Top-down control, dominated by zooplankton grazing, is one of the most important ecological mechanisms regulating the biomass and proportion of microorganisms in the ocean. Due to technological limitations, current methods cannot precisely quantify the in situ grazing rates on ammonia-oxidizing microorganisms (AOM) and nitrite-oxidizing bacteria (NOB) in the ocean, leaving the top-down control on marine chemoautotrophic nitrifiers unstudied. Combining stable isotope labelling with dilution assays, we developed the "New isotope Method" to quantify the in situ grazing rates of AOM and NOB in the ocean. The new proposed method was applied in field experiments and revealed strong top-down control at the bottom of the euphotic zone in the ocean for the first time, where on average, grazing consumed almost all of the daily growth of both AOM and NOB. Our study broadens understanding of the impact of grazing processes on N biogeochemical cycling processes in the ocean, elucidating that grazing might play a important role in shaping the balance between ammonia oxidation and nitrite oxidation and regulating two-step nitrification rates in the ocean.

nitrification, grazing, top-down control, nitrifier, nitrogen cycle, zooplankton