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{| class="model-ib" style="float:right; margin-left:1em; margin-bottom:1em;" ! N₂ fixation (acetylene reduction) |- | '''Approach:''' acetylene reduction to ethylene (ARA) |- | '''Context:''' incubation, ''in situ'' |- | '''Spatial scale:''' point sample |- |..."

2026-05-11T23:31:00Z

Created page with "{{BreadcrumbsPrimaryProduction}} * Page authors: PRIMO * Responsible curator: Hagen Buck-Wiese ---- __TOC__ <div class="model-box"> {| class="model-ib" style="float:right; margin-left:1em; margin-bottom:1em;" ! N2 fixation (acetylene reduction) |- | '''Approach:''' acetylene reduction to ethylene (ARA) |- | '''Context:''' incubation, ''in situ'' |- | '''Spatial scale:''' point sample |- |..."

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{{BreadcrumbsPrimaryProduction}}

* [[Page authors|Page authors]]: [[PRIMO]]
* [[Responsible curator|Responsible curator]]: [[User:Hagi BucknWise|Hagen Buck-Wiese]]
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<div class="model-box">
{| class="model-ib" style="float:right; margin-left:1em; margin-bottom:1em;"
! N2 fixation (acetylene reduction)
|-
| '''Approach:''' acetylene reduction to ethylene (ARA)
|-
| '''Context:''' incubation, ''in situ''
|-
| '''Spatial scale:''' point sample
|-
| '''Temporal scale:''' minutes to hours
|-
| '''Units:''' nmol N L-1 h-1 (after conversion from ethylene)
|-
| '''Community captured:''' bulk; single cell; slurries
|-
| '''Co-measurements:''' C2H2:N2 reduction ratio for calibration
|}
</div>
<div style="clear:both"></div>

== Method Overview ==

Nitrogenase, the enzyme responsible for N2 fixation, also reduces acetylene (C2H2) to ethylene (C2H4) with high affinity. The Acetylene Reduction Assay (ARA) exploits this by adding acetylene gas to sealed incubation vessels containing seawater samples. Ethylene produced during the incubation is measured by gas chromatography with flame ionisation detection (GC-FID) or photoionisation detection (GC-PID), and converted to an equivalent N2 fixation rate using a theoretical stoichiometry (3 or 4 mol C2H4 per mol N2, depending on whether the natural H2-producing reaction is considered)<ref name="Wilson2012">Wilson, S. T., Böttjer, D., Church, M. J., & Karl, D. M. (2012). Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic North Pacific Ocean. ''Applied and Environmental Microbiology'', 78(18), 6516–6523. https://doi.org/10.1128/AEM.01146-12</ref>.

ARA is simpler and faster than 15N2 methods, making it suitable for high-throughput screening or field assays where isotope-ratio mass spectrometry is unavailable.

=== Scale of measurement ===

Point sample; incubations of minutes to hours. Short incubation times reduce the risk of feedback effects of acetylene on the diazotroph community.

=== Data generated ===

Ethylene production rate, which is converted to an N2 fixation rate using the molar conversion ratio. Results are reported in nmol N L-1 h-1 or nmol N cell-1 h-1 for single-cell assays.

=== Units & currency ===

Units are nmol N L-1 h-1 (after conversion from ethylene). The currency is nitrogen (ethylene is the primary measured currency; nitrogen is derived).

=== Sample size ===

Typical samples are 10–200 mL in volume.

=== Repositories & databases ===

== Limitations ==

The theoretical C2H4:N2 conversion ratio (3:1 or 4:1) is not stable across environmental conditions, organisms, and nutrient states, introducing a systematic uncertainty in the conversion. Acetylene inhibits natural H2 production by nitrogenase, which affects the theoretical ratio. Dissolution of acetylene gas and potential bubble formation introduce variability across replicate bottles. Incubations longer than ~24 h can stress cells and alter the measured rate. ARA does not distinguish N2 fixation from other acetylene reduction reactions.

== Example Applications & Protocols ==

=== Classic examples ===
* Wilson et al. (2012) ''Comparative assessment of nitrogen fixation methodologies, conducted in the oligotrophic North Pacific Ocean'' <ref name="Wilson2012" />

=== Recent applications ===
*

=== Common calculations/conversions ===
* N2 fixation rate (nmol N L-1 h-1) = C2H4 production rate (nmol L-1 h-1) / conversion ratio (3 or 4).
* Empirical conversion ratios should be determined from parallel 15N2 measurements in the same environment when possible.

== References ==

[[Category:Main Pages|Model types]]

Acetylene Reduction Assays (ARA) - Revision history