Nitrogenous substrate preference in eucheumatoids: Varying ammonium and nitrate ratios affect the nutrient uptake and...

The availability and proportion of ammonium (NH₄⁺) and nitrate (NO₃^-) in the marine environment fluctuate due to various processes such as upwelling, regeneration, pollution, nitrogen fixation, among others, significantly impacting the physiology and biochemistry of seaweeds. Hence, this study aims to examine the effects of varying NH₄⁺: NO₃^- ratio on the growth, nutrient uptake, and biochemical composition (carbohydrate, protein, polyphenol, and pigment content) of commercially important eucheumatoid-crops Kappaphycus alvarezii, Kappaphycus striatus, and Eucheuma denticulatum. Apical portions of the seaweed samples were cultured under controlled laboratory conditions for 15 d in seawater media with a constant total nitrogen concentration of 40 μM but differing NH₄⁺:NO₃^- ratios (40:0, 30:10, 20:20, 10:30, 0:40). Growth differed among species, with K. alvarezii showing highest growth rate when cultured in media containing NH₄⁺ as the sole source of inorganic nitrogen and K. striatus displaying lowest growth in media with NO₃^- as the only inorganic nitrogen source. E. denticulatum had the highest range of growth rate, but showed no sensitivity to alterations in nitrogen species proportion. Generally, the NH₄⁺ uptake rates of eucheumatoids decreased as the proportion of NH₄⁺ in the treatment decreased, while their NO₃^- uptake rates increased as the concentration of NO₃^- became more dominant. Notably, eucheumatoids absorbed more total inorganic nitrogen when both nitrogen species were available than when only one was present. Nitrate reductase activities (NRA) were similar regardless of the NH₄⁺:NO₃^- ratio except for E. denticulatum which had the highest NRA when only NO₃^- was present in the media. Despite differences in growth, nutrient uptake and NRA, the biochemical composition of the three eucheumatoid species remained consistent across nitrogen ratios, indicating that inorganic nitrogen proportions had minimal impacts on resource allocation to biochemical constituents. Together, our results suggest that while physiological processes such as nutrient acquisition and enzymatic activity were sensitive to inorganic nitrogen proportions, these changes did not translate into significant alterations in the allocation of resources to biochemical constituents under the experimental conditions. Understanding these dynamics is crucial for the sustainable cultivation of these commercially important seaweeds under varying environmental conditions.