Coral Bleaching and Coral Diseases

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					Coral Bleaching and Coral
 Diseases: An Overview

          J. Kilic
             Phylum Cnidaria
•   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
   –   Hydrozoa - hydroids
   –   Scyphozoa – true jellies
   –   Cubozoa - Box jellies
   –   Anthozoa – anemones &
            Class Anthozoa
• 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
• 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
• The dinoflagellate enters the host in the egg or
  larval stage or the adult may engulf free algal
• 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.
              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
• 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
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
• 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
• 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
•   Viruses
•   Bacteria
•   Protozoan
•   Fungi
•   There are some diseases that appear to have
    no known pathogen associated with them
          Rapid Wasting Disease
• 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
        Lethal Orange Disease
Attacks the reef-building coralline algae Porolithon
• 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
• 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 &
   – 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
• 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
• Many are part of the normal flora and are
  – Control populations of harmful bacteria
      White Band Disease I
– 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
– 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
• 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
• 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
• Novel type of coral disease
• Caused by Halofolliculina corallasia, eukaryotic
• 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
•   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)

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