Phosphorus in Saccharina latissima: Initial uptake, storage, and possibilities for recycling in IMTA-systems

The potential future phosphorus scarcity is a challenge that can be directly linked to food security, and a range of solutions are needed in order to shift the anthropogenic phosphorus use into renewable and more sustainable forms. One suggestion is to exploit opportunities with recirculation of biological waste from aquaculture production by co-cultivation of kelp species like Saccharina latissima in integrated multi-trophic aquaculture (IMTA) systems. The aim of this study was to characterize the initial uptake and storage of phosphorus in S. latissima, and assess the recycling potential for phosphorus through the IMTA cultivation of this species.
   The initial uptake of dissolved inorganic phosphorus (DIP) in preconditioned S. latissima of different nutritional states, was characterized by increasing DIP availability. To explore the incorporation of phosphorus from salmon aquaculture, a cultivation study at Frøya was conducted by deploying seaweed along a gradient downstream from a salmon farm in a period from February to June.
   It was found that DIP initial uptake in S. latissima was strongly affected by the nutritional state of the kelp, with low DIP uptake levels for the phosphorus depleted group. The acclimatization of this group was found to be specifically harsh as the phosphorus depletion infuenced the physiological state of the kelp. The nutritionally saturated group expressed good levels of uptake, with a linear increase in uptake rate (y \(µg.g_{DW}^{-1}L^{-1}hour^{-1}\)) with increasing treatment concentrations (x \(µmolP.L^{-1}\)) expressed as y = 23.7x + 6.0. The maximum measured uptake rate was 52.4±6.7\(µg.g_{DW}^{-1}L^{-1}hour^{-1}\) for the 2.0 \(µmolP.L^{-1}\)treatment.
   The internal distribution in tissue content of phosphorus (P) and nitrogen (N) showed highest content near the meristematic tissue in growth phase individuals (April), indicating a prioritization of nutrients to facilitate growth. Biomass yields were significantly higher in the seaweed group integrated to the salmon farm at all sampling days, with a peak yield of 7.01±0.88 \(kg_{FW}.m^{-1}\) in June. This supported former studies showing increased biomass yield in close proximity to salmon farms.
   Biomass yield was promoted as the main component needed to estimate the magnitude of bioremediation and recirculation potential, as nutrient content was determined to be more applicable for evaluating nutritional state and possible limitations.
   From the results of this study, IMTA was recognized as a potential opportunity for indirect recirculation of phosphorus by re-introduction to a new value chain. However, in order to optimize both the bioremediatory and recycling potential for phosphorus through IMTA, organisms able to utilize particulate forms of phosphorus waste release should also be included.