Abstract: Filamentous thalli of the red alga Pyropia yezoensis exhibiting yellow-green spot disease, provided by Hanyong Aquaculture Company (Lianyungang City), were used as experimental material. The dominant pathogenic isolate, designated ZCX-1, was obtained from these diseased thalli and preliminarily identified as Ruegeria arenilitoris based on 16S rRNA gene sequence comparison and phylogenetic analysis. Antibiotic susceptibility was evaluated by the paper-disc diffusion assay, and the influence of different temperatures and pH values on antibacterial efficacy was investigated. Results of the susceptibility tests showed that among nine tested antibiotics, strain ZCX-1 was resistant to cephalothin; it was highly susceptible to neomycin, enoxacin, ampicillin, cefuroxime, and ceftriaxone (inhibition zone diameter D ≥ 20 mm). Ampicillin exhibited the greatest activity, with a mean inhibition zone diameter of 33 mm, while neomycin produced the smallest mean inhibition zone (22.33 mm). ZCX-1 showed intermediate susceptibility to lomefloxacin, streptomycin, and kanamycin (10 mm < D < 20 mm). Temperature-pH interaction assays indicated that ampicillin achieved the strongest inhibitory effect at pH 8.0 and 30 ℃, with an inhibition zone diameter of 38.7 mm; conversely, enoxacin showed the poorest inhibition at pH 7.2 and 40 ℃, yielding an inhibition zone diameter of only 10.0 mm.

Key words: Pyropia yezoensis, filamentous thalli, yellow-green spot disease, antibiotic susceptibility, Ruegeria arenilitoris, temperature, pH