Preference and uptake rates of nitrogen forms by the red seaweed Asparagopsis armata

The global aquaculture industry continues to grow rapidly, and it is important to improve sustainability through effective treatment of its waste products. Seaweeds have been used in Integrated Multi-Trophic Aquaculture (IMTA) systems as an effective tool to reduce the nutrient burden of the fish farm effluents. Species of the genus Asparagopsis armata are successful nitrogen biofilters of flow-through fish aquaculture systems because of their high capacity to take up ammonium, the main metabolic waste product of fish. However, the potential of A. armata as a biofilter of recirculating aquaculture systems (RAS), where effluents have higher concentration of nitrate than ammonium, is unknown. In this study, a series of experiments were designed to evaluate the preference and the uptake rates of nitrogen in the forms of ammonium, nitrate and amino acids, as well as the effect of the different nitrogen forms on the growth rates of A. armata. Nitrogen uptake rates were quantified based on the incorporation of 15N-labeled ammonium, nitrate and amino acids in the tissues, during the internally controlled phase, when uptake rates are stabilized and relatively constant. Our results revealed that inorganic and organic nitrogen sources can be simultaneously taken up by A. armata. The species exhibited a clear preference of ammonium over the two other forms of nitrogen (nitrate and amino acids), either when supplied separately or in combination. Notably, in the presence of multiple sources, amino acids were taken up at faster rates than nitrate by A. armata. The species showed a limited capacity to use nitrate when other nitrogen forms were present in the medium. The availability of the different nitrogen forms in the growth medium did not influence the growth rate of A. armata, either when these were supplied alone or in combination, and growth rates did not show an increasing pattern with nutrient concentration. This study confirms that A. armata is a suitable seaweed to be used as a biofilter in IMTA systems.