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Revisiting nitrification in the Eastern Tropical S outh P acific: A focus on controls
Author(s) -
Peng Xuefeng,
Fuchsman Clara A.,
Jayakumar Amal,
Warner Mark J.,
Devol Allan H.,
Ward Bess B.
Publication year - 2016
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2015jc011455
Subject(s) - nitrification , anoxic waters , ammonia monooxygenase , photic zone , ammonium , denitrification , nitrite , chemistry , ammonia , oxygen , environmental chemistry , population , nitrogen , nitrate , nutrient , biochemistry , organic chemistry , demography , phytoplankton , sociology
Nitrification, the oxidation of ammonium ( NH 4 + ) to nitrite ( NO 2 − ) and to nitrate ( NO 3 − ), is a component of the nitrogen (N) cycle internal to the fixed N pool. In oxygen minimum zones (OMZs), which are hotspots for oceanic fixed N loss, nitrification plays a key role because it directly supplies substrates for denitrification and anaerobic ammonia oxidation (anammox), and may compete for substrates with these same processes. However, the control of oxygen and substrate concentrations on nitrification are not well understood. We performed onboard incubations with 15 N‐labeled substrates to measure rates ofNH 4 +andNO 2 −oxidation in the eastern tropical South Pacific (ETSP). The spatial and depth distributions ofNH 4 +andNO 2 −oxidation rates were primarily controlled byNH 4 +andNO 2 −availability, oxygen concentration, and light. In the euphotic zone, nitrification was partially photoinhibited. In the anoxic layer,NH 4 +oxidation was negligible or below detection, but high rates ofNO 2 −oxidation were observed.NH 4 +oxidation displayed extremely high affinity for bothNH 4 +and oxygen. The positive linear correlations betweenNH 4 +oxidation rates and in situNH 4 +concentrations and ammonia monooxygenase subunit A ( amoA ) gene abundances in the upper oxycline indicate that the natural assemblage of ammonia oxidizers responds to in situNH 4 +concentrations or supply by adjusting their population size, which determines theNH 4 +oxidation potential. The depth distribution of archaeal and bacterial amoA gene abundances and N 2 O concentration, along with independently reported simultaneous direct N 2 O production rate measurements, suggests that AOA were predominantly responsible forNH 4 +oxidation, which was a major source of N 2 O production at oxygen concentrations > 5 µM.