Synergistic Effects of Viruses and Environmental Gradients on Carbon Cycling in a River Ecosystem.

Riverine ecosystems represent critical nodes in the global carbon cycle, where the mechanistic role of viruses in modulating eukaryotic carbon cycling remains underexplored, particularly across heterogeneous landscapes. Here, we applied metatranscriptomics to dissect how multi-scale environmental factors and viral gene activity jointly regulate the spatial transcription of carbon cycling genes in riverine eukaryotic communities along the Yongding River, China. Our analyses reveal pronounced spatial heterogeneity in both viral gene expression-notably major capsid proteins of large eukaryotic DNA viruses-and carbon fixation, conversion, and metabolism pathways, peaking in agriculturally impacted plain regions. Multivariate statistics and network analyses demonstrate that land use enhances viral gene activity, serving as biological amplifiers that modulate host carbon metabolism and transformation. Structural equation modeling further identifies a cascade in which cropland coverage elevates viral gene expression, ultimately driving a 1.8-fold increase in TCA cycle gene transcription in plain regions, whereas nitrogen loading at the site scale suppresses viral activity and carbon fixation. Phylogenetic analysis corroborates that virus-host specificity underpins these spatial patterns. Collectively, these findings advance a new model in which viruses act as key intermediaries, transmitting multiscale environmental signals to shape riverine carbon cycling. Our study highlights the urgency of incorporating viral ecology into predictive frameworks of riverine biogeochemical cycling under accelerating environmental change.