Nitrogen uptake responses of Gracilaria vermiculophylla (Ohmi) Papenfuss under combined and single addition of nitrate and ammonium

The ammonium (NH4+) and nitrate (NO3−) uptake responses of tetrasporophyte cultures from a Portuguese population of Gracilaria vermiculophylla were studied. Thalli were incubated at 5 nitrogen (N) levels, including single (50 μM of NH4+ or NO3−) and combined addition of each of the N sources. For the combined additions, the experimental conditions attempted to simulate 2 environments with high N availability (450 μM NO3− + 150 μM NH4+; 250 μM NO3− + 50 μM NH4+) and the mean N concentrations occurring at the estuarine environment of this population (30 μM NO3− + 5 μM NH4+). The uptake kinetics of NH4+ and NO3− were determined during a 4 h time-course experiment with N deprived algae. The experiment was continued up to 48 h, with media exchanges every 4 h. The uptake rates and efficiency of the two N sources were calculated for each time interval. For the first 4 h, G. vermiculophylla exhibited non-saturated uptake for both N sources even for the highest concentrations used. The uptake rates and efficiency calculated for that period (V0-4 h), respectively, increased and decreased with increasing substrate concentration. NO3− uptake rates were superior, ranging from 1.06 ± 0.1 to 9.65 ± 1.2 μM g(dw)−1 h−1, with efficiencies of 19% to 53%. NH4+ uptake rates were lower (0.32 ± 0.0 to 5.75 ± 0.08 μM g(dw)−1 h−1) but G. vermiculophylla removed 63% of the initial 150 μM and 100% at all other conditions. Uptake performance of both N sources decreased throughout the duration of the experiment and with N tissue accumulation. Both N sources were taken up during dark periods though with better results for NH4+. Gracilaria vermiculophylla was unable to take up NO3− at the highest concentration but compensated with a constant 27% NH4+ uptake through light and dark periods. N tissue accumulation was maximal at the highest N concentration (3.9 ± 0.25% dw) and superior under NH4+ (3.57 ± 0.2% dw) vs NO3 (3.06 ± 0.1% dw) enrichment. The successful proliferation of G. vermiculophylla in estuarine environments and its potential utilization as the biofilter component of Integrated Multi-Trophic Aquaculture (IMTA) are discussed.

► G. vermiculophylla has linear uptake of NH4+ and NO3- in first hours of exposure. ► G. vermiculophylla assimilates nitrogen at concentrations as high as 600 μM. ► High substrate concentration means low N uptake rate and high uptake efficiency. ► Uptake of NH4+ and NO3- decreased with time and N tissue accumulation. ► N uptake occurs in the dark, especially of NH4+. ► N tissue accumulation was maximal with higher N concentration and NH4+ enrichment.