Culture of the upper littoral zone marine alga Pseudendoclonium submarinum induces pathogenic interaction with the fungus Cladosporium cladosporioides

The green marine alga Pseudendoclonium submarinum (Willie 1901) occurs on hard substratum in the upper littoral in the North Atlantic Ocean. In recent field and laboratory studies, Pseudendoclonium submarinum grew normally in situ but declined in vitro. Microscopic examination showed the alga inoculum to be associated with a filamentous fungus, where the fungus grew in close association with the alga. Pseudendoclonium samples collected from Ipswich, MA during a two-year period were consistently associated with the fungus. In cultures with the fungus present, algal thalli declined and became necrotic, while the fungus produced hyphae and conidia. The fungus failed to grow in sterile algal medium in the absence of Pseudendoclonium but could be isolated and grown on standard mycological culture media. Light and scanning electron microscopy showed fungal hyphae infiltrated intercellular areas of the algal filaments but did not penetrate algal cell walls. Pathogenicity is apparently a result of the in vitro environment, and it is hypothesized that the fungus produces one or more secondary metabolites, which accumulate in culture. These metabolites produced by the fungus eventually killed algal cells. Fungal ribosomal DNA sequences were compared to Cladasporium sp. in GenBank, indicating that the fungus is Cladosporium cladosporioides (Fresenius) de Vries. Widely distributed, Cladosporium cladasporioides has been shown to be a facultative parasite, endophyte and saprophyte on terrestrial plants and some algae. The fungus has not been described in association with an ulvophyte, a green marine alga.