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 mu M 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 mu M NO3- + 150 mu M NH4+; 250 mu M NO3- + 50 mu M NH4+) and the mean N concentrations occurring at the estuarine environment of this population (30 mu M NO3- + 5 mu M 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 mu M 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 mu M g(dw)(-1) h(-1)) but G. vermiculophylla removed 63% of the initial 150 mu M 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. (C) 2011 Elsevier B.V. All rights reserved.