Protist size-dependent shifts of bacterial communities can reduce litter decomposition

Microbial-mediated litter decomposition drives carbon and nutrient cycling. This process can be top-down regulated by microbiome predators, particularly the diverse protists. Size has been suggested to determine predation impacts, but how protists of different size categories affect microbial-mediated litter decomposition remains unknown. Using a litter decomposition experiment with three protist size categories, we investigated protist size-dependent effects on microbial-driven litter decomposition. We found that protists of the large-size category created more structurally similar bacterial communities compared to the no-protist control. These protists of the large size category also reduced litter mass loss by 40%, while increasing microbial respiration by 17% compared to the no-protist control after five weeks of decomposition. In contrast, protists of the small-size category and protists of the medium-size category had no measurable impact on bacterial communities, litter mass loss, or microbial respiration. Random forest analysis identified Streptomyces as a major contributor to litter mass loss (explained 8% of litter mass), while the potential protist symbionts Taonella and Reyranella explained 8% and 6% of microbial respiration, respectively. These likely predation-resistant bacterial taxa were primarily enriched by protists of the large-size category. Our results indicate that protists, especially large ones, can alter litter decomposition by shaping microbiome composition. Future studies on litter decomposition and carbon cycling should incorporate protists and their traits, particularly size, to enhance our understanding of global carbon and nutrient cycling.