Pathogenicity of culturable bacteria isolated from an ice-ice-disease-infected Kappaphycus striatus green cultivar on the healthy brown cultivar

Abstract The ice-ice disease (IID) impacting the commercially important eucheumatoid seaweed Kappaphycus striatus is characterized by thallus bleaching (whitening), softening, and subsequent disintegration of the affected tissue. The occurrence of IID is thought to be stress-related release of dissolve organic carbon exudates that are utilized as substrate for microbial growth. The presence of pathogenic bacteria can subsequently induce IID manifestations that can jeopardize crop health. In this study, we investigated the role of pathogenic bacteria isolated from diseased green cultivar of K. striatus in inducing IID symptoms in healthy non-axenic brown cultivar of the same species under controlled laboratory conditions. Healthy branches of K. striatus were exposed to four distinct bacterial strains: three isolated from the diseased K . striatus ( Vibrio brasiliensis strain A8, V . brasiliensis strain B2 and V . chemaguriensis strain V1) and a positive control Cytobacillus solani strain-V2. Over a period of ten days, routine administration of each bacterium was conducted during the daily renewal of the filtered seawater medium. The occurrence of symptoms, i.e., typical of IID infection, was observed in samples subjected to additional bacterium but not under the control condition with natural microbiome. Our findings suggest that the presence of pathogenic bacteria, even under non-stressful laboratory conditions, can induce IID disease syndrome with various manifestations, including the development of wounds, abrasions, thallus bleaching, and fragmentation. The samples infected with the positive control C. solani strain-V2 exhibited the most severe bleaching at 80%, followed by V. brasiliensis strain-A8 at 60%, while V. brasiliensis strain-B2 and V, chemaguriensis strain-V1 both induced 50% thallus bleaching.