustain the coral and to repopulate zooxanthellae, as well as providing them
in a form that requires the least amount of energy to obtain and use. The best answer for
all of these requirements is to make sure that there is a good supply of dissolved
nitrogen in the water column. A high nitrogen level will probably not be beneficial once a
bleached coral recovers, but it can be helpful in the recovery process. Bingman correctly
notes that many aquariums are already many times higher than reefs in usable forms of
nitrogen (pers. comm.). In such cases, increasing the usable forms of nitrogen (nitrate
and ammonium) will probably not matter. However, many aquarists now keep aquariums where
nitrogen levels approach or are below average reef water levels, and in such cases
ammonium or nitrate can be fuel for zooxanthellae reproduction. For further information on
the role of nitrogen in zooxanthellae reproduction, see Marubini and Davies (1996),
Hoegh-Guldberg (1994), Hoegh-Guldberg and Smith (1989), and Mueller-Parker et al. (1994).
The Sinularia sp. pictured here is bleached,
although zooxanthellae are still visible in some of the branches on the left.
Proud sponsor of this column
Another problem that
occurs with bleaching is the way in which zooxanthellae are lost. Aquarists may be
familiar with brown mucous strands of zooxanthellae being released from the mouth of a
coral. Often, zooxanthellae removal or loss is a fairly controlled process with the
vacuoles containing the algal cells moving towards the outer cell membrane, fusing with
it, and then releasing the contents into the coelenteron. However, rapid bleaching or
severe stress results in a much more detrimental release, with the entire cell contents
being blown out into the coelenteron or, even more severely, the entire dermal cell being
detached and lost. It should be apparent that such traumatic reactions have an even
greater detrimental effect on a coral than the loss of algal cells alone. In such cases,
bleaching is often severe enough and with enough concomitant injury that recovery chances
are slim.
Corals bleach for a number of
reasons, some of which were described above as regulatory processes. In more detail, the
quantity and quality of photosynthetic products is a major factor. In particular, the
production of oxygen by zooxanthellae can be especially problematic. Excess oxygen,
especially in forms where singlet oxygen radicals are produced, or when coupled with water
to form hydrogen peroxide, can be damaging to coral tissue. Corals do produce enzymes to
detoxify these oxygen forms, but under conditions that produce bleaching, they may not be
able to handle the amount of oxygen produced. Therefore, bleaching occurs to prevent the
oxygen poisoning of the coral cells.
It is not well known yet if bleaching is
ultimately under coral or zooxanthellae control. There is evidence to support both views,
and perhaps various bleaching events depending on the circumstances, may be under the
control of both or either partner(s). Further research is required to determine these and
other aspects of the bleaching response.
Other Aspects of Bleaching:
The factors that can cause coral bleaching
are numerous. In the wild, the most established factor attributed to mass bleaching events
is a prolonged increase in temperature above normal levels. Temperature as a bleaching
cause may be synergistic with other factors, including reduced water motion, irradiance,
and nutrients. A list of factors shown to cause bleaching in various studies can be seen
in Table 1.
Table 1: Known Causes of Coral Bleaching.
The
following list has support in scientific studies as contributing alone, in combination, or
synergistically to the signs associated with coral bleaching.
High temperature - sustained or short-term increase
Drugs
Low water motion (stagnant water, doldrums)
Competition
High irradiance - sustained or a rapid increase
Sedimentation
Ultraviolet radiation - sustained high levels or a rapid increase
Starvation
Rapid change in temperature - higher or lower
Physical injury or stress
"But my coral still has a
light blue color," queries an aquarist, "it must not be bleached." Untrue!
Many of the bright colors found in corals are due to fluorescing proteins that are not
apart of the zooxanthellae. These pigment complexes lie in vesicles either above or below
the zooxanthellae within the animal tissue. They serve to modulate visible and ultraviolet
light in either enhancing or protecting roles. If zooxanthellae are lost, these pigments
can remain for quite some time. Because they are no longer serving their function and they
are metabolically costly to produce, these pigments will eventually be lost until they are
required again. If there is recovery they will be produced again by the recovered coral if
required. But, it takes some time for them to be metabolized (unless the bleaching event
resulted in the entire loss of cellular contents or cell detachment), and so a coral may
retain some colorful pigmentation even when bleached of zooxanthellae almost completely.
Conclusions and Notes for Aquarists:
In conclusion, bleaching is a common event in
both wild and aquarium corals. In many cases, minor bleaching may not even be noticed,
with zooxanthellae and coral pigments occurring in high enough densities to prevent
observation. When bleaching becomes severe enough, a paling or transparency of coral
tissue results and the coral presents a resulting pale or white appearance.
When this occurs, it can be very difficult to
assess whether or not coral tissue remains. In some cases, tissue expansion can be
apparent and it is obvious that there is coral tissue remaining, but that it is
transparent. In other cases, and especially when a stressor is still present, coral tissue
may not expand, or be reduced in mass, and remains tightly contracted. It is then very
difficult to determine if there is coral tissue remaining, or if tissue loss has occurred.
One of the fastest ways to assess this is to watch for the rapid colonization of diatoms
and other algae. These algae will not settle on coral tissue, but readily populate exposed
skeleton and should be visible to the eye within a day or so after skeletal exposure.
