The Impact of Visible Symptoms of Thallus Damage on the Phycobiota of Mediterranean Epiphytic Lichens

Lichens are excellent bioindicators of overall ecosystem health. The symbiotic nature of their thalli enables tracking changes in humidity, temperature, habitat disturbance and air pollution, often before larger plants do. Sensitive species usually show visible thallus damage, such as bleaching or changes in colour (including total or partial necrosis, and death of the photosynthetic component of the symbiosis), slow growth, and/or biases in reproductive strategies. Particularly, the extent to which these damages are associated with changes in the microscopic photosynthetic community inhabiting lichen thalli (phycobiota) remains poorly understood. Here, we combined Sanger and Illumina sequencing techniques to characterize the diversity and community structure of the eukaryotic phycobiome in selected epiphytic macrolichens showing different levels of thallus damage. Phylogenetic analyses revealed a high microalgal diversity, largely dominated by a few Trebouxia species, which are the most prevalent lichenized microalgae, accompanied by several low-abundance co-occurring genera. Notably, microalgal diversity peaked at intermediate levels of thallus damage. This pattern is consistent with disturbance-mediated modulation of microalgal community evenness rather than a categorical shift in symbiotic composition. These findings reveal previously unrecognized variability within the lichen phycobiota, providing new insights into the ecological dynamics and stress responses of these communities. In conclusion, our work offers a new perspective on the potential of lichens as sensitive bioindicators of air quality and ecosystem health.