Radiolabeled tracer method

• Page authors: Anabel von Jackowski, Hagen Buck-Wiese
• Responsible curator: Hagen Buck-Wiese

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Bacterial production
Approach: radiolabeled tracer incorporation
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours
Units: mol incorporated tracer L-1 h-1
Community captured: bulk, size-fractionated
Co-measurements: cell abundance

Method Overview

Bacterial secondary production is estimated by measuring the incorporation of radiolabeled precursors — most commonly ³H-leucine (into protein) or ³H-thymidine (into DNA) — into microbial biomass during short, dark incubations. Aliquots of seawater are amended with tracer concentrations of the radiolabeled substrate, incubated for a defined period, and then filtered or precipitated to collect macromolecular material. Radioactivity retained on the filter is measured by scintillation counting and converted to a production rate.

The leucine incorporation method is the most widely applied variant. Leucine is assumed to be incorporated exclusively into protein, and a theoretical or empirically determined conversion factor is used to translate leucine incorporation into units of carbon production. Size-fractionated filtration can separate bacterial from eukaryotic production.

Publication examples

• Kirchman (2001) Measuring bacterial biomass production and growth rates from leucine incorporation in natural aquatic environments
• Kirchman et al. (2009) Standing stocks, production, and respiration of phytoplankton and heterotrophic bacteria in the western Arctic Ocean

Scale of measurement

As a bottle-based incubation, the method yields a point measurement in space. Incubation durations are typically a few hours, aimed at keeping the measurement close to in situ rates while minimising bottle effects and isotope dilution.

Data generated

The method yields bacterial carbon production rates, i.e., the rate at which heterotrophic bacteria synthesise new biomass. When combined with standing stock estimates (bacterial biomass), specific growth rates (d^-1) can be derived.

Units & currency

Units are mol incorporated tracer L^-1 h^-1. For example, nmol Leucine L^-1 h^-1.

Sample size

Replicate small-volume subsamples (1–5 mL) are typically processed per station.

Limitations

Leucine is assumed to be incorporated exclusively into protein and not re-mineralised during the incubation. Intracellular isotope dilution from unlabeled leucine pools can cause underestimation, and empirical conversion factors between leucine incorporation and carbon production are variable across environments and must be determined locally for highest accuracy. Bottle incubation can alter community composition and substrate availability relative to in situ conditions.

Leucine incorporation can be overestimated see Giering & Events (2022) . ! Hydrographic setting !! Min !! First Qu. !! Median !! Third Qu. !! Max !! n |- | Coast and shelf || 0.21 || 0.98 || 1.35 || 2.47 || 36.40 || 160 |-

| Open ocean || 0.02 || 0.25 || 0.56 || 1.29 || 19.20 || 105 |-

| Mesopelagic || 0.13 || 0.33 || 0.54 || 0.63 || 2.38 || 15 |-

| All marine || 0.02 || 0.52 || 1.14 || 2.00 || 36.40 || 280 |-

| Freshwater || 0.18 || 0.88 || 1.15 || 2.41 || 8.60 || 16 |-

| Sediment || 0.21 || 0.24 || 0.82 || 0.89 || 1.45 || 16 |-

| All environments || 0.02 || 0.53 || 1.14 || 2.03 || 36.40 || 296 |}