The autecology and ultrastructural interactions between Mycosphaerella ascophylli Cotton, Lautitia danica (Berlese) Schatz, Mycaureola dilseae Maire et Chemin : and their respective marine algal hosts.
Three species of higher algicolous fungi were examined at the autecologica1, cultural, light
microscope, and scanning and transmission electron microscope levels. Fungal development
and the host-parasite relationship are described for each association. The seasonal
occurrence of marine fungi from intertidal populations of Dilsea carnosa, Ascophyllum
nodosum and Chondrus crispus was examined at Bembridge, Isle of Wight, UK. A
basidiomycete pathogen, Mycaureola dilseae, was found on D. carnosa, and two
ascomycetes, Mycosphaerella ascophylli and Lautitia danica, on A. nodosum and
C. crispus, respectively.
Mycaureola dilseae is host specific and exhibited a limited reproductive cycle with green
necrotic lesions and basidiomata observed only during September and October.
Mycosphaerella ascophylli is an obligate endophyte of A. nodosum, an association in which
the fungal and algal reproductive cycles were found to be synchronous with sporocarps
confmed to host receptacles. This fungus is also found in Pelvetia canaliculata. Sporocarps
of L. danica were recorded on cystocarpic C. crispus throughout the year, with a higher
incidence of the fungus on older fronds. Tetrasporic C. crispus was also infected and in both
cases the fungus was confmed to algal reproductive tissues.
Mycosphaerella ascophylli was isolated from A. nodosum and P. canaliculata tissues and
the anamorph, Septoria ascophylli was induced to sporulate. Growth of M. di/seae from
D. carnosa lesions was limited and Lautitia danica could not be isolated from infected
C. crispus tissues. Ascospore (L. danica and M. ascophylli) and basidiospore (M. di/seae)
cultures did not grow beyond the germ tube phase.
Hyphae of M. dilseae grew both inter- and intra-cellularly in D. carnosa. Penetration of
algal cells was initially achieved by fme bifurcated penetration hyphae and there is evidence
of mechanical pressure and localized enzyme action. The fungus caused a progressive
breakdown of algal cell walls and cell contents; particularly evident was the damage to
chloroplasts and dissolution of Floridean starch grains. Infection fmally resulted in the
formation on necrotic lesions, each surrounded by a ring of basidiomata. Transmission
electron microscopy showed the ascus of M. ascophylli to be bitunicate with a thick
endoascus and thin ectoascus. Intra-membranous haustoria were occasionally observed in
the outer cell wall of A. nodosum and P. canaliculata. Lautitia danica asci were bitunicate
and ascospores were covered with a mucilagenous layer. Penetration of host cells caused
extensive damage and blackening of host reproductive tissues. The relevance of these
results are discussed in relation to algal pathology and marine fungal ultrastructure.