Macroalgal eukaryotic microbiome composition indicates novel phylogenetic diversity and broad host spectrum of oomycete pathogens

Seaweeds are important components of marine ecosystems with emerging potential in aquaculture and as sources of biofuel, food products and pharmacological compounds. However, an increasingly recognised threat to natural and industrial seaweed populations is infection with parasitic single-celled eukaryotes from the relatively understudied oomycete lineage. Here we examine the eukaryomes of diverse brown, red and green marine macroalgae collected from polar (Baffin Island), cold-temperate (Falkland Islands) and tropical (Ascension Island) locations, with a focus on oomycete and closely related diatom taxa. Using 18S rRNA gene amplicon sequencing, we show unexpected genetic and taxonomic diversity of the eukaryomes, a strong broad-brush association between eukaryome composition and geographic location, and some evidence of association between eukaryome structure and macroalgal phylogenetic relationships (phylosymbiosis). However, the oomycete fraction of the eukaryome showed disparate patterns of diversity and structure, highlighting much weaker association with geography and no evidence of phylosymbiosis. We present several novel haplotypes of the most common oomycete Eurychasma dicksonii and report for the first time a cosmopolitan distribution and absence of host specificity of this important pathogen. This indicates rich diversity in macroalgal oomycete pathogens and highlights that these pathogens may be generalist and highly adaptable to diverse environmental conditions.