13-Carbon uptake

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

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Carbon fixation (13C stable isotope)
Approach: stable isotope tracer (13C) uptake
Context: incubation, in situ
Spatial scale: point sample; single cell
Temporal scale: hours
Units: µmol C L-1 d-1; fmol C cell-1 d-1 (single cell)
Community captured: all (bulk > 0.7 µm); single-cell resolution available
Co-measurements: background δ13C (POC), DIC concentration, DIC 13C:12C ratio before and after incubation; can couple with 15N uptake in the same assay

Method Overview

NaH¹³CO₃, a stable (non-radioactive) isotope of bicarbonate, is added to seawater samples at tracer or enrichment levels. After incubation, particulate organic carbon is collected by filtration (GF/F, GF/D, or polycarbonate filters) and the ¹³C:¹²C ratio is measured by isotope ratio mass spectrometry (IRMS) or elemental analyser–IRMS (EA-IRMS). Carbon fixation rates are calculated from the isotopic enrichment of the POC relative to the enrichment of the DIC pool.

The stable isotope approach eliminates radioactive waste and allows simultaneous coupling with ¹⁵N uptake measurements using the same incubation bottles. At single-cell resolution, nanoSIMS (nano-scale secondary ion mass spectrometry) can measure the ¹³C:¹²C ratio in individually identified cells, providing cell-specific carbon fixation rates.

Scale of measurement

Bulk filtration provides a community-level point measurement. Size fractionation (e.g., 0.7 µm and 2.7 µm) resolves production across size classes. At the single-cell level, nanoSIMS can provide taxon-specific rates with spatial resolution of tens of nanometers, but requires a separate cell identification step (FISH or cell sorting).

Data generated

The method yields carbon fixation rates in µmol C L^-1 d^-1 (bulk) or fmol C cell^-1 d^-1 (single cell). When coupled with ¹⁵N, simultaneous C and N assimilation rates are obtained.

Units & currency

Units are µmol C L^-1 d^-1 for bulk, or fmol C cell^-1 d^-1 for single-cell nanoSIMS.

Sample size

Typical samples range from 0.3 to 4 L for bulk measurements; nanoSIMS requires only a few mL but extensive instrument time for sufficient cell numbers.

Limitations

The natural abundance of ¹³C in the DIC pool (background δ¹³C) must be measured accurately; isotopic changes in the DIC pool during incubation can bias rate estimates if not corrected. For single-cell estimates, the initial cellular carbon content is assumed to be proportional to cell volume, introducing uncertainty for cell types with variable carbon densities. Bottle effects apply as in the ¹⁴C method.

Example Applications & Protocols

Classic examples

• Hama et al. (1983) Measurement of photosynthetic production of a marine phytoplankton population using a stable ¹³C isotope
• Slawyk et al. (1977) The use of the ¹³C and ¹⁵N isotopes for the simultaneous measurement of carbon and nitrogen turnover rates in marine phytoplankton

Recent applications

• Wu et al. (2022) Single-cell measurements and modelling reveal substantial organic carbon acquisition by Prochlorococcus
• Irion et al. (2021) Small phytoplankton contribute greatly to CO₂-fixation after the diatom bloom in the Southern Ocean

Common calculations/conversions

• C fixation rate = [(δ¹³C_POC,final − δ¹³C_POC,initial) / (δ¹³C_DIC − δ¹³C_POC,initial)] × [POC] / incubation time.