The hidden diversity of lichenised Trebouxiophyceae (Chlorophyta)

The class Trebouxiophyceae is comprised of coccoid to ellipsoid unicells, filaments, blades and colony-forming species of green algae occurring in diverse terrestrial and aquatic environments. Some representatives have evolved parasitic heterotrophic lifestyles, others have been investigated for their biotechnological potential and others have evolved as integral components of lichen symbioses. In this review, we provide an overview of the current understanding of diversity, taxonomy and evolutionary context for the important lichen-forming algal class Trebouxiophyceae (Chlorophyta). In particular, we focus on members of the family Trebouxiaceae (Trebouxiales), the best-known, most widespread and most species-rich group of terrestrial, lichenised green algae. Recent investigations on the diversity of lichen phycobionts demonstrate the importance of implementing integrative taxonomic approaches. Therefore, combining analyses of morphological and anatomical traits with genetic data has improved our perspective of diversity in lichenised algae. More accurate recognition of diversity in Trebouxiophyceae will enhance our understanding of phylogenetic relationships and trait evolution, specimen identification in genomic and meta-bar-coding studies and patterns of specificity and selectivity among the lichen symbionts. We conclude with a discussion of the roles and transformative potential of high-throughput sequencing in research related to lichen-associated algae.