Niche separation of nitrifier and phytoplankton by light in the sunlit ocean

The fate of ammonia is one of the least explored issues in marine nitrogen cycle studies. Ammonia uptake by phytoplankton and nitrification are two major ammonia consumption processes in global ocean. The long held idea that nitrification is inhibited by light and phytoplankton competition is recently challenged by the evidences on the widely distribution of ammonia oxidizing archaea (AOA) in global sunlit ocean. Whereas, the regulation of ammonia by nitrifier and phytoplankton in upper ocean and the role of light in determining their relative rate distributions are still unclear. High vertical resolution in situ nitrification rate and ammonia uptake rate by phytoplankton were measured by using 15NH4+ tracer (final concentration 10nM) from the subtropical to temperate Western Pacific during two cruises. Nitrification rate ranges from 0~25nmol N L^-1d^-1 with a maximum at around the PNM, and ammonia uptake ranges from 0~40nmol N L^-1d^-1 with high values in the surface. In all stations, integrated uptake rate by phytoplankton is an order of magnitude higher than nitrification rate above the depth of 5% of PAR, whereas integration of nitrification rate is 2~5 times higher than the uptake in the depth interval of 5% PAR down to 0.1% PAR. Since ammonium concentrations in the high light zone are not particularly low and no consistent vertical patterns were found between ammonium and any rate measures, such distinctive vertical distributions between rates of nitrification and ammonia uptake by phytoplankton implies that light is the main driver in the euphotic ocean for their niche separation . The nitrification rates remained low (<0.1 nmol N L^-1 d^-1) in the high light zone even when ¹⁵NH₄⁺ tracer was enriched to 100nM. This further supports our speculation. Basing on our observations we concluded that light not only separates the niche of nitrifier and ammonia assimilator, but also determines the zones for regenerated and recycled-nitrate production .