Photobiont Diversity in Lichen Symbioses From Extreme Environments

Fungal-algal relationships-both across evolutionary and ecological scales-are finely modulated by the presence of the symbionts in the environments and by the degree of selectivity and specificity that either symbiont develop reciprocally. In lichens, the green algal genus Trebouxia Puymaly is one of the most frequently recovered chlorobionts. Trebouxia species-level lineages have been recognized on the basis of their morphological and phylogenetic diversity, while their ecological preferences and distribution are still only partially unknown. We selected two cosmopolitan species complexes of lichen-forming fungi as reference models, i.e., Rhizoplaca melanophthalma and Tephromela atra, to investigate the diversity of their associated Trebouxia spp. in montane habitats across their distributional range worldwide. The greatest diversity of Trebouxia species-level lineages was recovered in the altitudinal range 1,000-2,500 m a.s.l. A total of 10 distinct Trebouxia species-level lineages were found to associate with either mycobiont, for which new photobionts are reported. One previously unrecognized Trebouxia species-level lineage was identified and is here provisionally named Trebouxia A52. Analyses of cell morphology and ultrastructure were performed on axenically isolated strains to fully characterize the new Trebouxia A52 and three other previously recognized lineages, i.e., Trebouxia A02, T. vagua A04, and T. vagua A10, which were successfully isolated in culture during this study. The species-level diversity of Trebouxia associating with the two lichen-forming fungi in extreme habitats helps elucidate the evolutionary pathways that this lichen photobiont genus traversed to occupy varied climatic and vegetative regimes.

Automated summary

This study investigated the diversity of Trebouxia spp. associated with the lichen-forming fungi Rhizoplaca melanophthalma and Tephromela atra in montane habitats worldwide. The greatest diversity of Trebouxia species-level lineages was found in the altitudinal range of 1,000-2,500 m. A total of 10 distinct Trebouxia species-level lineages were identified, including one previously unrecognized lineage.