Functional hierarchy and redundancy organize phosphorus cycling potential in <i>Peltigera</i> lichen microbiomes

Lichen symbioses host complex microbial communities whose functional organization remains poorly understood. In Peltigera lichens, bacterial partners mediate key nutrient transformations, but it is unclear whether the spatial distribution of phosphorus-cycling functions follows the hierarchical control previously observed for community composition. We hypothesized that Peltigera microbiomes follow a thallus-to-soil gradient of control, in which host-driven specialization within thalli transitions toward environmentally driven reconfiguration in the substrate and soil. To test this, we quantified five bacterial genes involved in phosphorus turnover (gcd, phoD, phoN, phnX, and appA) across thalli, underlying substrates, and adjacent soils of several Peltigera species collected along contrasting Patagonian bioclimates. Absolute and relative gene abundances, together with diversity and variance partitioning analyses, were used to evaluate the influence of host identity, edaphic properties, and climate. Gene profiles revealed a shift from host-associated specialization to environmentally filtered assemblages, indicating that symbiotic and abiotic factors jointly structure phosphorus-cycling potential. The coexistence of functional specialization and redundancy provides a plausible mechanism for sustaining phosphorus turnover under changing environmental conditions.