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| Titel |
Autoxidation and acetylene-accelerated oxidation of NO in a 2-phase system; implications for the expression of denitrification in ex situ experiments |
| VerfasserIn |
Shahid Nadeem, Peter Dörsch, Lars Bakken |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250080192
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| Zusammenfassung |
| Denitrification allows microorganisms to sustain respiration under anoxic conditions.
The typical niche for denitrification is an environment with fluctuating oxygen
concentrations such as soils and borders between anoxic and oxic zones of biofilms
and sediments. In such environments, the organisms need adequate regulation of
denitrification in response to changing oxygen availability to tackle both oxic and anoxic
spells. The regulation of denitrification in soils has environmental implications,
since it affects the proportions of N2, N2O and NO emitted to the atmosphere. The
expression of denitrification enzymes is regulated by a complex regulatory network
involving one or several positive feedback loops via the intermediate nitrogen oxides.
Nitric oxide (NO) is known to induce denitrification in model organisms, but the
quantitative effect of NO and its concentration dependency has not been assessed for
denitrification in soils. NO is chemically unstable in the presence of oxygen due to
autoxidation, and the oxidation of NO is accelerated by acetylene (C2H2) which is
commonly used as an inhibitor of N2O reductase in denitrification studies. As a first step
to a better understanding of NO’s role in soil denitrification, we investigated NO
oxidation kinetics for a closed “two phase” system (i.e. liquid phase + headspace)
typically used for denitrification experiments with soil slurries, with and without
acetylene present. Models were developed to adequately predict autoxidation and
acetylene-accelerated oxidation. The minimum oxygen concentration in the headspace
([O2]min, mL L-1) for acetylene-accelerated NO oxidation was found to increase
linearly with the NO concentration ([NO], mL L-1); [O2]min= 0.192 + [NO]*0.1
(r2=0.978). The models for NO oxidation were then used to assess NO-oxidation rates in
denitrification experiments with batches of bacterial cells extracted from soil. The batches
were exposed to low initial oxygen concentrations in gas tight serum flasks (with
and without C2H2), and monitored for O2, NO, N2O and N2 production while
depleting the oxygen and switching to anoxic respiration. Acetylene effectively
scavenged NO from the cultures until oxygen concentration reached below ~0.19 mL
L-1, and the estimated rate of acetylene-accelerated NO oxidation was more than
sufficient to explain an observed reduction of the N2O production induced by acetylene.
When [O2] reached below 0.19 mL L-1, the NO concentrations increased and
stabilized at the same level as in the treatments without acetylene, but the rate of
denitrification was much lower than without acetylene. The results indicate that the early
accumulation of 10-20 nM NO during oxygen depletion has a significant effect
on the expression of denitrification in soil communities. This warrants a greater
interest in NO as a regulator of denitrification in soils and shows that the acetylene
inhibition method may be problematic even for intentionally anoxic incubations, unless
precautions are taken to secure initial O2-concentrations below 0.19 mL O2 L-1. |
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