H2 supersaturation

Data types › Nitrogen Fixation › H2 supersaturation

• Page authors: PRIMO
• Responsible curator: Hagen Buck-Wiese

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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.

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.

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

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.