Bioavailability and recalcitrance of humic-bonded dissolved organic nitrogen for algae uptake: Insights into...

This study investigates the bioavailability of humic nitrogen (humic-N) to algae through controlled bioassay experiments. Algae were able to utilize dissolved organic nitrogen (DON) from both humic acid (HA) and fulvic acid (FA), with bacterial co-culture enhancing uptake. Bioavailable nitrogen (BAN) from HA accounted for ~20 % of total nitrogen, whereas FA reached ~45 %, with bacterial presence further increasing FA utilization by about 6-7 %. Metal ions (Cu²⁺, Fe³⁺) at elevated concentrations (0.25-2.50 mg/L) inhibited nitrogen uptake, with reductions in TDN degradation rate of up to ~40 % compared to controls. The Gamma model provided a better fit (R² = 0.85-0.99) than the 1-pool model, indicating rapid utilization of bioavailable humic-N in the early incubation stage. These findings highlight the limited yet measurable role of humic-N in supporting algal growth, the enhancing effect of bacteria, and the inhibitory influence of metals, providing insights for nutrient management in aquatic systems.