High ammonium suppresses nitrate transporter expression and constrains thermal plasticity in the green macroalga Codium cylindricum

Nitrogen source and availability can modulate thermal stress responses in marine macroalgae, yet the transcriptional mechanisms underpinning these effects in green seaweeds remain poorly understood. We tested how three N sources (ammonium, nitrate, urea) at low (5 M) and high (100 M) concentrations influence the thermal physiology and gene expression of Codium cylindricum. Thermal performance curves were constructed for growth, photosynthesis, and pigment content across a 5-29 {degrees}C gradient, coupled with transcriptomic analysis at three key temperatures. At high concentrations, ammonium reduced maximum growth rate by 25% and lowered the thermal optimum relative to nitrate and urea, while nitrate supported the broadest thermal performance. Urea constrained thermal breadth regardless of concentration. These responses were linked to differential expression of Nitrogen assimilation genes. Ammonium suppressed nitrate transporter genes (NRT) and nitrite transporter (FNT), and drove pigment accumulation with rising temperature despite reduced growth, suggesting resource reallocation from growth to photoprotection. In contrast, nitrate upregulated photosystem and phosphorylation-related genes. Our findings demonstrate that Nitrogen source and concentration jointly determine the thermal threshold of C. cylindricum, with high ammonium reducing both maximum growth and the temperature at which it is achieved. This highlights how shifting coastal nutrient regimes may alter macroalgal resilience under ocean warming.