Inhibitors with 14 or 13CO2 uptake

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

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Nitrifier dark CO2 fixation (inhibitor + tracer)
Approach: dark 14C or 13C uptake with nitrification inhibitors
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours to days
Units: nmol C L-1 d-1; mol C cell-1 d-1
Community captured: bulk, size-fractionated, single cell
Co-measurements: cell numbers, community composition

Method Overview

The chemoautotrophic CO₂ fixation attributable specifically to nitrifiers is estimated by comparing dark ¹⁴C or ¹³C-bicarbonate incorporation in paired incubations: one with nitrification inhibitors (ATU, octyne) and one without. The difference in dark carbon fixation between control and inhibited bottles is attributed to nitrifier chemoautotrophic production. This approach is based on the principle that nitrifying bacteria and archaea derive energy from NH₄⁺ or NO₂^- oxidation and fix CO₂ as their sole carbon source.

Scale of measurement

Point sample; dark incubations of hours to days. Requires strict exclusion of light to prevent phytoplankton photosynthetic CO₂ fixation from contaminating the signal.

Data generated

Nitrifier-specific dark carbon fixation rates in nmol C L^-1 d^-1; convertible to nitrification rates using the theoretical yield of 1 mol CO₂ fixed per ~7 mol NH₄⁺ oxidized (AOA; ratio differs for AOB).

Units & currency

Units are mol C L^-1 d^-1 or mol C cell^-1 d^-1. The currency is carbon.

Sample size

Typical samples are ~1 L in volume.

Limitations

The method assumes that nitrification is the primary driver of dark CO₂ fixation in the water column. Recent evidence suggests this assumption is increasingly questionable in some environments where other chemoautotrophs (sulphur oxidisers, hydrogen oxidisers) contribute substantially. Long incubations in the dark can alter community composition. The conversion factor between CO₂ fixation and N oxidation varies among nitrifier lineages and must be determined empirically.

Example Applications & Protocols

Classic examples

• Ward (2011) Measurement and distribution of nitrification rates in the oceans

Common calculations/conversions

• Nitrification rate from dark C fixation: assuming 1 mol CO₂ fixed per 7 mol NH₄⁺ oxidised, nitrification rate (nmol N L^-1 d^-1) = dark C fixation (nmol C L^-1 d^-1) × 7.