Abstract: Phycosphere bacteria play a crucial role in regulating the growth, metabolism, and environmental adaptability of microalgae, representing important microbial resources for the development of novel probiotics. In this study, we focused on the phycosphere bacteria of the model diatom Phaeodactylum tricornutum. Based on shotgun metagenomic sequencing, metagenome-assembled genomes (MAGs) were obtained through assembly and binning. Taxonomic annotation was performed using GTDB-TK, and systematic gene mining was conducted targeting genes involved in colonization and adhesion, bacteria–host interaction, antioxidant pigment biosynthesis, and B-vitamin biosynthesis. A total of 11 medium- and high-quality MAGs were recovered, affiliated with Pseudomonadota (9 MAGs) and Bacteroidetes (2 MAGs). Among them, Erythrobacter sanguineus (MAG11) exhibited the highest relative abundance (35.14%). Functional gene annotation revealed four potential colonization mechanisms, including LuxI/LuxR and QseB/QseC quorum sensing systems, bacterial chemotaxis, the tad tight adherence gene cluster, and the type IV secretion system, suggesting that multiple mechanisms may act synergistically to facilitate bacterial colonization in the phycosphere. Regarding probiotic functions, Algoriphagus marincola (MAG9) harbored the genetic potential to synthesize seven natural antioxidants, including astaxanthin, zeaxanthin, and β-carotene. Ten MAGs possessed complete biosynthetic gene pathways for at least one B-vitamin, among which MAG4 and MAG11 each encoded five complete B-vitamin pathways, while MAG8 was the sole MAG capable of de novo vitamin B12 biosynthesis. The MAGs identified in this study with the potential for multi-antioxidant pigment synthesis and multi-B-vitamin biosynthesis provide a genomic basis and candidate resources for the subsequent screening and development of functional probiotics.

Key words: Phaeodactylum tricornutum, phycosphere bacteria, metagenomics, colonization mechanism, probiotic function genes