Nitrous oxide measurements in the

North Pacific Subtropical Gyre

Future nitrous oxide measurements

Nitrifier-denitrification in the marine

environment

Sam Wilson, Daniela del Valle, Mariona Segura-Noguera, and David Karl Daniel K. Inouye Center for Microbial Oceanography: Research and Education, University of Hawaii, Honolulu, Hawaii

Greenhouse gas measurements in the North Pacific

Greenhouse gases in the ocean

Accurate measurements of the short and long-term variability in climatically important

trace gases, including nitrous oxide (N2O) will be increasingly important over the next few

decades.

Identifying the microorganisms and the associated metabolic pathways responsible for

producing N2O remains a key challenge.

Vertical profiles of N2O concentrations

Insights from N2O isotope and isotopomer analysis

References

The nitrifier-denitrification metabolic pathway

Optimal location for nitrifier-denitrification in the water column

Experimental evidence for nitrifier-denitrification:

production of nitrous oxide from nitrite amendments

from Popp et al (2002) Global Biogeochem Cycles 16:1064

A Scientific Committee of Oceanographic Research (SCOR) Working Group (#143)

“Dissolved N2O and CH4 measurements: Working towards a global network of ocean time

series measurements of N2O and CH4” has been established to improve and consolidate

oceanic measurements of N2O and CH4. The project is co-chaired by Sam Wilson

(University of Hawaii) and Hermann Bange (Geomar, Kiel) and will (i) conduct an

intercalibration exercise for discrete N2O and CH4 measurements (ii) conduct an overall

assessment on the status of dissolved N2O and CH4 measurements in the global oceans.

Dore, J.E., Popp, B.N., Karl, D.M. and Sansone, F.J. (1998) Nature 396:63-66.

Lomas, M.W., Lipschultz, F., 2006. Limnol. Oceanogr. 51 (5), 2453–2467.

Olson, R.J., 1981. J. Mar. Res. 39, 227–238.

Ostrom, N.E., Russ, M.E., Popp, B., Rust, T.M., Karl, D.M., 2000. Chemosphere – Global Change Science 2 (3-4),

281–290.

Popp, B.N., Westley, M.B., Toyoda, S., Miwa, T., Dore, J.E., Yoshida, N., Rust, T.M., Sansone, F.J., Russ, M.E.,

Ostrom, N.E., Ostrom, P.H., 2002. Global Biogeochem. Cycles, 16 (4), 1064.

Weiss, R.F., Van Woy, F. A. and Salameh, P.K. (1992) Scripps Institution of Oceanography Reference 92-11. doi:

10.3334/CDIAC/otg.ndp044

Wilson, S.T., D.A. del Valle, M. Segura-Noguera and D.M. Karl (2014) Deep Sea Res Part I. 85: 47-55.

An increase in N2O concentrations

was observed when NO2 was

added to the deployed traps. The

increase in N2O was greater in the

presence of NO2 than NH4,

suggesting a ‘nitrifier-denitrifier’

metabolic pathway whereby NO2 is

first reduced to NO and then to

N2O (Wilson et al. 2014).

In oxygenated seawater, N2O production is

typically ascribed to nitrification whereby

ammonia (NH3) is oxidized via hydroxylamine

to nitrite (NO2-) . The production of N2O via

NO2- metabolism is referred to as ‘nitrifier-

denitrification’ whereby NO2- is reduced to NO

and then to N2O analogous to the classic

denitrification pathway

NO2- concentrations form a distinct peak at the base of

the euphotic zone referred to as the primary NO2-

maximum (PNM). It has been proposed that the PNM

results from ammonia oxidizing microorganisms that

oxidize NH3 to NO2- (Olson, 1981) However other

studies indicate that the PNM most likely reflects an

imbalance in phytoplankton N exudation, due to

incomplete assimilation (Lomas and Lipschultz, 2006).

Changes in the nitrogen isotopic composition of N2O following the addition of 15N-labeled

substrates to seawater collected from a water-column depth of 150 m and particles collected

using sediment traps. The δ15N values of N2O are reported

in permil relative to atmospheric N2

and the uncertainty is represented by

standard deviation. The statistical

significance (Sig. level) of each

treatment in comparison with the

unamended sample is shown as ns =

not significant = P > 0.05, * P < 0.05,

** P < 0.01.

Station ALOHA in the oligotrophic North Pacific Ocean

has been the long-term time series monitoring station

for the Hawaii Ocean Time-series (HOT) program since

1988. Repeated measurements of core parameters,

including carbon dioxide (CO2), methane (CH4), and

N2O are conducted on a near-monthly basis.

Dissolved N2O concentrations in the surface

waters of the open ocean are typically

slightly super-saturated. N2O concentrations

increase with depth, particularly when O2

concentrations decrease and nutrient

concentrations increase. N2O is a by-

product of microbial nitrogen metabolism

and in oxygenated waters is thought to

derive from nitrification (Dore et al. 1998).

Four decades of N2O measurements in the North Pacific Subtropical Gyre surface water

are shown below. Before 1992, measurements were made with an underway equilibrator

(Weiss et al. 1992), while later analyses were conducted on preserved samples. Seawater

concentrations are increasing alongside the rising atmospheric concentrations, with low

seasonal cycling.

Isotope measurements indicate

nitrification in near-surface

seawater (Dore et al. 1998).

However between depths of 200

and 500 m in the water column the

Δ18O values alongside the δ15N

values are indicative of a nitrifier-

denitrifier mechanism producing

N2O (Ostrom et al. 2000, Popp et

al. 2002).