Mass balance approach

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

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Chemoautotrophic C fixation (mass balance)
Approach: mass balance synthesis from ocean carbon and respiration inventories
Context: in situ, data synthesis
Spatial scale: ocean-wide to basin
Temporal scale: > years
Units: Pg C yr-1
Community captured: bulk
Co-measurements: ocean respiration rates; nitrification rates; C:N ratios

Method Overview

Chemoautotrophic carbon fixation in the dark ocean is estimated by combining two constraints: (1) dark ocean community respiration rates (from AOU, ¹⁴C-bicarbonate fixation, or ETS measurements), which set the total organic carbon demand; and (2) the stoichiometric relationship between oxidation of reduced nitrogen compounds (ammonium, nitrite) and CO₂ fixation. The classic formulation calculates dark ocean chemoautotrophic production by dividing dark respiration by the Redfield C:N ratio and applying the theoretical yield of one mole of CO₂ fixed per ~10 moles of ammonium oxidized.

The approach synthesizes existing rate measurements and biogeochemical inventories to constrain the global magnitude of dark chemoautotrophic production, and is complementary to direct measurements (dark ¹⁴C fixation, nanoSIMS).

Scale of measurement

Ocean-wide to basin-scale synthesis; the method does not provide spatial or temporal resolution within the dark ocean, but constrains the global budget.

Data generated

Estimated chemoautotrophic carbon production in Pg C yr^-1, disaggregated by process (nitrification, sulphur oxidation, etc.) where sufficient data exist.

Units & currency

Units are Pg C yr^-1. The currency is carbon.

Sample size

Not applicable; synthesis approach using existing datasets.

Limitations

Relies on multiple assumptions: steady-state ocean, constant Redfield ratios, fixed stoichiometry of CO₂ fixation per mole of substrate oxidized. Uncertainty in both the respiration rate estimates and the stoichiometric constants propagates strongly into the chemoautotrophic production estimate. Emerging evidence suggests that nitrification may not dominate dark ocean chemoautotrophy in all regions or seasons, and that other pathways (sulphur oxidation, hydrogen oxidation) may contribute substantially.

Example Applications & Protocols

Classic examples

• Middelburg (2011) Chemoautotrophy in the ocean

Common calculations/conversions

• Chemoautotrophic C fixation = dark respiration (Pg C yr^-1) × (1 / Redfield C:N) × (1 / 10) for nitrification-based fixation.