Power to the fungus: how temperature tips the balance between fungal pathogens and coral hosts

Aspergillus sydowii is a fungus which causes Aspergillosis disease in the sea fan octocoral Gorgonia ventalina. Coral tissue necrosis can result from infections by Aspergillus spp., leading to declines in coral populations in the Caribbean, with huge losses of sea fan populations seen over the last few decades. Infections are thought to be associated with increased sea surface temperatures caused by anthropogenic climate change. The increased duration of higher temperatures compromises the immunity of corals and brings a higher risk of disease.

A. sydowii produces proteases which aid in colonisation of the host by breaking down host tissue to release nutrients for the fungus and by disrupting the immune function of the host. Meanwhile, healthy colonies of G. ventalina produce protease inhibitor to act against pathogen proteases and prevent colonisation by the fungal pathogen. A study by Mann et al. (2014) aims to demonstrate how environmental conditions, such as increased temperatures, can alter the balance of a relationship between host and pathogen, such as the “arms race” between A. sydowii and G. ventalina and result in the outbreak of infection and disease.

Five strains of A. sydowii were analysed: three from various tropical reefs, one from a human and another from stock spore solutions. A sydowii was cultured and protease activity quantified by calculating a ratio from the measurement of both colony growth and the clear zone around a colony, where the casein protein in the media had been broken down. Ten healthy and ten diseased G. ventalina samples were taken from healthy and diseased colonies reefs in the Florida Keys, at a range of depths between 3-6 m. Diseased coral samples were taken from purple-coloured lesions and healthy samples were taken at least 10cm from a lesion site. Coral health was determined visually. The sea fan colonies were gradually exposed over 2 hours to artificially elevated temperatures of 30-32°C for 14 days, with controls being exposed to 26-28 °C. In addition, the sea fan colonies were exposed to three different types of commercially- and fungal-derived proteases. Protease and protease inhibitors were determined with assays and quantified with a microplate reader.

The results revealed that there was a significant increase in extracellular protease activity with elevated temperature for all fungal strains, with those fungal strains at the highest temperatures producing the greatest protease activity. Overall, inhibitor activity in G. ventalina did not significantly increase with temperature stress. Protease inhibitor activity was higher in healthy sea fan colonies as opposed to diseased ones. There was inhibition of all three proteases by the sea fan colonies, however, this protease inhibition was lower within the lesion area of diseased colonies than in healthy tissues of diseased and healthy sea fan samples.

The fact that sea fan colonies have protease inhibitors that work effectively against a range of proteases, including non-fungal ones, suggests that sea fans may have the capacity to resist infection by A. sydowii. Nevertheless, elevated temperatures may tip the balance in favour of A. sydowii, as protease inhibitor activity may not be sufficient to counteract the effect of increased protease production by the pathogenic fungus. Another line of defence for G. ventalina may be the microbial holobiont associated with the coral, which may produce antimicrobial compounds that inhibit pathogens such as A. sydowii. The authors acknowledged that the coral extracts used contained all of the associated microbes which may influence this pathogen-host interaction. This needs to be considered when analysing these results, yet I feel that removing this holobiont would be counter-productive and unrealistic in terms of the environmental conditions that sea fan colonies inhabit.

 

Reference:

Mann, W.T., Beach-Letendre, J. and Mydlarz, L. (2014) Interplay between proteases and protease inhibitors in the sea fan - Aspergillus pathosystem, Marine Biology, 161, 2213-2220.
http://link.springer.com.plymouth.idm.oclc.org/article/10.1007/s00227-014-2499-2