Lichen thalli as substrates and microhabitats for slime moulds (Myxomycetes)

We conducted a structured, evidence-led narrative synthesis rather than a PRISMA-compliant systematic review. Literature was retrieved from Web of Science Core Collection, Scopus and Google Scholar, supplemented by targeted searches in the Biodiversity Heritage Library and manual screening of reference lists in key sources. Searches were conducted up to 5 January 2026 without year limits. The objective was to compile documented observations of myxomycete sporocarps (fruiting bodies of Myxogastria, distinct from fruiting structures of the lichen-forming fungus) on lichen thalli and evidence of lichen-associated trophic stages, rather than to estimate prevalence.

Search terms were organised into three keyword categories and combined in database-specific syntax. The first category captured target organisms (e.g., “myxomycetes”, “Myxogastria”, “Eumycetozoa”, “slime moulds”, “slime molds”). The second category captured the lichen substrate and thallus context (e.g., “lichens”, “lichen thallus”, “thalli”, “lichenicolous”, “epiphytic”). The third category captured the interaction and observation context (e.g., “sporocarps on lichens”, “fruiting on lichen thalli”,“myxomycete on lichen”, “lichen microhabitat”, “corticolous”). Additional targeted searches addressed recurrent taxa and substrates reported in the literature (e.g., “Licea parasitica” “Physcia” “Phaeophyscia” “Physconia”, “Cladonia”, “Lobaria”, “Peltigera”).

Records were screened in two stages. Titles and abstracts were first screened for relevance to myxomycetes and lichens; full texts were then assessed for explicit statements of sporocarps developing on lichen thalli, trophic stages detected within thallus-associated assemblages, or quantified co-occurrence patterns linked to shared abiotic drivers. Duplicate reports of the same observation were consolidated, retaining the earliest or most information-rich account.

Evidence was grouped into three classes. The first comprised records of sporocarps reported fruiting on lichen thalli in natural or managed habitats. The second comprised microecological detection or quantification of myxomycete trophic stages within lichen-associated microbial eukaryote assemblages, including studies reporting localisation within thallus microzones (Bock and Anderson 2015). The third comprised environmental-context studies documenting co-occurrence patterns of myxomycetes and lichens in bark-associated systems and relating these patterns to shared drivers such as air pollution and bark chemistry (Wrigley de Basanta 2000; Härkönen and Vänskä 2004).

Interpretation followed conservative rules. Parasitism was treated as unconfirmed unless a source provided direct evidence of feeding on lichen symbionts and/or measurable negative impacts on lichen condition attributable to myxomycete activity (Lawrey and Diederich 2003). The descriptor lichenicolous (occurring on or in association with lichens, unless direct trophic evidence is provided) was used cautiously because, in parts of the literature, it is applied in a purely occurrence sense (recurrent presence on lichen thalli) rather than demonstrated trophic exploitation of living lichen tissues (Lawrey and Diederich 2003; Kukwa and Czarnota 2006). Statements describing taxa as facultatively lichenicolous (recurrently associated with lichen thalli but not restricted to them) or reporting fructification on both lichen thalli and bare bark were treated as key qualifiers for inference (Kukwa and Czarnota 2006; Tsurykau 2017).

For each eligible sporocarp record, the following fields were extracted: myxomycete taxon; lichen thallus substrate taxon (genus or species, where provided); underlying substrate context (e.g., bark or soil where explicitly stated or ecologically implied by the lichen substrate group); source citation; and information on substrate flexibility (e.g., occurrence on bare bark as well as lichen thalli), lichen growth form (where stated), and habitat or regional context provided by the original authors. For microecological trophic-stage studies, extracted information included the lichen system studied and localisation context reported by the authors (Bock and Anderson 2015).

Literature-derived lichen substrates were grouped into two ecological groups of lichen taxa on which myxomycetes were reported: epiphytic (corticolous) and epigeic (terricolous). Where a lichen taxon can occur in more than one ecological setting, assignment followed the dominant ecological affinity in the context of the cited record; epixylic occurrences were treated together with corticolous records as bark–wood surface substrates, because both represent the surface mosaic relevant to bark- and wood-associated microsites.

Higher-level placement and terminology for Eumycetozoa within Amoebozoa followed modern classification and phylogenomic frameworks (Adl et al. 2012, 2019; Tekle et al. 2022) and a phylogenetic classification of Myxomycetes (Leontyev et al. 2019). Nomenclature for myxomycetes was checked against an online nomenclatural information system for Eumycetozoa to reduce ambiguity due to synonymy and historical usage (Lado 2005–2026). Current names of lichen taxa were standardised according to Index Fungorum (Index Fungorum 2025); original names used in cited sources are retained in square brackets where they differ.

