Nitrate Reductase Activity in Green Macroalgae as an Environmental Indicator of Temperature and Salinity Changes and Its Implication for Climate Change Projections

Temperature and salinity changes can affect nutrient assimilation dynamics in primary producers. Green macroalgae use nitrate as a main source of nitrogen for their metabolism. Nitrate needs to be reduced by nitrate reductase, before amino acids synthesis. Our aims were to study the effect of temperature and salinity changes on nitrate reductase activity (NRA) in Ulva rigida and Enteromorpha clathrata, and to assess if this enzyme can be used as an environmental indicator for changes in such abiotic factors. The study of NRA was carried out using potassium nitrate as substrate and propanol as a membrane permeabilizer, letting the produced nitrite to exit the macroalgae cells into the assay medium, allowing its quantification through a colorimetric method. This procedure was carried out at five temperatures (10, 20, 30, 35 and 40 °C) and three salinities of the assay medium (0, 15 and 36 g/kg). Results show that both Ulva and Enteromorpha have maximum NRA at salinities of 36 g/kg, although nitrate reduction can occur in freshwater or brackish water, but at significantly lower rates. NRA varied significantly with temperature for both macroalgae, although Ulva showed maximum NRA at 30 °C, while NRA peaked at 35 °C in Enteromorpha. Likely adequate models were tentatively fitted to NRA-temperature data at different salinities. NRA is a suitable proxy of the effects of temperature and salinity changes on the ability of green macroalgae to uptake and metabolize nitrogen nutrients and can thus be the base for macroalgae proliferation models under climate change model scenarios.