Bacterial Species and Their Importance in Coral Death 2

SEa_Fan_2.jpgTitle: Aspergillus sydowii as an Opportunistic Pathogen of Gorgonacea

Author: Patrick O'Meara

Date: 5/13/09

Abstract: The recent rise in coral species being the target of pathogens must be caused by exposure to a new pathogen, an increased virulence of a common pathogen, or weakening of the immune system of coral species. From the data obtained Aspergillosis is most likely caused by an increased virulence of Aspergillus sydowii [Kim 2006]. A. sydowii is ubiquitous to the marine environment and has not recently been introduced to or increased in concentration in coral reefs. Its recent emergence as a pathogen has been because of changes to the coral reef environment such as increased temperature [Israely 2001] and addition of nutrients which most likely increases the survivability of the fungus without increasing the resistance of the coral species to the pathogen.

Introduction: Coral reefs have experienced a number of die offs in recent years. The causes are still under debate. This new frontier of research is interesting because coral reefs play such an integral role in ocean ecology.

Discussion: Aspergillosis is an infection caused by fungi of the genus Aspergillus. All known Aspergillosis infections of Sea Fans have been aspergillosis_pflexaura.jpgshown to be caused by the specific mold Aspergillus sydowii (A. sydowii) [Kim 2006]. A. sydowii was shown to be the pathogen causing Aspergillosis of Sea Fans using Koch’s postulates.
1) The fungi is found in all reported cases of Aspergillosis.
2&3) When cultured from an infected Sea Fan the resulting pure culture was able to infect a healthy host.
4) The secondarily infected host was then shown to have the same fungal infection as the original host.

Aspergillus sydowii is an ascomycete that is very commonly found in soil samples. The Sea Fans that it is now infecting are part of the order Gorgonacea, they are commonly found in tropical oceans and make up a large part of what are commonly called coral reefs. Aspergillosis of Sea Fans causes purple patches and lesions to appear on the surface of the Sea Fan [Bruno 2003].
Aspergillosis of Sea Fans is a disease that has only become prevalent in recent years. This means that there has been a recent change in the interaction between the two species. One possible explanation of this change is that A. sydowii was recently introduced to the marine habitat. This would indicate that the recent rise in cases of Aspergillosis has been because of a new exposure of Sea Fans to the fungus.

asp_gvent_3_eweil.jpgIn order to determine when Sea Fans were first exposed to A. sydowii, the genetic makeup of A. sydowii in multiple locations around the world was compared. The results of the experiment showed that A. sydowii has a single global population that has been part of the marine habitat for a very long time [Rypien 2008]. This makes it unlikely that new exposure to the pathogen was the cause of the recent rise in cases of Aspergillosis. There must then, be some other factor that has changed the interaction between the two species, making A. sydowii more pathogenic.
Temperature is cited as increasing the virulence of pathogens in coral reefs but this researcher was unable to find a paper showing this effect in A. sydowii. There were however, many papers which reported this as one of the most likely causes of increased virulence.
A more likely candidate is an increase in nutrients in the environment. An experiment was Picture1.gifconducted that showed that Sea Fans exposed to additional nutrients, such as phosphates and nitrates, were significantly more susceptible to infection than Sea Fans exposed to nutrients or the disease alone [Bruno 2003]. This seems to show that Aspergillosis becomes more pathogenic when nutrients are added to the environment. This excess of nutrients can occur due to many factors, many of which are the result of Humanity. Pollution for example, is an excellent source of phosphate and nitrate.
This research showed a strong likelihood that A. sydowii is endemic to coral reefs but under “normal” circumstances does not act as a pathogen. Outside influences act to cause an increase in the pathogenicity of the fungus by increasing the fitness or survivability of the pathogen without increasing the fitness of survivability of the host

Literature cited:
Bruno, J. 2003. Nutrient enrichment can increase the severity of coral diseases. Published online in Ecology Letters 6, 1056*1061 (2003).
Israely, T. 2001. Growth, differentiation and death of Vibrio shiloi in coral tissue as function of seawater temperature. Published online in Aquatic Microbial Ecology 24, 1-8 (2001).
Kim, K. 2006. Longitudinal study of aspergillosis in sea fan corals. Published online in Diseases of Aquatic Organisms 69, 95-99 (2006).
Rypien, K. 2008. Globally panmictic population structure in the opportunistic fungal pathogen Aspergillus sydowii. Published online in Molecular Ecology 17, 4068-4078 (2008).
Shinn, E. 2000. African Dust and the Demise of Caribbean Coral Reefs. Published online in Geophysical Research Letters 27, 3029-3032 (2000

Bacterial Species and Their Importance in Coral Death