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 Portuguesepopulation ofGracilaria vermiculophyllawere studied. Thalli were incubated at 5 nitrogen (N) levels, includingsingle (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 estuarineenvironment of this population (30μM NO3−+ 5μM NH4+). The uptake kinetics of NH4+and NO3−weredetermined during a 4 h time-course experiment with N deprived algae. The experiment was continued up to48 h, with media exchanges every 4 h. The uptake rates and efficiency of the two N sources were calculated foreach time interval. For thefirst 4 h,G.vermiculophyllaexhibited non-saturated uptake for both N sources evenfor 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 weresuperior, ranging from 1.06± 0.1 to 9.65± 1.2μM g(dw)−1h−1, with efficiencies of 19% to 53%. NH4+uptakerates were lower (0.32± 0.0 to 5.75± 0.08μM g(dw)−1h−1) butG. vermiculophyllaremoved 63% of theinitial 150μM and 100% at all other conditions. Uptake performance of both N sources decreased throughoutthe duration of the experiment and with N tissue accumulation. Both N sources were taken up during darkperiods though with better results for NH4+.Gracilaria vermiculophyllawas unable to take up NO3−at thehighest concentration but compensated with a constant 27% NH4+uptake through light and dark periods. Ntissue accumulation was maximal at the highest N concentration (3.9± 0.25% dw) and superior under NH4+(3.57± 0.2% dw)vsNO3 (3.06± 0.1% dw) enrichment. The successful proliferation ofG. vermiculophyllainestuarine environments and its potential utilization as the biofilter component of Integrated Multi-TrophicAquaculture (IMTA) are discussed.