Transcript Coral Bleaching and Coral Diseases

Coral Bleaching and Coral
Diseases: An Overview
J. Kilic
Phylum Cnidaria
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Radial (or biradial) symmetry
Diploblastic tissue organization
Mesoglea between tissue layers
Gastrovascular cavity
Nerve net
2 body forms – polyp & medusa
Cnidocytes (w/nematocysts)
Phylum Cnidaria
Phylum Cnidaria
• 4 Classes
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Hydrozoa - hydroids
Scyphozoa – true jellies
Cubozoa - Box jellies
Anthozoa – anemones &
corals
Class Anthozoa
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Corals & sea anemones
All Marine
Colonial (corals) or solitary (anemones)
No medusa stage
Polyps have a mesenteries and a pharynx
leading to the GV cavity
• Amoeboid cells in the mesoglea
Zooxanthellae
• Algal symbionts
• Most cnidarians possess the dinoflagellate
Symbiodinium microadriaticum
• Within the vacuoles of gastrodermal cells (about
50 dinoflagellates)
• May contain as many as 30,000 symbionts per
mm3
• The dinoflagellate enters the host in the egg or
larval stage or the adult may engulf free algal
cells.
Zooxanthellae
• It is the pigments of the symbiotic algae that give
corals their coloration
• In most cases, the symbiosis is obligate
• The host coral must live in shallow, clear waters
(<75m) so the algae can photosynthesize.
• Products of photosynthesis are translocated to the
coral as carbon compounds.
• The algae utilizes the coral’s nitrogenous wastes
and acetate.
Zooxanthellae
Coral Bleaching
• Caused by the loss or large reduction in the
zooxanthellea (or their pigment).
• White calcium carbonate skeleton of the the coral
becomes visible
• Loss of zooxanthallea causes the corals to begin to
starve.
• Large number of environmental factors that may
cause coral bleaching
– Pollution, sedimentation, increased UV radiation,
freshwater runoff, salinity changes, changes in
atmospheric carbon dioxide
– Strongest correlation has been found between sea
surface temperatures (usually linked to ENSO) and
bleaching
Montastrea faveolata
Coral Bleaching
• In most species, temps above 32C along with
increased UV radiation can trigger bleaching.
• Although bleaching may be lethal, some corals do
recover.
• They may regain their symbionts when conditions
return to normal (if timely)
• During a bleaching event reproduction and growth
are negatively affected and corals tend to be more
susceptible to disease.
• If conditions remain stressed for extended periods,
death individual coral colonies or entire stretches
of reef may occur.
Bleached section of The
Great Barrier Reef off the
coast of Queensland, AU
1998 Massive Bleaching Event
• 1997-1998 experienced major bleaching events.
• Every coral region in the world effected by
bleaching in 1998 – the first global bleaching
event
• Triggered by severe ENSO conditions
• Summer 1997-1998 at The Great Barrier Reef was
the hottest on record
• 67% inshore reefs showed “high or extreme”
levels of bleaching (14% offshore)
• Sea temps were 1-2C above long-term averages
• On some reefs coral mortality reached up to 80%
More Recently…
• 2002 ENSO conditions are thought to have
triggered this major bleaching event.
• 2005 NOAA reported a major bleaching
event in the Caribbean.
– Bleaching was reported from the entire area, the
Florida Keys, Texas coast, Costa Rica, Tobago,
Panama etc…
– Bleaching coincided with areas that
experienced levels of “high thermal stress”
“The DHW accumulates any HotSpots greater than 1 °C over a 12- week window, thus showing how
stressful conditions have been for corals in the last three months. It is a cumulative measurement of the
intensity and duration of thermal stress, and is expressed in the unit °C-weeks. DHWs over 4 °C-weeks
have been shown to cause significant coral bleaching, and values over 8 °C-weeks can cause widespread
bleaching and some mortality.” --NOAA Coral Reef Watch
Coral Disease
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Viruses
Bacteria
Protozoan
Fungi
There are some diseases that appear to have
no known pathogen associated with them
Rapid Wasting Disease
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First observed in 1996
Leaves skeleton exposed with no living tissue
Appears on the coral head first
Although the skeleton appears normal, when
touched it simply crumbles
• Cause is not yet confirmed, however
– There has been observation of a filamentous fungus
present on infected corals
– Infected corals tend to be found where unfavorable
algal species occur, particularly those that are often
associated with excess nutrients from runoff and
sewage.
Lethal Orange Disease
Attacks the reef-building coralline algae Porolithon
onkodes
• Proceeds in an orange band leaving behind the
white skeleton
• Forms upright filaments and globules similar to
slime molds
• Coralline lethal disease is probably related but
lacks the orange band
• Believe to be a bacterial pathogen
Dark Spot Disease
• Circular or irregular shaped dark spots
appear on the surface of coral
• Usually begins as purple or gray lesions
• Sediment accumulates in the center of these
patches
• Cause is unknown, possibly a combination
of pathogens
Coral Viruses (Vega, 2008)
• Corals do not just have zooxanthellea as
symbionts, they also have an array of microbial
flora & fauna, much like we do
– The “coral holobiont” refers to the coral, zooxanthellea
& this normal flora & fauna.
