Response to James Cervino (Bacterial Bleaching of O.patagonica)

[Maoz Fine]

Dear James,

Thank you for your message.  your point of view is of graet intereste to
us, and I must say, to answer all your questions we need some more
research time.  Any way, we have tried to gather some of the information
we have so far in order to convince you and the other readers, that at
least in the case of O.patagonica a bacteria is the causetive agent for
bleaching.

	Coral bleaching is generally considered to be a phenomenon caused
by environmental stress, such as increased or decreased seawater
temperatures, increased solar radiation, including ultraviolet radiation,
pollution reduced salinity and combinations of these stresses. The
evidence supporting stress as the cause of coral bleaching is based both
on field studies and laboratory experiments. The field studies involve
correlations between environmental parameters and frequency of bleaching.
The most common factor believed to be responsible for extensive coral
bleaching is elevated sea temperature.  This is particularly important
because of the possible link between coral bleaching and global warming.
How does increased seawater temperature cause coral bleaching? In
principle, there are three possible targets: the coral animal, the
endosymbiotic algae and potential coral pathogenic microorganisms. There
are several reasons to assume that the coral animal is not the prime
target. If the increased temperature affected the coral physiology
directly, then one would expect that genetically identical coral species
exposed to the same temperature stress would all bleach. The possibility
that the endosymbiotic algae is the target of environmental stress,
resulting in coral bleaching, was implied in the adaptative bleaching
hypothesis of Buddemeir and Fautin (1993. Coral bleaching as an adaptive
mechanism. A testable hypothesis. BioScience 43: 320-325.). They suggested
that coral bleaching is a normal regulatory process by which genetic
variation among the zooxanthellae is allowed. Accordingly, increased sea
water temperature would lead to the loss of algae, allowing more
heat-resistant algae to form stable symbioses with the coral. Moreover,
the model of Ware et al (1996. Patterns of coral bleaching: modelling the
adaptivebleaching hypothesis. Ecological Modelling 84: 199-214.)  showed
how the adaptive bleaching hypothesis could explain some features of
bleaching events that are difficult to reconcile with mechanisms based on
invariant temperature tolerances of the two symbiotic partners.  Recently,
it has been shown that corals can host multi-species communities of
symbiotic algae (Rowan, R., N. Kowlton, A. Baker, and J.  Jara. 1997.
Landscape ecology of algal symbionts creates variation in episodes of
coral bleaching. Nature 338: 265-269.). The composition of these
communities followed a gradient of environmental parameters, and an
analyses of the symbionts before and after bleaching suggested that some
corals were protected from bleaching by hosting an additional symbiont
that was more tolerant to the stress condition. It should be pointed out
that the fact that different algae may make corals more resistant to
bleaching does not prove that the algae are the primary target of the
stress condition.

The third possible target for an environmental stress condition leading to
coral bleaching is potential pathogenic microorganisms. It is known that
stress conditions, especially temperature, can cause certain bacteria to
become pathogenic by "turning on" virulence genes (Colwell, R.R. 1996.
Global climate and infectious disease: The cholera paradigm.  Science 274:
2025-2031., Patz, J. A., R. Epstein, A. B. Thomas, and J.  M. Balbus.
1996. Global climate change and emerging infectious diseases.  JAMA. 275:
217-223.,Toren, A., L. Landau, A. Kushmaro, Y. Loya, 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.).We suggest that altering the surface bacterial population by
even small changes in the environmental conditions can lead to coral
bleaching. In the bleaching of the coral Oculina patagonica in the
Mediterranean Sea, the causative agent of the coral bleaching disease is
the bacterium Vibrio AK-1 (Kushmaro, A., Y. Loya, M. Fine, and E.
Rosenberg. 1996. Bacterial infection and coral bleaching. Nature.  380:
396., Kushmaro, A., E. Rosenberg, M. Fine, and Y. Loya. 1997. Bleaching of
the coral Oculina patagonica by Vibrio AK-1. Mar. Ecol. Prog. Ser.  147:
159-165.) and elevated seawater temperatures cause the bacterium to become
virulent (Toren, A., L. Landau, A. Kushmaro, Y. Loya, 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.).

Bleaching of Oculina patagonica in the Mediterranean Sea. Recent surveys
show that O. patagonica is abundant in wide areas along the Israeli coast
of the Mediterranean at a depth range of one to fifty meters. Most of the
bleaching colonies have been found in patchy formations at depths of one
to six meters. Bleaching of O. patagonica was first observed in 1993 and
since then has been continuously monitored. The number of bleached
colonies and the extent of bleaching increases rapidly in the summer
following rising sea temperatures. In each of the years, Mediterranean
seawater temperature off the coast of Israel increased from a minimum of
17=B1 0.2 C in February to a maximum of 29 =B1 0.2 C in August. The
percentage of colonies that showed bleaching increased from a minimum of
less than 10% in February/March to a maximum of 80% in August/September.
The frequency of bleaching began to increase in the Spring when the water
temperature reached 22-25 C and the corals began to recover in the late
Fall when the temperature again dropped below 25 C.

