White plague (WP)–like disease is one of the ecologically significant coral diseases. It causes tissue loss and affects many species from Caribbean Sea and other locations. This disease has been classified in three types, based on the rate of progression of tissue loss. A bacterium, Aurantimonas coralicida was proposed as a causative agent; but this view is challenged as A. coralicida has not always found to be associated with WP diseased coral colonies. There is lot of confusion about WP disease in corals; about its causative agent and also whether three types of WP are in fact, one disease or it represents different diseases.
This study investigated an outbreak of WP type l –like disease occurred in the US Virgin Islands in 2010, looking at totally different hypothesis of viruses as the causative agent of WP. Though, viruses have been characterized from healthy and stressed corals and also their symbiont zooxanthellae; very little is known about their role in coral diseases. The suspicion is on four main groups of viruses – bacteriophages (infecting coral associated microbes and not the corals), enveloped herpes-like viruses, nucleocytoplasmic large DNA viruses and single stranded DNA (ssDNA) viruses. Transmission electron microscopy was used to see if viruses are involved in bleaching and WP disease whereas replicated metagenomics was used to characterize viruses associated with the healthy, bleached and WP diseased corals.
Interestingly, in contrary to previous belief, bacterial pathogens were not found to be associated with WP samples rather; novel types of single stranded DNA viruses were shown as causative agents of WP. Likely host of this type of viruses appears to be coral animal itself. Higher number of these viruses was found in samples of diseased coral compared to healthy ones. Additionally, the viral community of this type of viruses associated with WP diseased samples was unique and different from same type of viruses collected from water column. Based on previous studies, the authors linked outbreaks of this type of viruses to anthropogenic environmental degradation and pollution.
The other group of DNA viruses – nucleocytoplasmic large DNA viruses were noted in abundance on samples of bleached corals, compared to non-bleached ones. This finding supports the possibility that this type of viruses infects algal symbionts (zooxanthellae) and hence may be involved in thermal stress-induced bleaching response.
Herpes viruses appeared to be common associates of corals, as the authors noted that this type of viruses dominated samples of healthy corals. Significant number of bacteriophages was also found from all coral and seawater samples.
In conclusion, various health states of coral (i.e. healthy, bleached and diseased) have different viral communities. WP diseased corals harbour highest diversity of eukaryotic viruses; bleached corals have intermediate viral diversity whereas healthy corals have the lowest viral diversity. Greater viral diversity of WP samples compared to healthy ones is not related to symbiont densities of both healthy and diseased samples.
This study made me think about mysterious nature of coral diseases. Just few days ago, I reviewed a paper (by Atad et al. 2012, and published the post on blog on 18 December 2013), which experimentally proved that a bacterial pathogen is behind the WP coral disease in the Red Sea. In the Caribbean, completely different bacterium was hypothesized as a causative agent of the same disease and this paper showed single stranded DNA viruses as the causative agent of the same disease! How can there be more than one type of causative agents behind single/same disease? How can there be variability among causative agents of the same disease in relation to geographic location? Or as the authors said WP represents more than one disease? This situation typically reflects etiologically complex nature of coral diseases and need for further research with more robust methodologies for better understanding of coral diseases.
Soffer, N., Brandt, M. E., Correa, A. M., Smith, T. B., & Thurber, R. V. (2013). Potential role of viruses in white plague coral disease. The ISME journal 1-13. doi:10.1038/ismej.2013.137.