The fundamental role of microbes in the physiology, development, ecology and evolution of algae is now well established, yet the detailed mechanisms of these interactions often remain to be elucidated. The oomycetes Eurychasma dicksonii and Anisolpidium ectocarpii are two obligate intracellular pathogens with a broad host range that we are using to investigate the immune responses of brown algae. We find that resistance to infection by Eu. dicksonii is mediated by the hypersensitive death of the algal cells attacked. This response is accompanied by the deposition of beta-1, 3-glucan in the cell wall and of blue-fluorescent metabolites, the production of reactive oxygen species, and the induction of markers usually associated with programmed cell death, such as DNA fragmentation and metacaspase expression. This hypersensitive response was observed in
ten algal species belonging to four different orders, demonstrating its broad conservation among brown algae (Phaeophyta). Furthermore, TEM and in vivo staining assays suggest that the induction of algal autophagy might be another line of defence against A. ectocarpii and possibly, Eu. dicksonii. Pilot investigation of the heritability of resistance against Eu. dicksonii in Ectocarpus fasciculatus is consistent with the hypothesis that resistance is a phenotypically stable, quantitatively inherited trait. Our current efforts focus on the development of high-throughput phenotyping bioassays to identify the loci underpinning the resistance of the brown algae Ectocarpus and Saccharina against these pathogens, with the long-term view of breeding disease-resistant kelps for aquaculture purposes.