Nitric oxide (NO) in the oxygen minimum zone off Peru

Abstract Nitric oxide (NO) is a short-lived compound of the marine nitrogen cycle. However, measurements of NO in seawater are analytically challenging and our knowledge about its oceanic distribution is, therefore, rudimentary. NO was measured in the oxygen minimum zone (OMZ) of the eastern tropical South Pacific Ocean (ETSP) off Peru during R/V Meteor cruise M93 in February/March 2013. NO concentrations ranged from close to or below the detection limit (0.5 nmol L −1 ) in the surface layer to 9.5 nmol L −1 in the OMZ. NO concentrations increased significantly when oxygen (O 2 ) concentrations dropped below 1–2 µmol L −1 . We found positive correlations between NO and NO 2 - as well as between NO and the abundance of archaeal amoA , a marker gene for archaeal nitrifiers. No trends between NO and nirS and hzo , marker genes for canonical denitrification and anammox, respectively, were found. To this end, we conclude that NO off Peru was mainly produced by archaeal nitrifier-denitrification at low O 2 concentrations in the OMZ.