Metagenomic analysis reveals fuzzy bacterial communities potentially supporting <i>Pyropia yezoensis</i> through vitamin <scp>B₁₂</scp> and indole acetic acid biosynthesis

Summary Algae and bacteria interact via various metabolic processes such as nutrient exchange and antimicrobial action. Since poor growth of cultivated algae can occur because of the absence of necessary bacteria, understanding algae‐bacteria interactions is crucial. In this study, metagenomic analysis was used to assess the bacterial species and metabolic functions associated with 34 cultivated conchocelis samples from the red alga Pyropia yezoensis . The analysis revealed Flavobacteriia, Alphaproteobacteria, and Gammaproteobacteria in all samples, though their relative dominance varied significantly. Functional gene analysis showed that all samples harbored genes for the synthesis of vitamin B 12 , which is considered an essential vitamin for P. yezoensis , and indole acetic acid (IAA), a phytohormone that promotes growth. The bacteria associated with P. yezoensis exhibited unique IAA biosynthesis genes in the tryptamine (TAM) and indole‐3‐acetonitrile (IAN) pathways, which were distinct from those found in symbiotic and pathogenic bacteria of terrestrial plants. Although vitamin B 12 and IAA biosynthetic genes were universally detected across the samples, the bacterial species harboring these genes varied between samples. Thus, although bacteria appear to confer beneficial metabolic functions to P. yezoensis hosts, the relationship between specific bacterial species and their functional roles remains fuzzy and variable.