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Algae are increasingly being used in human affairs. Alongside with higher volumes, threats for seaweed natural reservoirs and seaweed farming are starting to appear. Epidemics seems a key complication, similar to agriculture and animal aquaculture. However, phycopathologies are much more under documented, and future diseases are currently difficult to foresee. Thus, efforts should be stressed on biodiversity of algal pathogens and algal defense strategies against diseases. This thesis is divided in two parts: the first section comprises Chapters Two and Three, which describe novel algal pathosystems from cell biology and molecular phylogeny perspectives. Chapter Two characterizes the obligate parasite Maullinia braseltonii sp. nov., a phytomyxid infecting Durvillaea (bull kelp) in the Southern hemisphere. Chapter Three reports new records, cell plasticity and epidemiology of the endophyte Laminariocolax aecidioides infecting Lessonia berteroana (huiro negro) in northern Chile. The two hosts aforementioned have commercial importance, and remarkable roles in coastal ecology in temperate marine habitats in the Southern hemisphere. Chapters Four, Five and Six correspond to a second section and describe cellular processes in novel brown algal pathosystems, related with infection and immunity responses. In Chapter Four, using the Macrocystis/Anisolpidium as a model, microscopy evidence indicate that autophagy is a key cellular process used either to help starving pathogen syncytia to survive (controlled) or to kill themselves (abortive), probably induced by the host. Likewise, host autophagy was also widely observed and may be involved in the neutralization of early-stage oomycetes or nutrient recycling to mount defences. Chapter Five shows the results on a screening of ca. 40 strains from different brown algal orders, to phenotype cellular defense markers against two phylogenetic distant pathogens (Anisolpidium ectocarpii and Maullinia ectocarpii). Overall, responses such as cell wall re-arrangements, programmed cell death, autophagy and ROS accumulation were confirmed to be at least partially conserved across different Phaeophycean lineages. Chapter 6 reports a boost on the phlorotannin metabolism in the model brown alga Ectocarpus (e.g. physode formation and polyketide synthase gene expression) after pathogen challenging, suggesting a similar protective role of this molecule as against grazers and deleterious light regimes. Phlorotannins role however needs to be elucidated.