Ultraviolet radiation lowers upper thermal limits in marine diatoms

Escalating ultraviolet radiation (UVR) and ocean warming are expected to interactively shape phytoplankton performance, yet their combined effects on diatom thermal plasticity remain poorly resolved. We quantified how UVR alters growth‐based thermal performance curves in three model marine diatoms grown across species‐specific temperature gradients (5–28°C) under UV‐excluded (PAR) and UV‐present (PAB) solar radiation. UVR generally suppressed growth most strongly at thermal extremes and consistently lowered the critical thermal maximum ( CT max ) in all three species, indicating reduced tolerance to high temperature. UVR also narrowed thermal breadth in two species and reduced thermal tolerance in two species, whereas effects on maximum growth rate ( r max ), optimum temperature ( T opt ), and thermal safety margin (TSM) were species specific. These results indicate that UVR modifies growth‐based thermal performance rather than acting solely as an additive stressor, bringing populations closer to their upper thermal limits under the tested conditions. Incorporating UVR‐sensitive thermal traits into predictive frameworks may help constrain projections of phytoplankton productivity under warming and changing light climates.