Ammonia oxidation and nitrite oxidation are the two sequential steps of marine nitrification, and their coupling determines the efficiency of inorganic nitrogen transformation in the euphotic zone. Light is considered a key environmental factor regulating this coupling. However, current understanding of light inhibition relies largely on changes in bulk rates, which cannot reveal whether light alters the substrate utilization kinetics of nitrifying microorganisms, thereby limiting mechanistic understanding of light-regulated coupling between ammonia oxidation and nitrite oxidation.Using ¹⁵N stable isotope tracers, we determined the Michaelis–Menten kinetic parameters (K_m, V_max, and α) for ammonia oxidation and nitrite oxidation from matrix incubations with five light intensities and five substrate concentrations in the South China Sea. The two processes exhibited distinct kinetic responses to light. In particular, the responses of K_m and α to increasing irradiance differed between ammonia oxidation and nitrite oxidation, suggesting that light influences not only nitrification rates but also the substrate utilization kinetics of the two processes. These findings provide kinetic evidence for the differential effects of light on the two nitrification steps and improve our understanding of environmental regulation of nitrification coupling in the euphotic ocean.

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