Seasonal ammonium uptake kinetics of four brown macroalgae: implications for use in integrated multi-trophic aquaculture

The combined culture of fed species (bivalves, fish) and macroalgae, known as integrated multi-trophic aquaculture (IMTA), has been suggested as a method of mitigating localised nitrogen (N) increase from aquaculture, whilst simultaneously culturing macroalgae for commercial applications. The development of IMTA requires an understanding of the N ecophysiology of candidate macroalga species. We examined seasonal variations in ammonium (NH₄⁺) uptake kinetics, carbon to nitrogen (C:N) ratio, pigment content and soluble tissue N of four macroalgae of the phylum Ochrophyta, Ecklonia radiata, Macrocystis pyrifera, Lessonia corrugata, and Phyllospora comosa, from Tasmania, Australia. This study aimed to determine, (1) if the N physiology of the four macroalgal species was suitable for IMTA applications and (2) whether the species had seasonal variations in N ecophysiology which would influence their suitability for IMTA. Macrocystis pyrifera, L. corrugata, and E. radiata exhibited saturable NH₄⁺ uptake kinetics, with a maximum uptake rate (V_max) during spring, summer and autumn of 200, 45.8 and 45 μmol gDW^-1 h^-1 and half-saturation constants (K_s) of 361.3, 104.2 and 121 μM, respectively. Phyllospora comosa exhibited biphasic uptake patterns for three out of four months sampled. There were no noticeable seasonal patterns in pigment content or soluble tissue N for any species. C:N ratios increased from spring (October) to autumn (March) in both E. radiata (28.34 – 47.83) and P. comosa (24.99 – 51.62), indicating progressive N limitation though summer and into autumn. Results suggest that M. pyrifera and P. comosa are most suitable for IMTA due to their high NH₄⁺ uptake potential.