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

Abstract

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 (50μM 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 (450μM NO3−+150μM NH4+; 250μM NO3−+50μM NH4+) and the mean N concentrations occurring at the estuarine environment of this population (30μM NO3−+5μM NH4+). The uptake kinetics of NH4+ and NO3− were determined during a 4h time-course experiment with N deprived algae. The experiment was continued up to 48h, with media exchanges every 4h. The uptake rates and efficiency of the two N sources were calculated for each time interval. For the first 4h, 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 (V0–4h), respectively, increased and decreased with increasing substrate concentration. NO3− uptake rates were superior, ranging from 1.06±0.1 to 9.65±1.2μMg(dw)−1h−1, with efficiencies of 19% to 53%. NH4+ uptake rates were lower (0.32±0.0 to 5.75±0.08μMg(dw)−1h−1) but G. vermiculophylla removed 63% of the initial 150μM 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.