Differential surface microbial community and thalli metabolome as early indicators of disease in red algae Pyropia yezoensis

To understand pathogenic mechanisms and suggest molecular biomarker candidates for the disease status of edible red algae, Pyropia, we analyzed relationships between disease symptom severity, changes in microbial communities, and the metabolic dysregulation of the red algae with 16S and 18S ribosomal RNA gene amplicon sequencing and mass spectrometry-based metabolomics. Our observations of disease symptoms and microbial communities indicated that Pythium was the major pathogen. As the disease progressed, the prokaryotic diversity and composition of the microbial communities were clearly distinct from diseased thalli compared with healthy and weakly diseased thalli. The results of the microbial correlation network analysis showed the co-occurrence of known pathogenic microbes in the diseased thalli. Similarly, the results of the metabolomic analysis showed that the relative abundance of annotated metabolites differed in the diseased thalli compared with the healthy and weakly diseased thalli. In particular, three purine metabolism-related metabolites—hypoxanthine, guanine, and inosine—increased significantly in the diseased thalli. All the affected metabolites in abundance regulation, including the three metabolites, are known to be related to biotic stress and defense mechanisms. This study contributes to filling the gaps in research on molecular and microbial events, especially the links between pathogenic and non-pathogenic microbes, metabolites, and symptom severity in Pyropia. Additionally, the results indicate that the observed prokaryotic diversity reduction and co-occurrence of the known microbial pathogens are new non-target indicators that could be beneficial in cases where no known pathogens indicate pathogenicity. Dysregulated purine metabolism-related metabolites and other stress- and defense-related metabolites are potential molecular indicators of disease status of Pyropia.