Macroalgal growth in temperate coastal ecosystems is primarily regulated by light and inorganic nitrogen availability. The effect of light (photon irradiance) on NO₃ ⁻ and NH₄ ⁺ uptake, and NO₃ ⁻ assimilation, were studied in the red macroalga, Hemineura frondosa, which does not operate a carbon concentrating mechanism (non-CCM). Non-CCM macroalgae grow in low and high light environments but become increasingly dominant with depth, suggesting a mechanism for ‘preserving energy’ under low light levels. H. frondosa was acclimated to limiting (30 µmol photons m^{− 2} s^{− 1}) and saturating (150 µmol photons m^{− 2} s^–1) irradiances for 8 days. Then, NO₃ ⁻ and NH₄ ⁺ uptake rates were measured under limiting and saturating irradiances at six concentrations ranging from 2–64 μM. NO₃ ⁻ uptake did not follow saturating uptake kinetics at both irradiances suggesting multiple uptake mechanisms. NH₄ ⁺ uptake saturated at concentrations <32 µM under limiting but not under saturating irradiance. Saturating irradiance resulted in greater maximum uptake rates of both NO₃ ⁻ and NH₄ ⁺. There was no evidence that irradiance regulated NO₃ ⁻ reduction by nitrate reductase. Also illustrated is the importance of measuring nitrate reductase activity on fresh material, as freezing in liquid nitrogen and storage at −80°C for 7 days caused a 65% decline in activity. Photosynthetic pigments, soluble tissue nitrogen and % total tissue nitrogen were all higher in limiting irradiance. In this first study of the nitrogen physiology of a non-CCM seaweed, we show that light regulates NO₃ ⁻ and NH₄ ⁺ uptake but not NO₃ ⁻ assimilation.