High-light correlates with adaptive dominance of comammox Nitrospira clade B in alpine rivers

Solar radiation is a key abiotic factor in sunlit fluvial ecosystems, yet its role in the lineage distribution and adaptive evolution of complete ammonia-oxidizing (comammox) Nitrospira remains poorly resolved. Here we show that total solar radiation is a major environmental correlate of comammox Nitrospira clade B (COMB) distribution across four major alpine rivers (Yangtze, Yellow, Yarlung Tsangpo, and Lancang) originating from the Qinghai-Tibet Plateau. COMB dominated the comammox guild in plateau reaches, accounting for 70.8% of planktonic and 69.3% of benthic communities respectively, with its relative abundance positively correlated with solar irradiance along the elevational gradient. Population genomics revealed accelerated evolutionary turnover and candidate positive selection on stress-resistance genes, consistent with long-term pressure from high solar irradiance. Genome‑scale metabolic modeling further showed that network coupling between peroxide‑scavenging pathways and core energy metabolism sustains in silico viability under high H₂O₂ load, with NADPH expenditure reflecting metabolic trade‑offs. Controlled light microcosms confirmed stoichiometrically complete nitrification, stable COMB DNA dominance (70.7-76.3%), and wavelength-specific transcriptional reweighting. Together, these findings identify alpine rivers as a candidate global COMB hotspot and position total irradiance as a key environmental correlate of COMB assembly along elevational gradients.