<i>Ascophyllum</i> and its symbionts. X. Ultrastructure of the interaction between <i>A. nodosum</i> (Phaeophyceae) and <i>Mycophycias ascophylli</i> (Ascomycetes)

The symbiosis of a brown alga, Ascophyllum nodosum (L.) Le Jolis and its obligate fungal symbiont, Mycophycias ascophylli (Cotton) Kohlmeyer and Volkmann-Kohlmeyer, was studied using transmission electron microscopy. A high quality of cell preservation was achieved after propane-freezing and freeze substitution; this allowed us to observe the interaction of the symbiosis without extensive artifacts. The fungus was found in the middle portion of cortical-cell walls, and at the edge of medullary cells and air-bladder filaments, but never close to host cell protoplasm. Host cell-wall modification was limited to a short distance around the hyphae. A sheath with electron-dense materials around the fungus was found in the older hyphae, but not in the hyphal tips. A range of hyphal ultrastructure was observed from cells with dense cytoplasm, absent to slight vacuolation and with well-defined organelles, to highly vacuolated cells with little cytoplasm and poorly defined organelles, to senescent cells that were often collapsed with no recognizable organelles. No sign of typical cytological resistance responses was observed in host cells, thus confirming the nonantagonistic nature of the two symbionts.