Temporal shifts in the dominance of tolerant taxa reshape microbial functional potentials from substrate utilization to stress resistance in sediments contaminated with sulfonated methyl phenolic resin.

Sulfonated methyl phenolic resin (SMP), a widely used additive in water-based drilling fluids, accumulates in sediments due to its low degradability. Bioremediation is a promising strategy for SMP removal, but the microbial responses to SMP remain unclear. Here, we conducted microcosm experiments to compare microbial composition and functional potentials between early and late exposure stages across a SMP concentration gradient. The results showed that SMP exposure reduced microbial diversity and increased the relative dominance of putatively tolerant taxa. Microbial response asynchrony increased and was positively correlated with resistance potential. Life-history proxies indicated contrasting patterns between exposure stages, with lower 16S rRNA gene copy number and longer predicted minimum doubling time at the early stage and the opposite pattern at the late stage. Additionally, degradation- and resistance-related functional genes were enriched at the early and late stages, respectively. At the metagenome-assembled genome (MAG) level, abundant MAGs at the late exposure stage exhibited high resistance potential. Overall, our results showed that SMP could exert a double-edged sword effect on microorganisms, acting as both a stressor and a carbon source, thereby inducing stage-specific microbial functional transitions.