Physiological and biochemical mechanisms underlying nitrogen-mediated cadmium tolerance in Ulva prolifera and Ulva linza

Cadmium (Cd) contamination coupled with nitrogen (N) eutrophication in coastal ecosystems poses complex challenges to marine primary producers. However, the modulatory role of N availability in macroalgal Cd 2+ tolerance remains inadequately understood. This study investigated the physiological, biochemical, and subcellular phenotypic mechanisms underlying N-mediated Cd 2+ resistance in two dominant green tide-forming macroalgae, Ulva prolifera and Ulva linza . Both species were exposed to factorial combinations of three N levels (0, 500, 1000 μM NaNO 3 ) and three Cd 2+ concentrations (0, 20, 80 μM CdCl 2 ) for 7 days. Results demonstrated that N supplementation significantly alleviated Cd 2+ -induced growth inhibition. Under 20 μM Cd 2+ stress, 1000 μM N enhanced relative growth rates by 84% in Ulva prolifera and 154% in Ulva linza compared to N-depleted controls. N supply effectively protected photosystem II (PSII) function, elevating maximum photochemical efficiency (Fv/Fm), effective quantum yield, and photosynthetic pigment contents. Notably, N enrichment concomitantly increased total Cd 2+ accumulation while mitigating its toxicity. Subcellular fractionation revealed that under N deficiency, Cd²⁺ was predominantly bound to the cell wall fraction (>60% of total cellular Cd²⁺). N supplementation significantly increased absolute Cd²⁺ content across all fractions but decreased its relative proportion in the cell wall, driving a redistribution toward the intracellular soluble fraction.This redistribution coincided with a remodelling of the osmotic solute profile: N application increased soluble proteins and free amino acids while reducing soluble sugars and stress-induced proline accumulation. Ulva linza consistently demonstrated greater physiological metabolic resilience than Ulva prolifera . Collectively, these phenotypic data reveal an apparently paradoxical but ecologically meaningful pattern in which N availability decouples Cd²⁺ accumulation from physiological toxicity, coincident with subcellular Cd²⁺ redistribution, osmotic solute remodelling, and photosynthetic protection. This study provides insights relevant to optimizing phycoremediation strategies in heavy metal-contaminated, eutrophic coastal waters.