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Coral Bleaching and Coral Diseases: An Overview


Lethal Orange Disease Attacks the reef-building coralline ... and values over 8 C-weeks can cause widespread bleaching and some mortality. --NOAA Coral Reef ... – PowerPoint PPT presentation

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Title: Coral Bleaching and Coral Diseases: An Overview

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)

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Phylum Cnidaria
Phylum Cnidaria
  • 4 Classes
  • Hydrozoa - hydroids
  • Scyphozoa true jellies
  • Cubozoa - Box jellies
  • Anthozoa anemones corals

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

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  • 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

  • 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
    (lt75m) so the algae can photosynthesize.
  • Products of photosynthesis are translocated to
    the coral as carbon compounds.
  • The algae utilizes the corals 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 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

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.

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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
  • 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 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
  • Cause is unknown, possibly a combination of

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
  • 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 archaea.
  • 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
  • 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
    upif 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 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
  • 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 fungiin 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

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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)
  • 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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