Microplastic structure correlates with Anna Karenina destabilization in soil fungi via dispersal limitation.

Microplastic pollution can destabilize soil fungal communities, but how the intrinsic chemistry of different polymers governs this process remains unclear. Here, by combining microcosm experiments with null-model-based assembly analysis (iCAMP), we show that the Anna Karenina Principle (AKP)-a pattern of stress-induced community disintegration-extends from host-associated microbiomes to microplastic-polluted soils. Polystyrene (PS), which carries a π-conjugated aromatic backbone, induced a markedly stronger AKP in fungal communities than did linear, non-aromatic polyethylene (PE). This difference was associated with a shift from homogeneous selection (HoS) toward dispersal limitation (DL): the inferred contribution of DL reached 47.0% under PS versus only 21.9% under PE. Exogenous indole, a common microbial signaling molecule, alleviated the PS-induced AKP with high efficiency-an effect we attribute to specific π-π interactions between the aromatic polymer and indole. Adsorption/desorption kinetics revealed that PS accumulates 44% more indole than PE, and first-principles Density Functional Theory (DFT) calculations further demonstrated that this enhanced affinity originates from strong electrostatic complementarity and orbital hybridization at the PS-indole interface (non-dispersion energy three times that of PE), rather than from universal van der Waals forces. This specific interaction creates an "interfacial locking" effect that translates a transient chemical pulse into a persistent signaling reservoir on PS surfaces. Consequently, PS surfaces act as localized hotspots that concentrate the signal, thereby reducing DL and marginally enhancing the relative contribution of HoS. In contrast, indole had only weak effects in PE systems. Notably, a single pulse of indole produced divergent community states that remained detectable after eight weeks, far exceeding the expected half-life of the free molecule-suggesting that the PS surface may translate a transient chemical cue into a prolonged ecological trajectory shift, consistent with an interface-mediated legacy effect. Our findings identify polymer π-conjugation as a key determinant of fungal community destabilization and reveal that modulation by signaling molecules depends critically on the chemistry of the co-existing microplastic. This work provides a mechanistic framework linking pollutant interfacial properties to the ecological processes that govern soil fungal microbiome stability.