	Vibrio AK-1 is the causative agent of bleaching of O. patagonica.
Koch's postulates were applied to demonstrate that a particular Vibrio
strain, initially referred to as AK-1, was the causative agent of the
coral bleaching disease of O. patagonica (Kushmaro, A., Y. Loya, M.  Fine,
and E. Rosenberg. 1996. Bacterial infection and coral bleaching.  Nature.
380: 396., Kushmaro, A., E. Rosenberg, M. Fine, and Y. Loya.  1997.
Bleaching of the coral Oculina patagonica by Vibrio AK1. Mar.  Ecol. Prog.
Ser. 147: 159-165.). First, the microorganism was found to be present in
all 28 diseased (bleached) corals examined and absent in all 24 healthy
corals examined. Second, the bacterium was obtained in pure culture. The
bacterium was identified as a new species of Vibrio by classical
biochemical tests, fatty acid profile and 16S rDNA. Third,=20 pure
cultures of Vibrio AK-1 caused the bleaching disease in controlled aquaria
experiments. As few as 120 bacteria caused 83% of the corals to bleach in
20 days at 29 C. None of the corals that were not inoculated with bacteria
showed any signs of bleaching. Fourth, addition of antibiotics to the
aquaria completely blocked the Vibrio AK-1 induced bleaching.

	The effect of temperature of V. shiloi induced coral bleaching. As
mentioned above, bleaching of O. patagonica in the Mediterranean Sea is
correlated with increased seawater temperatures. To examine if Vibrio AK-1
infection of O. patagonica is also temperature regulated, a series of
aquaria experiments were performed at different temperatures. No bleaching
occurred at 16 C. At 20 C, bleaching was slow. At 25 C and 29 C, bleaching
was rapid and extensive, reaching 80% and 100%, respectively, after 45
days. No added bacteria controls showed no bleaching at all four
temperatures tested. Thus, a similar pattern of temperature dependence on
bleaching was observed in laboratory infection experiments and in field
observations.

	In the case of adhesion of Vibrio AK-1 to O. patagonica, the
process is both bacteria-specific and host-specific (Toren, A., L. Landau,
A.  Kushmaro, Y. Loya, 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). Approximately 80% of the input
Vibrio AK-1 cells adhered to the O. patagonica in 6 h, whereas several
other marine bacteria failed to adhere to the coral. Several lines of
investigation indicated that adhesion of Vibrio AK-1 to O. patagonica
involved a b-D-galactoside-containing receptor on the coral surface.
First, 50 =B5m methyl-b -D-g-alactopyranoside completely inhibited
adhesion, whereas several other sugars tested had no effect on the binding
of Vibrio AK-1 to O. patagonica. Second, addition of methyl-
b-D-galactopyranoside solutions to the coral, after the bacteria were
allowed to adhere for 6-12 h, resulted in desorption of the bacteria from
the coral surface. Addition of the inhibitor after 12 h did not release
the bacteria, indicating that Vibrio AK-1 had become irreversibly
associated with the coral.

	The discovery that the causative agent of the bleaching of the
coral O.  patagonica is the bacterium Vibrio AK-1 and that the infection
is temperature regulated, raised four major questions:

1. How general is the phenomenon? Are bacteria the causative agents of
coral bleaching in other parts of the world? If so, which bacteria are
involved?

2. How do environmental factors, such as changes in sea water temperature
and radiation trigger the infection?

3. How is the disease transmitted?

4. What are the mechanisms by which bacterial infection causes the loss of
the endosymbiotic algae?

Regarding the first question - there have been a few studies that show
changes in the bacterial population of corals following bleaching events
(Ritchie, K.B., J.H. Dennis, T. McGrath, and G.W. Smith. 1994. Bacteria
associated with bleached and nonbleached areas of Montastrea annularis.
In: Kass L. (ed) Bahamian field station. Prof. 5th Symp. Nat. Hist.
Bahamas, San Salvador, Bahamas, p. 75-79., Upton, S.J., and E.C. Peters.
1989. A new and unusual species of Coccidium (Apicomplexa,
Agamococcidiorida) from Caribbean scleractinian corals. J. Invert. Pat.
47: 184-193. However, except for the Vibrio AK-1 / O. patagonica
interaction, there have been no demonstrations that the bacteria
associated with the bleached corals are the causative agents of the
disease.

	The fact that bacteria are the causative agents of at least
certain coral diseases does not diminish the critical importance of
environmental factors and their effect on the disease process. Thus, it is
important both to discover the causative agents for coral diseases and to
study the effect of environmental factors on the pathogen/host
interaction.

Regards
Maoz Fine