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Marine macroalgae play a key role in the coastal carbon cycle, contributing to the coastal primary productivity and playing key roles in coastal fisheries. Gracilaria lemaneiformis (Bory) has been commercially farmed for food and agar production and is playing a significant role in the remediation of the eutrophicated waters along the Chinese coast. Solar ultraviolet radiation (UVR, 280–400 nm) is known to inhibit the photosynthesis of macroalgae, whereas nitrogen availability may alter the sensitivity of the algae to UVR. Here, we cultured thalli under solar radiation with or without UVR in NH₄⁺ enriched or ambient seawater for 16 days, and measured photosynthesis, pigments and nitrogen uptake, in order to investigate combined effects of NH₄⁺ enrichment and UV radiation on Gracilaria lemaneiformis. The results showed that UV-B (280-315 nm) significantly reduced the net photosynthetic rate of this alga. This inhibition was alleviated by enrichment with ammonia, which also caused a decrease in dark respiration. The presence of both UV-A (315-400 nm) and UV-B stimulated the accumulation of UV-absorbing compounds. However, this stimulation was not affected by enrichment with ammonia. The content of phycoerythrin (PE) was increased by the enrichment of ammonia only in the absence of UVR. Ammonia uptake and the activity of nitrate reductase were repressed by UVR. However, exposure to UVR had an insignificant effect on the rate of nitrate uptake. In conclusion, increased PE content associated with ammonia enrichment played a protective role against UVR in this alga, and UVR differentially affected the uptake of nitrate and ammonia. In coastal waters, ammonia often shows pulsed high levels due to temporary inputs from runoff or intensive aquaculture. The enrichment of ammonia can increase the content of PE which plays a protective role in addition to energy capture as a key antenna pigment, with the result that it relieves the photosynthetic damage caused by solar UVR.