Biology and biochemistry of host-pathogen interactions in marine brown algae

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Associations between marine algae and symbiotic or parasitic microorganisms are widespread but poorly understood. This study aims to utilize modern biological and biochemical techniques to analyze the responses of brown algal hosts to pathogen and epibiont attacks. A 3-year field study along the European Atlantic coast found that Pylaiella littoralis populations experienced significant epidemics from the parasites Eurychasma dicksonii, Chytridium polysiphoniae, and Anisolpidium rosenvingei. Laboratory investigations revealed that Eurychasma has a broader host range than Chytridium, affecting various brown algal orders and tolerating a wider temperature range. Phylogenetic analyses indicated that Eurychasma dicksonii is part of the Oomycota, positioned between terrestrial plant pathogens and marine heterotrophic picoplankton, while Chytridium polysiphoniae is more closely related to certain Chytridiomycota genera. Eurychasma infections initially have a modest effect on host physiology, enhancing photosynthetic capacity, whereas Chytridium causes immediate detrimental effects, leading to rapid cell death. Biochemical responses of kelp sporophytes (Laminaria digitata, Macrocystis pyrifera) to pathogen attacks were also studied. Oligoalginates act as elicitors, triggering an oxidative burst and resistance to pathogenic endophytes. This oxidative response, involving K+, Cl-, and Ca2+ channels, is characteristic of the alginate-rich