Thallus growth stage and geographic origin shape microalgal diversity in the Ramalina farinacea lichen holobionts

Abstract

[eng] Lichen symbioses are microecosystems hosting many other living organisms besides the two majorlichen symbionts (i.e., lichenized fungi [the mycobiont] and green microalgae or cyanobacteria[the photobiont]). Recent investigations evidenced that other fungi, non-photosynthetic bacteria, andmicroalgae co-inhabit within the lichen thalli, but their diversity and their roles are still underinvestigated.Here we present an ad hoc stratified sampling design and in-depth Illumina paired-end metabarcodingapproach to explore microalgal diversity in lichen thalli of the model species Ramalina farinacea fromdifferent ecologies. Lichen thalli were surveyed according to three different sizes, and differentthallus parts were considered for molecular, bioinformatics, and community diversity analyses. Theresults revealed that microalgal diversity strongly depends on the growth stage of the thalli, thegeographic area, and the habitat type. The results also show that microalgal diversity does not vary along thethallus branches (lacinias)—that is, it does not correlate with the apical growth and founder effects—and that there is no balanced co-presence of two main photobionts as previously established in R. farinacea.The sampling design performed here minimizes bias in the assessment of photobiont diversity in lichensand is proposed to be reliable and applicable to further study microalgal diversity in lichen symbioses.