Sulfated oligosaccharides mediate the interaction between a marine red alga and its green algal pathogenic endophyte

The endophytic green alga Acrochaete operculata completely colonizes the sporophytes of the red alga Chondrus crispus; however, it does not penetrate beyond the outer cell layers of the gametophytes. Given that the life cycle phases of C. crispus differ in the sulfation pattern of their extracellular matrix carrageenans, we investigated whether carrageenan fragments could modulate parasite virulence. lambda-Carrageenan oligosaccharides induced release of H2O2, stimulated protein synthesis, increased carrageenolytic activity, and induced specific polypeptides in the pathogen, resulting in a marked increase in pathogenicity. In contrast, kappa-carrageenan oligosaccharides did not induce a marked release of H2O2 from A. operculata but hindered amino acid uptake and enhanced their recognition by the host, resulting in a reduced virulence. Moreover, C. crispus life cycle phases were shown to behave differently in their response to challenge with cell-free extracts of A, operculata. Gametophytes exhibited a large burst of H2O2, whereas only low levels were released from the sporophytes.