Increased UV radiation may reshape phytoplankton community structure by facilitating bloom formation of the invasive cyanobacterium Raphidiopsis raciborskii

Raphidiopsis raciborskii is an invasive cyanobacterium that has rapidly expanded to higher latitudes and elevations in recent decades, replacing traditional dominant species such as Microcystis aeruginosa in many freshwater ecosystems. Unlike other cyanobacterial blooms, its deep yellow and diffusely distributed blooms make lake governance considerably more difficult. While this expansion is often attributed to climate warming, rising temperatures alone cannot fully explain the sustained competitive advantage of R. raciborskii in high-altitude and high-latitude regions. Increasing evidence suggests that UV radiation acts as an important environmental stressor shaping algal community dynamics and facilitating its successful invasion. In this study, we combined long-term field monitoring from two high-altitude plateau lakes in China (Lakes Yilong and Dianchi) with laboratory monoculture and co-culture experiments simulating progressively increasing UV radiation under natural conditions. Field surveys showed that R. raciborskii dominated cyanobacterial biomass in both lakes. Machine-learning models exhibited high predictive performance, and SHAP analyses consistently identified UV radiation as the most important model predictor for R. raciborskii relative biomass. We note that field UV is strongly temporally co-varying with water temperature and total solar radiation; causal evidence for UV effects comes from constant-temperature laboratory experiments. Model-derived reference threshold analyses indicated that its dominance risk increased markedly when monthly total UV radiation exceeded 2.0 MJ m −2 . Laboratory experiments revealed that enhanced UV radiation significantly suppressed the growth and photosynthetic efficiency of M. aeruginosa , whereas R. raciborskii maintained stable growth and gradually became dominant in mixed cultures. This competitive advantage may be mediated by multiple mechanisms. Under early-stage, low UV conditions, R. raciborskii showed early extracellular release of CYN together with increased accumulation of photoprotective pigments (e.g., β-carotene, zeaxanthin, lutein), which we propose as potential mechanisms enhancing antioxidant defenses, though causal evidence for CYN function remains to be verified. Under later-stage, high UV conditions, its dominance was primarily maintained by sustained high pigment levels, which preserved photosynthetic activity and antioxidant capacity. Overall, our results demonstrate that UV radiation acts as a key selective pressure reshaping cyanobacterial community structure and maintaining the competitive advantage of R. raciborskii in high-UV freshwater ecosystems.