Shredder identity matters: taxon-specific gut microbiota in aquatic shredders drives bacterial diversity during leaf...

Microbial communities play a key role in organic matter (OM) decomposition and nutrient cycling in headwater streams. Macroinvertebrate shredders enhance these processes by producing large amounts of fine particles from microbe-colonized OM through shredding and egesta, yet how gut and faecal microbiota vary among shredder taxa remains largely unknown. Thus, we examined composition, diversity, and predicted functions of bacterial communities in leaves, guts, and faecal pellets of three Trichoptera shredders (Allogamus, Potamophylax, and Sericostoma) across two larval stages using 16S rRNA metabarcoding. Bacterial community composition differed significantly among the shredder taxa and between sample types, while ontogeny had only a minimal effect. Sericostoma showed the lowest diversity but the most distinct microbiota in their gut and faecal pellets, while Allogamus and Potamophylax hosted more diverse and overlapping bacterial communities. Carnobacterium and Tyzzerella were specialized bacteria in the shredder guts. Functional predictions showed clear metabolic transitions from aromatic compound degradation in leaves to amino acid and carbohydrate metabolism in guts and continued degradation of plant-derived substrates in faecal pellets, reflecting the shift from environmental to host-associated microbial processes during gut passage. Unique pathways in Sericostoma guts, such as palmitate and peptidoglycan biosynthesis, suggest host-specific adaptations. Overall, shredder identity rather than ontogenetic stage was the dominant factor structuring bacterial assemblages and functions, emphasizing the taxon-specific role of shredders as mediators of microbial dynamics, linking microbial structure with ecosystem-level processes in the stream detritus-food-web.