From coasts to clades: Trebouxia tabarcae sp. nov. enhances the diversity of known lichen microalgae

Abstract

Trebouxia is globally recognized as the most prevalent genus of green microalgae inhabiting lichen thalli. Despite this, the genus remains taxonomically underexplored, since only 34 species are presently accepted while numerous species-level phylogenetic lineages have yet to be formally described. One of these lineages, repeatedly detected in previous molecular studies and commonly referred to as Trebouxia sp. A48, has been reported from multiple lichen-forming fungi, particularly within Teloschistaceae across the Mediterranean region, but has lacked formal taxonomic treatment. In this study, an integrative taxonomic approach including phylogenetic and several microscopical analyses was applied to describe Trebouxia tabarcae sp. nov., a new microalgal species isolated from a thallus of Xanthoria parietina collected on the Mediterranean island of Tabarca (Alicante, Spain). Ultrastructurally, the species exhibits a crenulate-type chloroplast and crenulata -type pyrenoid, which shows distinct structural deviations in symbiosis compared to cultured cells, highlighting morphological differences between symbiotic and free-living states. Phylogenetic analyses based on the nrITS barcode placed the new species into Trebouxia clade A, with the closest described relatives being T. maresiae , another microalga that occurs in maritime lichens, as well as T. decolorans and T. crenulata. Furthermore, ecophysiological experiments revealed that T. tabarcae sp. nov. tolerates relatively high temperatures, which is in line with its tendency to thrive in coastal environments. The formal description of this species contributes to the advancement of the systematics and taxonomy of Trebouxia and supports the accurate identification of lichen phycobionts.