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Host-pathogen interactions have been widely studied in humans and terrestrial plants, but are much less well explored in marine systems. Here we show that a marine macroalga,Delisea pulchra, utilizes a chemical defence - furanones - to inhibit colonization and infection by a novel bacterial pathogen,Ruegeriasp. R11, and that infection by R11 is temperature dependent.Ruegeriasp. R11 formed biofilms, invaded and bleached furanone-free, but not furanone-producingD. pulchrathalli, at high (24°C) but not low (19°C) temperatures. Bleaching is commonly observed in natural populations ofD. pulchranear Sydney, Australia, during the austral summer when ocean temperatures are at their peak and the chemical defences of the alga are reduced. Furanones, produced byD. pulchraas a chemical defence, inhibit quorum sensing (QS) in bacteria, and this may play a role in furanone inhibition of R11 infection of furanone-free thalli as R11 produces QS signals. This interplay between temperature, an algal chemical defence mechanism and bacterial virulence demonstrates the complex impact environmental change can have on an ecosystem.