Grace E. Edwards1, Wouter Visch1, Quinn Fitzgibbon1, Gregory Smith1, Catriona L. Hurd1.
1Institute for Marine and Antarctic Studies, University of Tasmania, Hobart Tasmania 7001, Australia.
Farming extractive species such as macroalgae that take up nitrogen (N), with fed species (e.g., lobsters, finfish, prawns) is termed integrated multi-trophic aquaculture (IMTA). In onshore recirculating aquaculture systems (RAS), high concentrations of nitrogenous waste can become toxic to the cultivated species and macroalgae offer a method of mitigating nitrogen waste. To utilise macroalgae as a biofilter for nitrogen waste, an understanding of their nitrogen ecophysiology is needed. Using time-course and multiple flask uptake experiments, we determined the N ecophysiology of the tropical brown macroalga, Sargassum siliquosum, to assess its potential use as a biofilter in an onshore RAS with the tropical rock lobster, Panulirus ornatus. Uptake of both ammonium and nitrate showed saturating kinetics: for ammonium Vmax =110.9 µmoles g-1 DW h-1 and Ks = 37.9 µM, and for nitrate Vmax = 152.7 µmoles g-1 DW h-1 and Ks = 58.52 µM. Urea uptake was highly variable with both positive and negative (i.e. urea release from tissue) rates recorded; when positive values were considered, Vmax = 55.5 µmoles g-1 DW h-1 and Ks = 17.65 µM. We used uptake rates from our study and literature values of excretion rates for by barramundi, tropical rock lobsters, slipper lobsters, mudcrabs and tiger prawns, to determine the stocking density of S. siliquosum that would offset ammonium excretion in RAS systems. The findings serve as a reference for future studies on integrating Sargassum species into RAS and IMTA systems.