The caddisfly gut microbiome contributes to the consumption of riparian leaf litter within rivers.

Ecosystem functions can be regulated by biodiversity ranging from broad to narrow scales, including variation within a species. For most multicellular life, intraspecific variation is determined not only by an organism's phenotype and genotype, but also variation imparted by their microbiome. Here, we investigate how leaf decomposition in rivers is affected by phenotypic variation in plant secondary metabolites (PSMs) and antibiotic-induced variation within the gut microbiomes of aquatic macroinvertebrate decomposers. We found that consumption by Dicosmoecus caddisflies was inhibited by the presence of dietary PSMs from riparian Alnus rubra trees, irrespective of the state of their gut microbiome. We further compared consumption of diets prepared from 20 different local and non-local A. rubra trees. We found that antibiotic-treated and control caddisflies consumed similar amounts of diets containing local A. rubra PSMs. However, compared to caddisflies with an intact gut microbiome, antibiotic- treated caddisflies consumed less of non-local A. rubra diets. Thus, decomposition by caddisflies could be regulated by both host adaptation to local resources and bacterial-mediated digestion of non-local resources. Overall, our study suggests that gut microbiomes can facilitate adjustment of macroinvertebrate decomposers to novel plant phenotypes, which may arise from shifting spatial distributions of plants in response to global change.