Diel variations in nutrient physiology among commercially important eucheumatoids Kappaphycus alvarezii, K. striatus and Eucheuma denticulatum.

Main conclusion: Eucheumatoids exhibited significant day and night nutrient uptake, with NH₄⁺ increasing linearly, NO₃^- following Michaelis-Menten kinetics, higher internal NH₄⁺, NO₂^-, PO₄^-3 at night, and peak nitrate reductase activity during day. Nutrient physiology of aquatic phototrophs is mostly measured during daytime or experimentally under saturating irradiance with the assumption that nutrient uptake is tightly coupled with photosynthetic activities. At night or under dark condition, when photosynthesis ceases, little is known on the nutrient physiology of these macrophytes. Here, we examined the nutrient uptake of commercially important eucheumatoids Kappaphycus alvarezii, K. striatus, and Eucheuma denticulatum during night and daytime, considering conditions that extend beyond just the presence or absence of light. The NH₄⁺ and NO₃^- uptake of the three eucheumatoid species were higher during the day (53.4-58.2% of the total TIN absorbed), although nighttime uptake (41.8-46.6% of the total TIN absorbed) also contributed significantly to the overall uptake rates. For both night and daytime, the NH₄⁺ uptake of the three eucheumatoid species increased linearly with the substrate concentration while NO₃^- uptake followed the Michaelis-Menten saturation kinetics. Higher V_max were observed for K. striatus (5.670 ± 0.474 µmol h^-1 g^-1 DW) and E. denticulatum (8.058 ± 1.632 µmol h^-1 g^-1 DW) during nighttime, while nighttime K_m values of the three species (13.028-32.387 µM) were higher compared to their daytime values (6.397-9.238 µM). Regardless of night- or day-sampling time, the nutrient (NH₄⁺, NO₃^-, NO₂^- and PO₄^-3) concentration in the seaweeds' internal pools were higher than their concentrations in the surrounding seawater, highlighting their capacities to store inorganic nutrients. Higher amounts of NH₄⁺, NO₂^- and PO₄^-3 in the seaweeds' internal pools were observed during nighttime while NO₃^- in the internal pools were similar at both periods. Lastly, the three eucheumatoid species exhibited higher nitrate reductase activities (NRAs) during the day. Our study showed that algal nutrient uptake and nitrogen metabolism are independent of light and photosynthesis. Understanding the influence of night and daytime in nutrient uptake can provide insights in the sustainable nutrient management for commercial crop production and/or in incorporating eucheumatoids in an integrated multi-trophic aquaculture (IMTA) system. In terms of accounting for the bioremediation potentials of the above species, it is important to measure both nighttime and daytime nutrient uptakes.