Antimicrobial and Growth-Promoting Properties of Cultured Seaweeds Confer Resistance and Attraction to Ice-Ice Disease-Causing Bacteria: A Proposed Seaweed-Bacteria Pathosystem Model

Cultivated seaweeds, especially the eucheumatoids and gracilarioids, suffer from frequent incidence of ice-ice disease outbreaks, which affect their health, growth, and product quality. Microbial load (both epiphytic and endophytic) is observed to be one to two orders of magnitude higher in seaweeds with ice-ice condition as compared to healthy fronds. Acetone extracts of healthy Kappaphycus alvarezii showed inhibition of antibacterial activity of up to 40% of bacterial strains used in the bioassay, consisting of Gram-positive and Gram-negative strains. Microbial inhibition is higher in extracts from healthy cultivars than from cultivars with ice-ice disease. Meanwhile, the filtrates from the culture medium used in culturing the seaweed also showed growth-promoting ability on the same bacterial strains used in the antimicrobial assay. These episodes of growth inhibition and growth promotion of microorganisms in cultivated seaweeds could determine whether or not the seaweed host can be at risk of microbial infection leading to an ice-ice disease. A model of seaweed-microbe pathosystem based on boom-or-bust episodes is therefore proposed for farmed seaweeds, which means that seaweed cultivars that remain healthy protect themselves from bacterial attack through the production of antimicrobial compounds but enhance bacterial growth when they are weak through the release of nutrient-rich exudates.