H2 supersaturation: Difference between revisions
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{{DISPLAYTITLE:H<sub>2</sub> supersaturation}} | {{DISPLAYTITLE:H<sub>2</sub> supersaturation}} | ||
{{BreadcrumbsNFixation}} | {{BreadcrumbsNFixation}} | ||
{{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;" | |||
! H<sub>2</sub> supersaturation (N<sub>2</sub> fixation proxy) | |||
|- | |||
| '''Approach:''' dissolved H<sub>2</sub> gas measurement by reduction gas analyzer | |||
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| '''Context:''' ''in situ'' | |||
|- | |||
| '''Spatial scale:''' point sample to continuous underway | |||
|- | |||
| '''Temporal scale:''' minutes | |||
|- | |||
| '''Units:''' H<sub>2</sub> saturation (%) | |||
|- | |||
| '''Community captured:''' bulk | |||
|- | |||
| '''Co-measurements:''' temperature, pressure, salinity (for equilibrium saturation calculation) | |||
|} | |||
</div> | |||
<div style="clear:both"></div> | |||
== Method Overview == | |||
Nitrogenase produces molecular hydrogen (H<sub>2</sub>) as a stoichiometric by-product of N<sub>2</sub> fixation. Dissolved H<sub>2</sub> supersaturation relative to the atmospheric equilibrium therefore indicates that N<sub>2</sub> fixation is occurring in the water column. Dissolved H<sub>2</sub> is measured using a UV reduction gas analyzer interfaced with a membrane inlet system that equilibrates the dissolved gas phase with a carrier gas stream. Samples of ~1 mL of seawater are sufficient per measurement, and the method can be adapted for continuous underway operation<ref name="Moore2014">Moore, C. M., Seeyave, S., Hickman, A. E., Allen, J. T., Lucas, M. I., Planquette, H., Pollard, R. T., & Poulton, A. J. (2014). Extensive hydrogen supersaturations in the western South Atlantic Ocean suggest substantial underestimation of nitrogen fixation. ''Journal of Geophysical Research: Oceans'', 119, 4340–4350. https://doi.org/10.1002/2014JC010017</ref>. | |||
=== Scale of measurement === | |||
Sample volumes are ~1 mL, but continuous membrane inlet systems allow underway surveys at kilometer-scale resolution. Because H<sub>2</sub> is produced and consumed rapidly, the signal represents a near-instantaneous snapshot of diazotrophic activity, though air-sea exchange integrates over the mixed layer gas exchange timescale. | |||
=== Data generated === | |||
H<sub>2</sub> supersaturation expressed as a percentage above atmospheric equilibrium. Conversion to N<sub>2</sub> fixation rates requires knowledge of the H<sub>2</sub>:N<sub>2</sub> production ratio and the air-sea gas transfer velocity for H<sub>2</sub>. | |||
=== Units & currency === | |||
Units are H<sub>2</sub> saturation (%). The currency is hydrogen. | |||
=== Sample size === | |||
Typical samples are ~1 mL per discrete measurement. | |||
=== Repositories & databases === | |||
== Limitations == | |||
The ratio of H<sub>2</sub> production to N<sub>2</sub> fixation varies among diazotroph taxa (some unicellular diazotrophs may have uptake hydrogenases that recycle H<sub>2</sub>) and with physiological state. No data exist on H<sub>2</sub> release rates for several important unicellular diazotroph groups. Converting H<sub>2</sub> supersaturation to a quantitative N<sub>2</sub> fixation flux requires gas transfer parameterisations that introduce significant uncertainty. The method cannot identify which taxa are responsible for the observed supersaturation. | |||
== Example Applications & Protocols == | |||
=== Classic examples === | |||
* Moore et al. (2014) ''Extensive hydrogen supersaturations in the western South Atlantic Ocean suggest substantial underestimation of nitrogen fixation'' <ref name="Moore2014" /> | |||
=== Recent applications === | |||
* | |||
=== Common calculations/conversions === | |||
* H<sub>2</sub> supersaturation (%) = ([H<sub>2</sub>]<sub>measured</sub> / [H<sub>2</sub>]<sub>equilibrium</sub> − 1) × 100; equilibrium calculated from temperature, salinity, and atmospheric H<sub>2</sub> mixing ratio (~0.53 ppm). | |||
* N<sub>2</sub> fixation flux = k × ρ × [H<sub>2</sub>]<sub>sat</sub> × ΔH<sub>2</sub> / H<sub>2</sub>:N<sub>2</sub> ratio. | |||
== References == | |||
[[Category:Main Pages|Model types]] | |||
Latest revision as of 16:33, 11 May 2026
Template:BreadcrumbsPrimaryProduction
| H2 supersaturation (N2 fixation proxy) |
|---|
| Approach: dissolved H2 gas measurement by reduction gas analyzer |
| Context: in situ |
| Spatial scale: point sample to continuous underway |
| Temporal scale: minutes |
| Units: H2 saturation (%) |
| Community captured: bulk |
| Co-measurements: temperature, pressure, salinity (for equilibrium saturation calculation) |
Method Overview
Nitrogenase produces molecular hydrogen (H2) as a stoichiometric by-product of N2 fixation. Dissolved H2 supersaturation relative to the atmospheric equilibrium therefore indicates that N2 fixation is occurring in the water column. Dissolved H2 is measured using a UV reduction gas analyzer interfaced with a membrane inlet system that equilibrates the dissolved gas phase with a carrier gas stream. Samples of ~1 mL of seawater are sufficient per measurement, and the method can be adapted for continuous underway operation[1].
Scale of measurement
Sample volumes are ~1 mL, but continuous membrane inlet systems allow underway surveys at kilometer-scale resolution. Because H2 is produced and consumed rapidly, the signal represents a near-instantaneous snapshot of diazotrophic activity, though air-sea exchange integrates over the mixed layer gas exchange timescale.
Data generated
H2 supersaturation expressed as a percentage above atmospheric equilibrium. Conversion to N2 fixation rates requires knowledge of the H2:N2 production ratio and the air-sea gas transfer velocity for H2.
Units & currency
Units are H2 saturation (%). The currency is hydrogen.
Sample size
Typical samples are ~1 mL per discrete measurement.
Repositories & databases
Limitations
The ratio of H2 production to N2 fixation varies among diazotroph taxa (some unicellular diazotrophs may have uptake hydrogenases that recycle H2) and with physiological state. No data exist on H2 release rates for several important unicellular diazotroph groups. Converting H2 supersaturation to a quantitative N2 fixation flux requires gas transfer parameterisations that introduce significant uncertainty. The method cannot identify which taxa are responsible for the observed supersaturation.
Example Applications & Protocols
Classic examples
- Moore et al. (2014) Extensive hydrogen supersaturations in the western South Atlantic Ocean suggest substantial underestimation of nitrogen fixation [1]
Recent applications
Common calculations/conversions
- H2 supersaturation (%) = ([H2]measured / [H2]equilibrium − 1) × 100; equilibrium calculated from temperature, salinity, and atmospheric H2 mixing ratio (~0.53 ppm).
- N2 fixation flux = k × ρ × [H2]sat × ΔH2 / H2:N2 ratio.
References
- ↑ 1.0 1.1 Moore, C. M., Seeyave, S., Hickman, A. E., Allen, J. T., Lucas, M. I., Planquette, H., Pollard, R. T., & Poulton, A. J. (2014). Extensive hydrogen supersaturations in the western South Atlantic Ocean suggest substantial underestimation of nitrogen fixation. Journal of Geophysical Research: Oceans, 119, 4340–4350. https://doi.org/10.1002/2014JC010017