The potential of the red alga Gracilaria bursa-pastoris as a potential biofilter component was tested in an integrated multi-trophic aquaculture systems (IMTA). Nutrient uptake characteristics of the species in media with inorganic nitrogen showed inhibition of uptake of nitrate by ammonium based on significant differences between their mean uptake rates (19.26 μmol g dry wt^-1 h^-1 for nitrate and 66.11 μmol g dry wt^-1 h^-1 for ammonium) and oscillations in their substrate concentrations within the 48-h incubation period. With the red sea bream (Pagrus major) effluent water, G. bursa-pastoris reduced 35.63 ± 11.10 %, 29.14 ± 19.65 % and 55.67 ± 13.80 % for nitrate, phosphate and ammonium, respectively. Mean uptake rate for ammonium (758.35 ± 378.65 μmol kg wet wt^-1 h^-1) in the effluent water was higher compared to nitrate and phosphate and supported high growth rates (11.70 % d^-1) and N:P ratio (19.64). Dark uptake of nutrients and suppression of ammonium over nitrate was observed in both inorganic N media and fish effluents.
This study shows that G. bursa-pastoris is an efficient biofilter based on its affinity for ammonium, capacity for dark uptake and high growth rate in fish effluents. The biomass obtained can provide additional revenue through its use as human food, source of agar and as feed for abalone.