Unveiling the ecological processes driving soil and lichen microbiome assembly along an urbanization gradient

Global biodiversity loss is accelerating due to the transformation of natural landscapes into agricultural and urban areas. Yet, research on the urbanization impact on environmental and host-associated microbiomes, particularly on the ecological processes that mediate their assembly and function, remains scarce. This study investigated the effects of an urbanization gradient on the diversity and assembly processes of the soil microbiome and the microbiomes of three epiphytic lichen species (Candelaria concolor, Physcia adscendens, and Xanthoria parietina). Our findings revealed that the urbanization gradient shaped the soil microbiome, while the lichen microbiomes exhibited strong host specificity and showed no significant changes in diversity along the urbanization gradient. Heterogeneous selection and dispersal limitation primarily governed the soil community assembly and higher community turnover in medium- and highly urbanized zones compared to low-urbanized zones, indicating an increased influence of environmental pressures, altered resources, and habitat fragmentation in more urbanized areas. The lichen microbiome assembly in each species was primarily governed by undominated processes regardless of urbanization level, indicating that both selection and stochasticity contributed to, but neither dominantly influenced, their assembly. The lichen microbiomes further revealed species-specific co-occurrence networks, with microbial compositional signatures and potential functions being essential for lichen fitness and urban ecosystem health. Taken together, our study contributes to understanding how microbial communities are assembled in urban environments, bridging the gap between conceptual theories and empirical findings in the urban ecology of soil and lichen-associated microbiomes.