Alcobiosis, an algal-fungal association on the threshold of lichenisation.

Alcobiosis, the symbiosis of algae and corticioid fungi, frequently occurs on bark and wood. Algae form a layer in or below fungal basidiomata reminiscent of the photobiont layer in lichens. Identities of algal and fungal partners were confirmed by DNA barcoding. Algal activity was examined using gas exchange and chlorophyll fluorescence techniques. Carbon transfer from algae to fungi was detected as 13C, assimilated by algae, transferred to the fungal polyol. Nine fungal partners scattered across Agaricomycetes are associated with three algae from Trebouxiophycae: Coccomyxa sp. with seven fungal species on damp wood, Desmococcus olivaceus and Tritostichococcus coniocybes, both with a single species on bark and rain-sheltered wood, respectively. The fungal partner does not cause any obvious harm to the algae. Algae enclosed in fungal tissue exhibited a substantial CO2 uptake, but carbon transfer to fungal tissues was only detected in the Lyomyces-Desmococcus alcobiosis where some algal cells are tightly enclosed by hyphae in goniocyst-like structures. Unlike lichen mycobionts, fungi in alcobioses are not nutritionally dependent on the algal partner as all of them can live without algae. We consider alcobioses to be symbioses in various stages of co-evolution, but still quite different from true lichens.

Additions to the taxonomy of Lagarobasidium and Xylodon (Hymenochaetales, Basidiomycota).

Lagarobasidium is a small genus of wood-decaying basidiomycetes in the order Hymenochaetales. Molecular phylogenetic analyses have either supported Lagarobasidium as a distinct taxon or indicated that it should be subsumed under Xylodon, a genus that covers the majority of species formerly placed in Hyphodontia. We used sequences from the ITS and nuclear LSU regions to infer the phylogenetic position of the type species L.detriticum. Analyses confirm Lagarobasidium as a synonym of Xylodon. Molecular and morphological information show that the traditional concept of L.detriticum covers at least two species, Xylodondetriticus from Europe and X.pruinosus with known distribution in Europe and North America. Three species currently placed in Lagarobasidium are transferred to Xylodon, viz. X.magnificus, X.pumilius and X.rickii. Three new Xylodon species are described and illustrated, X.ussuriensis and X.crystalliger from East Asia and X.attenuatus from the Pacific Northwest America. The identity of X.nongravis, described from Sri Lanka, is discussed.