Vermicomposting supports richer and more complex microbial communities than passive composting across grape marc varieties

The wine industry generates unsustainable quantities of grape marc. Passive composting is commonly used to stabilize these byproducts, but vermicomposting is another alternative, yet direct comparisons of how these two processes shape grape marc microbiomes under the same operating conditions remain scarce. We compared the effects of passive composting and vermicomposting on grape marc bacteriomes and mycobiomes derived from Vitis vinifera cv. Albariño (raw) and Mencía (raw and distilled) using amplicon sequencing (16S rRNA and ITS). After 63 days of processing, Pseudomonadota and Bacteroidota dominated the bacteriomes across processes, while Basidiomycota and Ascomycota dominated the mycobiomes. Vermicomposting consistently altered community composition and enriched taxa associated with organic matter degradation and nutrient cycling. Vermicomposts also contained greater proportions of unique bacterial and fungal OTUs than passive composts across the three grape marc types, indicating extensive community turnover. Hundreds of bacterial and fungal genera and OTUs differed significantly between processing methods, reflecting taxonomic restructuring. Moreover, vermicomposting increased microbial richness by up to 76.6% and complexity (i.e., inverse Simpson) by up to 64.0%. Both bacteriomes and mycobiomes showed clear separation between passive composting and vermicomposting in multivariate ordinations, with community variation largely explained by processing method. Overall, our results suggest that vermicomposting is not simply an alternative stabilization process but a distinct biological transformation that generates more diverse and compositionally distinct microbiomes than passive composting. This supports the use of vermicomposting to valorize grape marc into biologically enriched organic amendments with potential benefits for soil health, vineyard productivity, and wine quality.