Not contagious after all?

The decline of Caribbean corals over the last 50 years has been severe. Many factors have been identified as causes, among them coral diseases. One of those is the Dark Spot Syndrome (DSS), which affects sclerotinian corals in the Carribbean. Due to its wide prevalence, DSS is thought to be an indicator of coral reef health. However, a vector or pathogen has yet to be found.

In their paper Randall et al. (2016) set out to identify the mechanics of DSS transmission. Their aims were to test direct and waterborne transmission of DSS on Siderastrea siderea. Additionally, they also compared the bacterial communities on healthy, disease-exposed and diseased corals after the experiment. 

Fragments of diseased and apparently healthy coral colonies of S. siderea were collected at Wonderland Reef in the lower Florida Keys in 2013.

In the first experiment, waterborne transmission was examined using aquarium tanks in a cascading design. The water could be exposed to diseased fragments in the ‘upper’ tanks for 6 minutes and continue to flow into the ‘lower’ tanks. Afterwards, the bacterial communities on diseased, exposed and healthy corals were sequenced with Illumina to detect potential bacterial pathogens. In a second experiment, direct transmission was tested by placing either diseased or healthy fragments directly onto a healthy coral colony and subsequent monitoring for 28 days.

No transmission was detected in the first experiment, nor did the dark-spot lesions of the diseased corals change in size, colour or shape. The analysis of the bacterial communities showed no significant differences between the corals. However, the relative abundance of 9 bacterial taxa was significantly increased in diseased corals. These were identified as potential pathogens, although it seems as if the transmission was impaired. Yet, most of the observed taxa were not proposed as potential pathogens by earlier studies. In the second experiment, the coral colonies showed no dark-spot lesions after 28 days. 

The scientists were not able to demonstrate visual transmission of DSS to healthy corals in this study. Moreover, the bacterial communities differed considerably from data collected in previous studies. In their analysis, the authors acknowledge that the differences may be due to how bacterial communities change in space and time. Additionally, the so called ‘tank effect’ and different sequencing methods may have influenced the results. Furthermore, DSS may have multiple causes or need a vector that was not present in this experiment. The incubation time of 28 days may also have been too short to see visual transmission and the authors concede that the macroscopic diagnosis of DSS is highly subjective. However, DSS seems less transmittable than other coral diseases and does not seem to always be contagious

This is an interesting study which seems to refute a popular theory on the transmission of a prevalent coral disease. Understanding the transmission and the causes of coral diseases is vital in order to predict future outbreaks and to prevent the spread of diseases. However, there is some debate whether DSS is an infectious disease or rather a stress response of the coral. Based on the data presented in this paper, the latter seems more reasonable. 

Reviewed Article:

Randall, C. J., Jordán-Garza, A. G., Muller, E. M., & van Woesik, R. (2016). Does Dark-Spot Syndrome Experimentally Transmit among Caribbean Corals?. PloS one, 11(1), e0147493. Link: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0147493

Further Reading: 

Meyer, J. L., Rodgers, J. M., Dillard, B. A., Paul, V. J., & Teplitski, M. (2016). Epimicrobiota Associated with the Decay and Recovery of Orbicella Corals Exhibiting Dark Spot Syndrome. Frontiers in Microbiology, 7, 893. Link: http://journal.frontiersin.org/article/10.3389/fmicb.2016.00893/full