The first claim of lichenisation for this fungus was made in 1970 by Rolf Singer, who wrote:
Marasmiellus affixus (Berk.) Sing. also is lichenised and grows in association with a crustaceous lichen (sic) on the surface of Eucalyptus saplings in Australia.
That was all, with no reference to any investigation of a fungal-photobiont connection. In 1973 he gave some more information:
I have received material of a crustaceous organism consisting of Coccomyxa imbedded in basidiomycetous hyphae, and these fruiting in the form of a pleurotoid Marasmiellus of section Marasmiellus which was identified as Marasmiellus affixus (Berk.) Sing. by me. It was collected by W.A. Weber, F.15045, 19 October 1967 in New South Wales: Bradford Distr., between Major's Creek and Araluen, Jan. 6, 1846, on saplings of Eucalyptus robertsonii.
This Marasmiellus belongs in the same group as the original Marasmiellus described by Murrill. It has a poor Rameales-structure in the surface layers of the pileus, and ellipsoid spores (5)-6.5-7.6 x 3.8-4.5 μ, inamyloid; the hyphae with walls 0.2-0.5 μ thick, with clamp connections. In the present monograph we would place it near or in stirps Inoderma.
The carpophores are gregarious and found exclusively where there is a covering of algae. The mycelial mat interweaving with the Coccomyxa is in places so scattered that in rather large accumulations of algal cells only a few mycelial threads can be seen whereas in others the mycelium is so thick that the algal cells are completely covered by it and the crustaceous base on top of the eucalyptus (sic) bark appears white instead of green. The crust is of indistinct morphology and therefore the association of alga and fungus appears to be less strict and obligatory than in Botrydina and Coriscium [material seen by us (1970) and the ones studied by Heli Heikkillä & Paavo Kallio (1966)]. On the other hand some cells of the alga are occasionally found in the trams of the fruiting body.
The 1846 date of collection of the type material has inadvertently remained in Singer's main text, rather than in the footnote which gives the details about the type material. Once that error is ignored the remaining words seem to constitute the only detailed published analysis of Marasmiellus affixus, and seemingly an analysis based on just one collection. The 2001 paper by Oberwinkler gives a good summary of the state of basidiolichen knowledge at the beginning of the 21st century, has diagrams that show the details of fungus-photobiont connections in various species but does not mention Marasmiellus affixus. The detailed molecular analyses of basidiolichens published by Lawrey and colleagues in 2009 do not include Marasmiellus affixus.
Singer's comments are illuminating and suggest lichenisation but do not constitute proof. He published no observations about the nature of any hyphal-algal connection and, in essence, assumed lichenisation based on proximity of hyphae and algal cells. The occurrence of some algal cells in the trama of the fruiting body is not significant, since foreign bodies occasionally appear in fungal fruiting bodies. The situation is best summarised by saying that lichenisation in Marasmiellus affixus is plausible but not proven.
Lawrey, JD; Lücking, R; Sipman, HJM; Chaves, JL; Redhead, SA; Bungartz, F; Sikaroodi, M & Gillevet, PM. (2009). High concentration of basidiolichens in a single family of agaricoid mushrooms (Basidiomycota: Agaricales: Hygrophoraceae). Mycological Research, 113, 1154-1171.
Oberwinkler, F. (2001). Basidiolichens, Chapter 12 in Hock, B (ed.) The Mycota IX: Fungal Associations, Springer, Berlin.
Singer, R. (1970). Omphalinae (Clitocybeae-Tricholomataceae Basidiomycetes). Hafner Publishing Company, New York. [=Flora Neotropica, Monograph No. 3]
Singer, R. (1973) The genera Marasmiellus, Crepidotus and Simocybe in the Neotropics. Cramer [=Beihefte zur Nova Hedwigia, Heft 44].