sam wilson, daniela del valle, mariona segura-noguera, and...
TRANSCRIPT
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).