However, this too can be deceiving. Sometimes, bleached coral tissue was present but then
died as a result of the bleaching, and the skeleton is now exposed. Also, recovery from
bleaching can sometimes occur quickly, and the recovering and reproducing brown
zooxanthellae within the tissue can be mistaken for diatoms and other brown algae on
exposed skeleton. Conversely, brown diatoms are often mistaken for zooxanthellae recovery.
Furthermore, initial populations of diatoms are soon replaced by other algae, many of
which may be unicellular green types that frequently give aquarists the false impression
of recovery. Aquarists often report that their coral is recovering because they see a
greenish color returning to the tissue, but it is often just green algae growing on
exposed skeleton.
Whether or not a coral recovers from bleaching is mostly
a factor of the subsequent conditions following the bleaching event and the severity of
the bleaching itself. There are no hard and fast rules to determine whether or not a coral
will recover, and time is often the only indication and cure. Because a coral appears
white, however, does not necessarily indicate that bleaching has occurred. The same signs
of a pale or white coral can also be indicative of tissue recession, competition,
predation, environmental stress, and disease. Despite the difficulty of always being able
to recognize bleaching, it is the still the easiest of these "white" coral
problems to identify. In the next article, I will discuss some of the other causes of
"white" corals and their recognition in aquariums.
Euphyllia
parancora shows bleaching, but retains flourescing
protiens..
Websites with further information on coral
bleaching:
Literature Used (not exhaustive, but
useful for anyone interested in aspects of coral bleaching, and including excellent
summary papers):
Brown, B. (1997). "Coral bleaching:
causes and consequences." Proceedings of the 8th International Coral Reef
Symposium, Panama.
Brown, B. E. (1995). "Mechanisms of
bleaching deduced from histological studies of reef corals sampled during a natural
bleaching event." Marine Biology122: 665-663.
Brown, B. E. and L. S. Howard (1985).
"Assessing the effects of ‘stress’ on reef corals." Advances in
Marine Biology. London, Academic Press, Inc. 22: 1-63.
Brown, B. E. and M. Le Tissier (1992).
"Quantification of coral bleaching." Proceedings of the Seventh International
Coral Reef Symposium, Guam, University of Guam Press.
Bunkley Williams, L. and E. H. J. Williams
(1988). "Coral reef bleaching: current crisis, future warning." Sea Frontiers(March-April):
81-87.
Fagoonee, I., H. B. Wilson, et al. (1999).
"The dynamics of zooxanthellae populations: a long-term study in the field." Science283(5 February 1999): 843-845.
Fitt, William K., et al. 2001. "Coral
bleaching: interpretation of thermal tolerance limits and thermal thresholds in tropical
corals." Coral Reefs 20: 51-65.
Fitt, W. K., H. J. Spero, et al. (1993).
"Recovery of the coral Montastrea annularis in the Florida Keys after the 1987
Caribbean "bleaching event"." Coral Reefs12: 57-64.
Gates, R. D., G. Baghdasarian, et al. (1992).
"Temperature stress causes host cell detachment in symbiotic cnidarians: implications
for coral bleaching." Biological Bulletin182: 324-332.
Glynn, P. W. and L. D'Croz (1990).
"Experimental evidence for high temperature stress as the cause of El Nino-coincident
coral mortality." Coral Reefs8: 181-191.
Harriott, V. J. (1985). "Mortality rates
of scleractinian corals before and during a mass bleaching event." Marine Ecology
Progress Series 21: 81-88.
Hoegh-Guldberg, Ove. 1999. "Climate
change, coral bleaching and the future of the worldís coral reefs." Mar. Freshwater
Res. 50: 839-866
Hoegh-Guldberg, Ove. 1994. "Population
dynamics of symbiotic zooxanthellae in the coral Pocillopora damicornis exposed to
elevated ammonium {(NH4)2SO4} concentrations." Pac
Sci 48: 263-72.
Hoegh-Guldberg, Ove, and G. Jason Smith.
1989. "Influence of the population density of zooxanthellae and supply of ammonium on
the biomass and metabolic characteristics of the reef corals Seriatopora hystrix and
Stylophora pistillata." Mar Ecol Prog Ser 57: 173-86.
Hoegh-Guldberg, O., L. R. McCloskey, et al.
(1987). "Expulsion of zooxanthellae by symbiotic cnidarians from the Red Sea." Coral
Reefs5: 201-204.
Hoegh-Guldberg, O. and G. J. Smith (1989).
"The effect of sudden changes in temperature, light and salinity on the population
density and export of zooxanthellae from the reef corals Stylophora pistillata
Esper and Seriatopora hystix Dana." Journal of Experimental Marine Biology
and Ecology129: 279-303.
Kleppel, G.S., R.E. Dodge, and C.J. Reese.
1989. "Changes in pigmentation associated with the bleaching of stony corals." Limnol
Oceanogr 34: 1331-5.
Kushmaro, A., Banin, E., Stackebrandt, E.,
and Rosenberg, E. (2001) "Vibrio shiloi sp. nov: the causative agent of
bleaching of the coral Oculina patagonica." Int J Sys EvolMicrobiol 51:
1383-1388.
Marubini, F., and P.S. Davies. 1996.
"Nitrate increases zooxanthellae population density and reduces skeletogenesis in
corals." Mar Biol 127: 319-28.
Muller-Parker, G., et. al. 1994. "Effect
of ammonium enrichment on animal and algal biomass of the coral Pocillopora
damicornis." Pac Sci 48: 273-83.
