The Gorgon’s Gaze: Do stony defences protect sea fans against fungal pathogens?

Gorgonian sea fans (Cnidaria: Octocorallia) are sessile, colonial organisms common in tropical and subtropical seas. Of these, Gorgonia ventalina has piqued the interests of marine microbiologists due to its susceptibility to aspergillosis – a fungal disease caused by the pathogenic ascomycete Aspergillus sydowii. Fungal infections by this species devastated populations of G. ventalina in the Florida Keys, causing mass tissue necrosis. Although there is evidence that this epizootic is subsiding, a proper understanding of resistance mechanisms to coral fungal infections in general may help the control of future outbreaks. Gorgonian sea fans produce fine, calcium carbonate structures known as ‘sclerites’ in and between their tissue-layers. While previous studies have provided evidence for a structural and anti-predatory function for sclerites, their role in the resistance of fungal infections had not been previously investigated.

To remedy this, Toledo-Hernández and colleagues (2016) isolated environmental fungi from the tissue of healthy G. ventalina specimens and, using growth rates, estimated pathogenesis and rDNA ITS sequencing, identified four species to investigate (two of which were Aspergillus congeners). The coenochyme of G. ventalina was simulated by filling volumetric pipettes with agar and the effect of sclerites in fungal defence was analysed by augmenting the model cnidarians with sclerites from either healthy or diseased sea fans (to investigate whether antifungal defences were induced), half of which were incubated with acetone to remove associated polar compounds and investigate potential biochemical defences. Relative to the no-sclerite control, fungal penetration took significantly more time in agar tubes layered with sclerites, although the health of the donor coral did not influence fungal penetration. Interestingly, for some fungi, sclerites treated with acetone were less effective at resisting fungal infection than those that were not, hinting at the presence of an associated biochemical factor.

This paper provides a neat model to investigate the role of sclerites in resisting fungal infection in G. ventalina, however I am sceptical as to what can be applied to the in vivo biology of this sea fan. The modelled tissue system was two-dimensional, greatly simplified and did not control for the shape and arrangement of sclerites, which would affect the packing density and efficiency of the sclerites as a physical barrier. Also, the fungi isolated (none of which were A. sydowii) were isolated from healthy tissue leaving much to be desired as to how appropriate these chosen species are to model pathogenic fungal penetration.

Leaving these discrepancies aside, the importance of these sclerites should still not be exaggerated as a major defence against fungal infection. Following five days, all modelled treatments had been penetrated by fungi and, in marine systems, a vast number of taxa have been shown to be endolithic in hermatypic corals (as reviewed by Golubic et al, 2005) and possess cellular mechanisms to degrade calcium carbonate structures, rendering sclerites ineffective against any pathogen with such capabilities.

Overall, this study hints at a potential role of sclerites in resistance to fungal infection, however much more work is needed in vivo to corroborate these findings. I would be particularly interested to see the biochemical defences suggested by this study examined using mass spectrometry to identify any potential antifungal metabolites. Such compounds may prove very useful in combatting fungal infections in corals worldwide. 

Reviewed Paper: Toledo-Hernández, C., Ruiz-Diaz, C. P., Ramírez-Camejo, L., Paniagua, R., Sabat, A. M., Roberson, L., & Bayman, P. (2016). The role of sclerites in the defense against pathogens of the sea fan Gorgonia ventalina (Octocorallia).Journal of Experimental Marine Biology and Ecology, 483, 20-24. http://www.sciencedirect.com/science/article/pii/S0022098116301058

Endolithic Marine Fungi: Golubic, S., Radtke, G., & Le Campion-Alsumard, T. (2005). Endolithic fungi in marine ecosystems. Trends in microbiology, 13(5), 229-235. http://www.sciencedirect.com/science/article/pii/S0966842X05000776