ENDOPHYTIC ALGAE OF CHONDRUS-CRISPUS (RHODOPHYTA) .5. FINE-STRUCTURE OF THE INFECTION BY ACROCHAETE-OPERCULATA (CHLOROPHYTA)

The sequence of events taking place during infection of the algal host Chondrus crispus by the algal endophyte Acrochaete operculata was characterised at the ultrastructural level. Infection required settlement of quadriflagellate zoospores on the outer cell wall of the host. The host outer cell wall did not represent a barrier against penetration by the endophyte, which digested its way through. The extent of further spread of A. operculata in C. crispus was determined by the life-history phase of the host. In sporophytic fronds, colonisation of inner tissues occurred by digestion of the intercellular matrix, and infection spread throughout the frond. In gametophytic thalli, infection remained localised, forming a characteristic papule. In spite of these differences, degenerative changes at the cellular level were similar in the two phases of the host. Damage was caused by compression and by penetration of host cells. Final breakdown of the host resulted from the combined effects of A. operculata and bacteria. Bacteria alone did not cause damage to healthy C crispus, but they accelerated cell wall degradation in the presence of A. operculata. Thus, A. operculata should be considered a primary pathogen of C. crispus, causing direct cellular damage and facilitating secondary infections.