Ammonia-oxidizing archaea (AOA), the dominant nitrifiers in the ocean, are strongly constrained by light regimes that are being reshaped by climate-driven changes in upper-ocean stratification and mixing. However, how AOA sustain metabolism during recurrent light exposure at the base of the euphotic zone remains unresolved. Here, we exposed cultures of Nitrosopumilus maritimus SCM1 to low-light (~1% photosynthetically active radiation, PAR) and high-light (~15% PAR) diel cycles to characterize their physiological and molecular responses. While high light nearly arrested both ammonia oxidation and carbon fixation and induced pronounced oxidative damage, low light reduced these processes without causing significant H₂O₂ accumulation or sustained lipid peroxidation. These responses suggest that light constrains AOA primarily through direct photoinhibition, with ROS-mediated oxidative stress becoming pronounced only under high irradiance. Ammonia oxidation was more light-sensitive than carbon fixation, a disparity that was reflected under low light by transient C/N stoichiometric imbalance and enhanced dissolved organic carbon release. This release was accompanied by the irradiance-dependent accumulation of putative extracellular polymeric substances on cell surfaces, as observed by scanning electron microscopy. Multi-omics profiling further identified retention of high-affinity ammonium uptake, activation of phosphate acquisition, and enhanced adhesion as additional putative mechanisms of light acclimation. Additionally, in situ light-manipulation experiments showed increased substrate affinity for ammonia oxidation under low irradiance (≤10% PAR), consistent with AOA acclimation near the lower euphotic boundary. These findings identify natural light as an intensity-dependent constraint that also induces acclimation states supporting AOA persistence and nitrification near the euphotic-zone base.
 

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