Lichen protists
Lichens, traditionally considered a simple symbiosis between a fungal host and algal symbiont, are increasingly recognized as micro-ecosystems hosting diverse microbial and mesofaunal communities. In this study, we characterised the eukaryotic diversity associated with two common British lichens, Xanthoria parietina and Punctelia subrudecta, using high-throughput sequencing of a partial fragment of the 18S rRNA gene. Sampling across different substrates—native and non-native trees and metal railings—revealed diverse fungal, protist, and metazoan communities. Chlorophyta (48%) and fungi (37%) dominated the sequence libraries, with the fungal host Xanthoria parietina accounting for 74% of fungal sequences, while Punctelia subrudecta was minimally represented (<7%). Fungal communities differed significantly by substrate, with Xanthoria on trees enriched in Chaetothyriales and Hypocreales, while lichens on railings hosted unique Micareaceae taxa. Protist communities were dominated by Cercozoa and Amoebozoa, varying significantly between habitats and lichen species. Metazoan communities comprised mites, tardigrades, rotifers, and collembolans, with distinct differences across substrates; for instance, railing-associated Xanthoria showed high mite diversity, including known lichenivores. Additionally, Trebouxiophyceae algal symbionts displayed significant variation, with Xanthoria parietina consistently hosting Trebouxia asymmetrica, while Punctelia subrudecta also supported Symbiochloris symbiontica and other algae. Meiofaunal diversity, particularly rotifers and tardigrades, was a consistent feature across all samples, highlighting lichens as ecological hotspots. Our findings underscore the complexity of lichen-associated communities, which vary by species and environment. These results support the concept of lichens as holobionts, emphasizing their ecological significance and the need for further research into their microbial and faunal symbioses.