Across the assembled evidence, the most defensible interpretation is substrate use and possible microhabitatlevel facilitation, rather than obligate antagonism. Here, substrate use refers to the occurrence or placement of sporocarps on a lichen thallus without assuming trophic dependence on the lichen symbiosis, whereas microhabitat facilitation refers to locally favourable conditions created primarily by surface microrelief and microbial surface films. Sporocarp records across multiple taxa indicate that lichen thalli can act as microtopographically complex surfaces suitable for sporocarp placement and possibly for trophic activity in thallus-associated boundary layers (Kocourková 2000; Ronikier et al. 2017). Among architectural traits, surface roughness and related microrelief currently provide the most cautious explanation. In such cases, the thallus may function primarily as a fruiting platform rather than as the trophic substrate itself. This distinction is important because sporocarps are reproductive structures, and their position may reflect microhabitat selection for exposure and dispersal rather than the primary feeding substrate of preceding trophic stages (Stephenson and Studlar 1985; Ing 1994; Stephenson 2023).

Microecological work provides direct support for lichens as microhabitats capable of supporting trophic stages, with such stages detected and quantified in thallus microzones (Bock and Anderson 2015). At the same time, the available evidence does not justify interpreting these associations as parasitic, symbiotic or obligately dependent. This is particularly relevant for facultatively lichenicolous taxa, which occur recurrently on lichen thalli but are also known from non-lichenised substrates, such as bare bark. None of the reviewed sources provides direct evidence of feeding on lichen symbionts or measurable negative effects attributable to myxomycete activity. For this reason, lichenicolous should be used here only in the broad occurrence-based sense (Lawrey and Diederich 2003; Kukwa and Czarnota 2006; Czyżewska et al. 2008).

The compiled records (Tab. 1) show a pronounced taxonomic signal, with L. parasitica emerging as the most consistently documented myxomycete associated with lichen thalli. This signal should be interpreted together with substrate flexibility: several accounts report fructification both on lichen thalli and on bare bark, supporting interpretation of L. parasitica as facultatively lichenicolous rather than obligately dependent on the lichen symbiosis (Kukwa and Czarnota 2006; Czyżewska et al. 2008; Tsurykau 2017). The range of recorded corticolous lichen substrates also suggests that ecological context within bark-associated mosaics may be more important than a narrow host range (Kukwa 2005; Kukwa and Czarnota 2006; Czyżewska et al. 2008; von Brackel 2013; Etayo and López de Silanes 2024).

Beyond L. parasitica, records of Leocarpus fragilis, Listerella paradoxa, Lamproderma arcyrioides, Comatricha laxa and Didymium spp. indicate that sporocarp development on lichen thalli is not restricted to a single myxomycete species. Macrolichen thalli and Cladonia podetia are particularly relevant in this respect because they provide exposed surfaces for sporocarp placement, but the present evidence remains occurrence-based rather than experimental (Eliasson and Gilert 1982; Záhorovská and Lisická 2002; Drozdowicz et al. 2003; von Brackel 2013; Kluša 2023).

The evidence base indicates that lichen thalli should be treated as a distinct microhabitat compartment within bark-lichen mosaics, especially in forest contexts where bark, dead branches and decaying wood jointly structure cryptogam layers and moisture dynamics. Three non-exclusive pathways can be distinguished conceptually: sporulation on thallus surfaces, possible microbiome-mediated trophic use of thallus-associated boundary layers and shared abiotic filtering across bark-lichen systems. The microbiome-mediated pathway is plausible because lichen thalli host structured microbial communities and surface films whose distribution and accessibility may be shaped by thallus microrelief (Aschenbrenner et al. 2016; Hawksworth and Grube 2020; Grimm et al. 2021; Morillas et al. 2022), while myxomycete trophic stages are phagotrophic and embedded in microbial food webs (Clark and Haskins 2015; Li et al. 2022; Peng et al. 2024). Shared abiotic filtering is supported by studies showing concordant responses of corticolous lichens and myxomycetes to bark-associated environmental conditions. A long-term comparison from Helsinki reports concurrent increases in corticolous myxomycete richness and corticolous lichen richness over decades, alongside improvement in air quality and stabilisation of bark pH (Härkönen and Vänskä 2004). A gradient study from Madrid similarly reports fewer corticolous myxomycetes in more polluted zones on holm oak (Quercus ilex L.) bark, supporting a shared-sensitivity framework in which both groups respond to abiotic constraints on bark microhabitats (Wrigley de Basanta 2000). In this framing, lichen thalli are not only potential microhabitats in their own right but also indicators of broader bark-surface conditions that may constrain both groups.

Future work should prioritise designs that explicitly discriminate boundary-layer grazing on lichen-associated microbiomes, opportunistic use of adjacent bark–lichen mosaics with sporulation on thallus surfaces and possible direct trophic interaction with lichen symbionts. This requires time-resolved microscopy and controlled in situ observations on living thalli coupled with stratified sampling of thallus microzones, alongside surveys that compare forest, plantation and urban systems under contrasting bark chemistry and pollution regimes (Bock and Anderson 2015; Grube 2018; Morillas et al. 2022). Integrating lichen growth form, surface roughness or microrelief and microzone sampling into myxomycete field protocols would test whether the apparent prominence of physcioid epiphytes and macrolichen substrates in published records reflects an ecological association or observation bias (Czyżewska et al. 2008; Ronikier et al. 2017). Given the current evidence base, the most cautious working hypothesis is that L. parasitica is more often reported from corticolous lichen systems, whereas terricolous lichen systems more often contain records of taxa such as Listerella paradoxa and Leocarpus fragilis.