Lichen biomonitoring revisited: a multi-level framework incorporating symbiotic response and microbiome dynamics as early-warning indicators.

Lichens are among the most widely used biological indicators of environmental quality because they integrate atmospheric pollutants and respond sensitively to environmental change. However, recent advances in lichen biology have expanded biomonitoring beyond traditional measures of pollutant accumulation, physiological injury, and community composition, creating the need for an updated synthesis. This review integrates current evidence on the biological basis, response mechanisms, environmental applications, and emerging frontiers of lichen biomonitoring. Classical approaches have been successfully applied to monitor air pollution, heavy metals, nitrogen deposition, and other environmental stressors, while recent studies demonstrate that symbiotic interactions and lichen-associated microbiomes provide additional, potentially earlier indicators of ecosystem disturbance. Building on this evidence, we propose a multi-level framework integrating six complementary biological domains: pollutant accumulation, physiological responses, community dynamics, functional traits, symbiotic interactions, and microbiome dynamics. This framework links established and emerging response pathways into a unified biomonitoring concept while acknowledging remaining challenges, including environmental variability, species-specific responses, methodological standardization, and geographic bias. Integrating conventional and emerging biological endpoints provides a more comprehensive and mechanistic foundation for lichen biomonitoring in a rapidly changing environment.