Nitrogen excretion by the lobsters Panulirus ornatus and Thenus australiensis and uptake by the brown algae Sargassum siliquosum: Implications for integrated recirculated aquaculture systems

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 recirculating aquaculture systems (RAS), high concentrations of nitrogenous waste can become toxic to the fed species and macroalgae offers a method of mitigating N waste. To utilise macroalgae as a biofilter for N waste, an understanding of their N ecophysiology of both the fed and extractive species is needed. In this study we first determine the ammonium and urea excretion rates for emerging aquaculture candidates the tropical rock lobsters, Panulirus ornatus (TRL) and slipper lobster, Thenus australiensis (SL). Using multiple flask uptake experiments, we then determined the N ecophysiology of the tropical brown macroalga, Sargassum siliquosum, to assess its potential use as a biofilter in a lobster RAS. Routine (basal) N excretion was predominantly ammonium with only a minor contribution of urea (8 and 13% for TRL and SL respectively). For both lobster species, ammonium and urea excretion substantially increased post-prandial, peaking at levels 3.6 times greater than basal amounts for TRL and 3.2 times greater for SL, and remained elevated for between 32 and 38 h. Uptake of both ammonium and nitrate by S. siliquosum showed saturating kinetics, where the kinetic parameter V_max is the maximum rate of uptake, K_s is the half-saturation constant and the V_max:K_s-ratio is a proxy for the algae's affinity of uptake for that respective nutrient: for ammonium V_max = 110.9 μmoles g⁻¹ dry weight (DW) h⁻¹ and K_s = 37.9 μM and for nitrate V_max = 152.7 μmoles g⁻¹ DW h⁻¹ and K_s = 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, V_max = 55.5 μmoles g⁻¹ DW h⁻¹ and K_s = 17.65 μM. We used uptake and excretion rates from our study to derive a unit mass stocking ratio of algae to lobster (algae: animal) to offset N pollution in RAS. The ratios of TRL to algae to offset NH₄⁺ and urea post-prandial were 5.15E-04 and 3.09E-04 respectively and for SL, 4.46E-04 and 6.31E-04 respectively. The findings serve as a reference for future studies on integrating Sargassum species into RAS and IMTA systems.