Conversion of organic nitrogen into N 2 in the oceans: where does it happen? and how? Yuan-Hui (Telu) Li Department of Oceanography University of Hawaii at Manoa

Outline 1. Nitrogen cycle in the oceans: 2. Three end-member mixing model and the aerobic partial nitrification hypothesis. 3. Nitrate deficits by the aerobic partial nitrification and the anoxic denitrification. 4. Conclusions 5. Acknowledgement

Air

1. Nutrient cycle in the ocean: Redfield Ratios and Nitrification by nitrifying bacteria [oxic]: 138 O 2 + (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )  H 2 PO 4  + 16 NO 3  CO H H 2 O  P\  N\  C org \-  O 2 = 1\16\106\138 or r p = -  O 2 /  P = 138 r n = -  O 2 /  N = 8.63 r c = -  O 2 /  C org = 1.30 phytoplankton

Denitrification by denitrifying bacteria [anoxic and suboxic] phytoplankton 94.4 NO H + + (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 )  H 2 PO 4  N CO H 2 O  P\-  N\  C org \  N 2 = 1\94.4\106\55.2 Anaerobic ammonia oxidation (anammox) by anammox bacteria: NH NO 2 -  N H 2 O

2. Three end member mixing model (Li and Peng, 2002) 1 = f 1 + f 2 + f 3 (1)  = f 1  1 + f 2  2 + f 3  3 (2) S = f 1  S 1 + f 2  S 2 + f 3  S 3 (3) O 2 + r n  NO 3 = (NO) = f 1  (NO) 1 + f 2  (NO) 2 +f 3  (NO) 3 (4) O 2 =  0 +  1  +  2  S - r n  NO 3 (4a) where, r n = -  O 2 /  NO 3 Similarly O 2 = A 0 + A 1  + A 2  S – r p  PO 4 (5a) where, r p = -  O 2 /  PO 4 Also: DA =  0 +  1  +  2  S +  3  O 2 (6a) where, r c = 1/(  3 – 0.5/r n ); r c = -  O 2 /  C org DA = (DIC – Alk/2)

2a. Aerobic Partial nitrification hypothesis: Unidentified bacteria have evolved in a low oxygen (but oxic) and high nitrate environment (such as in oxycline, marine snow and fecal pellets, sediments) to utilize both oxygen and nitrate as terminal electron acceptors during oxidation of organic matter, and convert some organic nitrogen into N 2, N 2 O, and NO.

3. Nitrate deficit by partial nitrification (dN) and denitrification (dN”) N a = 16(P ) N b =  P  P  P 3 When N b  N  N a dN = N a - N ; When N < N b dN = N a - N b dN” = N b - N ; N* by Deutsch et al (2001): N* = (N - N a ’) N a ’ = 16(P ) -N*  dN + dN”

i7n (  mol/kg)

Additional support for the aerobic partial nitrification hypothesis: 1. Schmidt et al (2004) showed that a wild-type of Nitrosomonas europaea in chemostat cell cultures can produce nitrogen gases (N 2, NO, and N 2 O) during aerobic (O 2 ~ 125  M) oxidation of ammonia, using genes encoding reduction enzymes such as nitrite reductase, nitric oxide reductase etc. For example, NH 4 + (ammonia monooxygenase)  NH 2 OH (hydroxylamine oxidoreductase)  NO 2  (nitrite reductase)  NO (nitric oxide reductase)  N 2 O (not yet identified nitrous oxide reductase)  N Aerobic and anaerobic ammonia oxidizing bacteria are coupled in suspended organic particles in a low-oxygen (O 2 ~ 5  M) CANON reactor (Nielsen et al., 2005) to produce N 2

3. Codispoti et al. (2001) estimated the excess N 2 in the water column of the Arabian Sea, using the Ar/N 2 ratio in the water column and in the air. They found that the excess N 2 is substantially greater than the N 2 produced by the denitrification process.

Acknowledgement: Ms Lauren Kaupp patiently showed me how to use the Ocean Data View program, which was provided by Dr. Reiner Schlitzer. Discussions with Drs. James Cowen, David Karl, Marcel Kuypers, Fred Mackenzie, Hiroaki Yamagichi and Wajih Naqvi were most fruitful. Many thanks to Professor Yoshiki Sohrin for kindly inviting me here. This work is supported by a NOAA grant to Y.H. Li and T.H. Peng.

-dN\O2 =(6  1)\130

r p = -O 2 /P; r n = -O 2 /N; r p /r n = N/P

Redfield ratios: P\N\C org \-O 2 = 1\16\106\138; (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) Antarctic Indian Ocean: P\N\C org \-O 2 = 1\(15  1)\(83  2)\(134  9) Deep equatorial Indian Ocean: P\N\C org \-O 2 = 1\(10  1)\(94  5)\(130  7) Average remineralization ratios for the warm water mass: P\N\C org \-O 2 = 1\(15.6  0.7)\(110  9)\(159  8) Anderson’s (1995) remineralization ratios and phytoplankton formula: P\N\C org \-O 2 = 1\16\106\150; (C 106 H 48 )(H 2 O) 38 (NH 3 ) 16 (H 3 PO 4 )

1. The remineralization ratios (P\N\C org \-O 2 ) of organic matter in the oxygenated regions of Indian Ocean change systematically with latitude and depth. 2. The average remineralization ratios for the Indian warm water masses (potential temperature   ~ 10°) are P\N\C org \-O 2 = 1\(15.6  0.7)\(110  9)\(159  8). These are comparable to the traditional Redfield ratios P\N\C org \-O 2 = 1\16\106\138, and are in good agreement with Anderson’s (1995) values of P\N\C org \-O 2 = 1\16\106\150 within the given uncertainties. 5. Conclusions