The role of microorganisms in coral bleaching

The Role of Microorganisms in Coral Bleaching

This paper looks at how microbes, specifically Vibrio shiloi are the causative agents behind bleaching of the coral species Oculina Patagonica.  The secreted CaCO₃ exoskeleton, supporting the coral tissue layers, form a substrate not only for the photosynthetic zooxanthellae, but also for various bacteria, archaea and viruses. Environmental stressors are known to affect coral growth and survival and to be involved in bleaching of O. patagonica. for instance low salinity, sediment covering, exposure to excess heavy metals, species dependent poisons and changes in water temperatures have been shown to interfere with coral health. Recently, the influence of biotic factors, namely the infection by harmful bacteria, has been investigated in the context of coral bleaching..

In a related previous study, Kushmero et al. found a correlation between the presence of V. shiloi and coral bleaching.. In a further paper by Rosenberg and Falkovitz it is seen that the infection within the coral occurs by Vibrio after penetration of the epidermis, where the pathogen is thought to produce a peptide toxin which prevents zooxanthellae from photosynthesising.

Within the same study it was also noticed that the bleaching effects caused specifically by the Vibrio only occurred during the summer months when water temperatures are above the survival threshold of 25°C. With this in mind it is thought that the infection of O. patagonica by the Vibrio is temperature dependent, as during the winter months with water temperature <25°C these infections are not seen, but this does not mean that the vibrio is unable to survive as  V. Shiloi have a further trick up its sleeve in order to make sure it does not suffer mortality  in the inhospitable environment. The survival of  V. Shiloi is dependent on the bacterium’s ability to inhabit a particular  marine fireworm known as Hermodice carunculata (Sussman et al. 2003), and it is this that allows it to maintain life during the winter temperatures.

 After studying this particular phenomenon for over ten years, Rosenberg et al.  discovered that the corals had begun to build up a resistance to the V. shiloi which causes a problem within this line of research as the relationship between V. shiloi and the coral O.patagonica has been used as a model system demonstrating Koch’s postulates. This is a system designed by Robert Koch in 1984 based on 4 criteria that link a causative microbe to a disease. However, the development of resistance to V. shiloi is in contradiction to Koch’s first criterion, by this I mean that one of the criteria states  the microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.

Further studies focus on two different approaches, one will look strictly at temperature and light, and how this affects the symbiotic algae by inhibiting photosynthesis and by favouring reactive oxygen species production and thus  promoting coral bleaching. Another direction will be to delve deeper into how high temperatures cause microbial shifts leading to the disruption of the crucial balance between symbionts and how this is linked to coral bleaching. Is it possible that resident microbial communities help the corals to build up  immunity to these pathogens as additional symbiotic function as in a study by Ritchie 2006  resident microbial communities most defiantly contribute to the corals resistance to harmful bacterial infection, so it is fair to assume that during the bleaching process the corals themselves lose their antibacterial qualities and have much greater risk to infection.    

Please see the paper below if you are interested further  in the paper this blog is based on.

Rosenberg E, Kushmaro A, Kramarsky-Winter E, Banin E, Yossi L (2009) The role of microorganisms in coral bleaching. International Society for Microbial Ecology (ISME) Journal 3: 139–146