MGS of of microbiota: seawater
The epiphytic bacterial communities on mature sporophytes samples collected in situ were subjected to metagenomic sequencing. Surface-associated debris was first removed by rinsing the S. japonica with sterile seawater, followed by swabbing the thallus surface with sterile cotton. The swabs were then transferred to centrifuge tubes and stored at -20 C for subsequent analysis. A total of 37 samples were analyzed, in-cluding 14 replicates from diseased S. japonica, 14 replicates from healthy S. japonica, and 9 replicates of seawater. Microbial DNA was extracted from both S. japonica and seawater samples using the FastDNA SPIN Kit for Soil (MP Biomedicals, USA). The qualified DNA was frag-mented using the fragmentation reagents from the library construction kit. Adapters were then ligated to the fragmented DNA using the kits ligation reagents. The ligated products were purified using magnetic beads, followed by two rounds of size selection to enrich for fragments of the desired size. The size-selected libraries were amplified by PCR using the kits amplification reagents and subjected to a final purification step with DNA magnetic beads. The concentration and fragment size distribution of the fi-nal libraries were quality-controlled using Qubit reagents and a bioanalyzer, respec-tively.The V3-V4 hypervariable region of the bacterial 16S rRNA gene was amplified with the primers 338F/806R. Qualified libraries were subjected to paired-end sequenc-ing (2  150 bp) on an Illumina NovaSeq 6000 platform, generating raw shotgun met-agenomic reads.