Nitrogen source and availability regulate plastic population dynamics in the marine diatom Thalassiosira weissflogii

Variation in nitrogen (N) availability significantly influences population dynamics and the productivity of marine phytoplankton. As N availability in the ocean is conditioned by the N source, it is important to understand the capacity of phytoplankton organisms to adjust their physiology and dynamics under different N conditions. We investigated the growth dynamics of Thalassiosira weissflogii, a coastal diatom, in response to different N sources (Nitrate, NO₃⁻; Ammonium, NH₄⁺; urea, CH₄N₂O) and availabilities (45 and 5 μM). Our findings demonstrate that T. weissflogii can display plastic adjustments in population dynamics to different N sources. These responses evidenced a greater preference for NH₄⁺ and urea than NO₃⁻, particularly under high N availability. The relative growth rate (μ) is higher (1.18 ± 0.01) under NH₄⁺-high treatment compared to NO₃⁻-high (1.01 ± 0.01). The carrying capacity (K) varied only among concentrations, indicating equal N utilization efficiency for biomass production. No effects of N source were detected under the low concentration, suggesting that the preference for NH₄⁺ and urea was diminished by limited nitrogen supply due to potential interactions. These results provide valuable insights into the physiological flexibility of T. weissflogii to varying N conditions, shedding light on the ecological success and resilience of this species in highly variable coastal environments.