Although the marine N 2 -fixers Trichodesmium spp. are affected by increasing pCO 2 and by ultraviolet radiation (UVR) in their habitats, little is known on their potential responses to future ocean acidification in the presence of UVR. We grew Trichodesmium at two pCO 2 levels (410 and 1000 μatm) under natural sunlight, documented the filament length, growth, and chlorophyll content after its acclimation to the pCO 2 treatments, and measured its carbon and N 2 fixation rates under different solar radiation treatments with or without UVR. We showed that the elevated pCO 2 did not significantly alter the diazotroph's growth, filament length, or pigment content, and its photosynthetic rate was only affected by solar radiation treatments rather than the pCO 2 levels. The presence of UV-A and UV-B inhibited photosynthesis by 10-22% and 17-21%, respectively. Inhibition of N 2 fixation by UV-B was proportional to its intensity, whereas UV-A stimulated N 2 fixation at low, but inhibited it at high, intensities. Elevated pCO 2 only stimulated N 2 fixation under moderate levels of solar radiation. The simulated depth profile of N 2 fixation in the water column showed that UV-induced inhibition dominated the combined effects of elevated pCO 2 and UVR at 0-30 m depth and the combination of these factors enhanced N 2 fixation at 30-60 m depth, but this effect diminished in deeper water. Our results suggest that Trichodesmium could be influenced more by UVR than by pCO 2 and their combined action result in negative effects on N 2 fixation under high solar radiation, but positive effects under low to moderate solar radiation.