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Pioneering post-genomic approaches for studying algal host-pathogen interactions: characterization of the Ectocarpus-Eurychasma model

Final Activity Report Summary - ECTOPATH (Pioneering post-genomic approaches for studying algal host-pathogen interactions: characterisation of the Ectocarpus-Eurychasma model)

Do algae ever get sick?
Brown algae make up over 70% of the biomass of our rocky seashores. Like any other living organism, they are plagued by diseases caused by fungi, bacteria or viruses. Among them, the parasite Eurychasma dicksonii is very common and widespread. It belongs to a group of fungal-like organisms (the oomycetes) that infect a remarkable palette of hosts, from algae, plants, invertebrates, fungi, to fishes and mammals. They are responsible for devastating diseases for aqua- or agriculture, such as grape downy mildew or potato late blight. The latter causes 100Mt losses of potato production every year, which could feed half a billion people, notwithstanding the ecological consequences of widespread pesticide use. Many other species cause significant economic or environmental damage, especially when recently introduced or when new epidemic patterns induced by climate change lead to the infection of previously unchallenged populations. On the long term, the project aspires to elucidate the mechanisms of the Eurychasma-caused disease in brown algae to get a better understanding of how oomycetes manage to infect their hosts, as part of the ever-going quest for new or improved treatments against the diseases that they cause. My research also supports the study of Eurychasma biology in the field, and of its impact on the functioning of marine coastal ecosystems.

Pathogenicity and immunity, the yin and yang of disease
Pathogenicity: how do organisms like Eurychasma successfully infect their host?
As for any other pathogen, the survival of Eurychasma relies on the pathogenicity toolbox it uses to penetrate, develop and reproduce within algae. Disrupt one of these processes, and the disease is overcome. Hence, most of my work has focussed on developing laboratory techniques to characterise this toolbox: laboratory cultures of the parasite within the alga, a precise method to quantify infection severity, protocols for large scale protein and metabolite analysis.

Immunity: how do algae defend themselves against infection?
Although not always successful, algae actually do have an immune system and try to defend themselves against pathogens. The project's results so far show that some of them indeed resist Eurychasma infection attempts by inducing an array of defence mechanisms. We have indications that one of these is the production of antimicrobial compounds, which we would like to identify further. Indeed, the striking diversity of the hosts attacked by oomycete pathogens implies that the latter can overcome virtually any immune system, supposedly by targeting conserved defence mechanisms. Conversely, successful defences mounted by a resistant host might be efficient against a wide range of bugs. Therefore, we are keen on identifying these algal defence compounds and testing whether they could be useful to combat other pathogens, such as the potato blight and the malaria agents. The results also demonstrate the existence of a genetically-determined immunity in brown algae, most probably mediated by programmed cell death and conserved across this whole group of organisms. Therefore, the laboratory model offers a unique potential to advance the understanding of disease and immunity in so-called protists.

What is exciting about it?
At first sight, seeking solutions against terrestrial diseases from the sea might seem a bit far-fetched. However, it is exactly because so little is known (and exploited) from the marine environment that so much remains to be discovered. What can we expect from it? As a researcher, the intellectual thrill of diving into the unknown and building new knowledge; as a citizen, the hope of making a useful contribution to society.