Light regulates inorganic nitrogen uptake and storage, but not nitrate assimilation, by the red macroalga Hemineura frondosa (Rhodophyta)

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.