– Viruses present as a part of this normal state are often
those that infect protozoans, metazoans, bacteria &
archaea
– Certain viruses and bacteria may be detrimental in
times of stress
• Temperature, nutrient levels, DOM
Herpes Viruses
• Elevated abundance when temperature
stress is applied
• Herpes viruses tend to be under control as
long as the coral is not stressed
• Once stressed or compromised, the viruses
become much more active
• Positive correlation has been found between
herpes genes and presence of coral tumors
Geminivirus
• Single stranded DNA plant virus
• Increased abundance with increased
nutrients levels (ie fertilization runoff)
• Symbiodinium abundance is negatively
correlated with certain Geminiviruses
– Zooxanthellea being reduced or lysed as a
result of viral infection
Bdellovibrio Phages
• Virus that infects bacterivorous bacteria
• Increased numbers in the presence of
increased DOM (carbon source)
• Negative correlation between Bdellovibrio
phages and heterotrophic bacteria
• Suggests that Bdellovibrio phages kill the
“good” bacteria that eat the “bad”
Coral Bacterial Infections
• The good-guys: on 1 cm2 of coral, there
may be 10 million bacteria and 1 billion
archaea.
• Many are part of the normal flora and are
symbiotic
– Control populations of harmful bacteria
White Band Disease I
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Slow acting (1 cm/day)
Attacks Acroporid (branching) corals only
Tissue slowly peels off
White bands found at the base and middles of
the coral
– Gram negative rod shaped bacteria has been
associated with the disease.
White Band Disease II
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Fast acting (up to 10 cm/day)
Affects all corals Acroporid and non-Acroporid
Bleaching edge that precedes the dead egde
Bleaching edge may arrest and necrosis may
catch up…if so, WBD I & II look very similar
– Bacteria in the genus Vibrio have been found in
the bleaching edge
Black Band Disease
• Affects a large variety of corals
• Slow acting
• Black ring about a cm wide moving across the
coral surface.
• Leave behind bare skeleton
• Caused by a number of bacteria resemblinga
bacterial mat
– Sulfur-reducers
– Cyanobacteria
Red Band Disease
• Host corals are limited to stag, star & brain
corals
• Brick red or dark brown microbial mat that
advances across the surface of corals
• Bacterial components of the microbial mat
seem to differ from those found in black
band disease
Black Aggressive Band Disease
• Attacks a large variety of corals
• Similar to BBD, but the band is much
thinner
• Actually a gray band
• Cyanbacterium from the genus Spirolina is
the most probable cause
• Although others such as Ballesteros sp.
have not been entirely ruled out
Yellow Band Disease
• Yellow botch disease
• Yellow pox disease
• Distinctive yellow band that proceeds
across the surface of the coral
• Leaves behind a skeleton that is stained
yellow (penetrates a few mm)
• Bacterial pathogen is Vibrio sp.
Skeleton Eroding Band – A
protozoan
• Novel type of coral disease
• Caused by Halofolliculina corallasia, eukaryotic
protozoan
• Damages not only the living tissue but also the
skeleton of the coral.
• Attacks a variety of corals
• Colonies of black loricea (shields or houses)
• When they reproduce asexually, they release
chemicals toxic to the coral tissue.
Aspergillus – A fungus
• Aspergillus is a ubiquitous genus of Ascomycetes
soil fungi…in terrestrial ecosystems
• Has been found in marine environments including
coral reefs
• Now known as the cause of brown sea fan disease
• First observed in 1995 when a large percentage of
purple sea fans appeared stuffed with material and
were turning brown.
– That material was fungal hyphae
Scolecobasidium – A fungus
(Raghukumar 1991)
• In the Bay of Bengal, 5 species of coral
were regularly found with necrotic patches
• Sections of the patches showed a dark
brown hyphal network
• Scolecobasidium a basidiomycete fungus
was the causative agent
• In contrast to most marine fungi identified
to date being Ascomycetes
Citations
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Raghukumar, Chandralata & Raghukumar, S. Fungal Invasion of Massive Corals. 1991 Marine
Ecology 12 (3):251-260
Kohlmeyer, B & Kohlmeyer, J. Mycological Research News, Letters: Fungi from Coral Reefs: A
Commentary. 2003. Mycological Research 107 (4) 385-387
Bruno, John F., Petes, Laura E., Harvell, C. Drew, Hettinger, Annaliese. Nutrient Enrichment can
Increase the Severity of Coral Diseases. 2003. Ecology Letters 6: 1056-1061
Vega Thurber, R., Barott, K., Rodriguez-Brito, B., Liu, H., Hall, D., Edwards, R.A., Desnues, C.,
Angly, F., Haynes, M., Wegley, L., and Rohwer, F. MetagenomicAnalysis Indicated that
Stressors Induce Production of Herpes-like Viruses in the Coral Porites compressa. (in
review, PNAS)
http://www.noaanews.noaa.gov/stories2005/s2526.htm
http://www.reef.crc.org.au/publications/brochures/1998event.htm
http://www.marinebiology.org/coralbleaching.htm
www.sbg.ac.at/ipk/avstudio/pierofun/aqaba/disease1.htm
http://www.livescience.com/environment/070620_microbes_corals.html