design: DNA was extracted using the DNeasy PowerSoil Pro Kits (Qiagen, Germany) following the manufacturer's instructions and using 200 mg (fresh weight) of the Ulva thallus. To sequence the V4 region of the bacterial 16S rRNA gene, Nextera barcoded two-step PCR libraries (Illumina, USA)  using the locus-specific primer pair 515F_5N (5-GTGYCAGCMGCCGCGGTAA-3) and 806R (5-GGACTACNVGGGTWTCTAAT-3).