Another One Bites the Dust

Terrestrial dust provides biologically important nutrients to the marine environment and facilitates the transportation of microorganisms. In the aftermath of a huge dust storm on the east coast of Australia in 2009, masses of fungal spores and mycelia formed a “floating raft” that covered an area stretching across coastal waters from Sydney to Brisbane, covering an area equivalent to 25 times that of the UK. Using molecular sequencing of three different genes these spores and mycelia were identified as Aspergillus sydowii. The pathogenic strains of A. sydowii have been associated with huge sea fan mortality in the Caribbean and are known for having an adverse effect on Symbiodinium dinoflagellate coral endosymbiont motility.  

Molecular genetic analyses have shown no clear difference between pathogenic and non-pathogenic strains of A. sydowii. Hayashi et al (2016) therefore set out to examine the metabolic profile of isolates from the 2009 Australian dust storm aftermath and compare them to the metabolic profiles of those from terrestrial habitats. They also wanted to explore the fungal diversity on the 2009 “raft” and further look at how A. sydowii metabolites affected various strains of Symbiodinium, in terms of photosynthetic performance by measuring the maximum quantum yield (Fv/Fm).

The metabolites were isolated using High Performance Liquid Chromatography (HPLC) and analysed against a library of known metabolites and a type species library. Strains of the Symbiodinium dinoflagellate were obtained from the Australian National Algal Culture Collection (ANACC) and selected based on genetic clade, growth rate and geographic region (Clade A – Heron Island GBR Australia, Clade C – Hawaii US, Clade A1 – Palau). Four typical A. sydowii metabolites (sydowinol, sydowinin A, sydowinin B and sydowic acid), as well as crude extracts of A. sydowii were added to individual microplates and cultured cells of Symbiodinium were added. The maximum quantum yield was measured after every second day over 8 days.

Results showed that, of the raft species, A. sydowii was the most dominant but other species of Aspergillus along with species of Penicillium and Cladiosporium accumulated as secondary colonisers. HPLC analyses showed that >50% major metabolites were shared between terrestrial and marine strains of A. sydowii but the marine had a more streamlined metabolic pathway, which suggested intensive strain selection on marine adaptation. Also, all metabolites reduced Fv/Fm of Symbiodinium dinoflagellates with sydowinol and sydowic acid the most active in doing so. Crude extracts exhibited less clear effects on Symbiodimium Fv/Fm. Finally Symbiodinium Clades C and A exhibited high and moderate sensitivities, respectively, with Clade A1 showing low sensitivity to metabolites.

From this study the authors suggested that the Great Barrier Reef has not experienced significant coral disease events compared to those experienced in the Caribbean due to the higher octocoral diversity found there. Of the 8 clades of Symbiodinum, this study only looked at 3 and so although this study provides quite a good basis for further investigation into this area, some of the conclusions drawn cannot made based on the data collected in this study. Further studies in Symbiodinium clades could yield better data for a more direct comparison with Caribbean corals but in general this paper is a good foundation for future study.

Paper reviewed

Hayashi, A., Crombie, A., Lacey, E., Richardson, A.J., Vuong, D., Piggot, A.M., Hallegraeff, G. (2016). Aspergillus Sydowii Marine Fungal Bloom in Australian Coastal Waters, Its Metabolites and Potential Impact on Symbiodinium Dinoflagellates. Marine Drugs 14 (3) 59