Calcification (13-Carbon uptake)

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

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Calcification rate (13C)
Approach: stable isotope (13C-bicarbonate) incubation; separation of PIC and POC fractions
Context: incubation, lab
Spatial scale: point sample; single cell
Temporal scale: hours
Units: µmol C L-1 d-1
Community captured: all (> 0.7 µm bulk); single-cell resolution available
Co-measurements: background δ13C (POC), DIC, Chl; 13C:12C of POC and PIC before and after incubation

Method Overview

NaH¹³CO₃ is added to seawater samples incubated under simulated in situ conditions. After incubation, two subsamples are collected: one filtered for total particulate material (POC + PIC), and one where carbonate is dissolved by acidification before filtration (POC only). The ¹³C enrichment in each fraction is measured by EA-IRMS. The difference in ¹³C incorporation between the total and acidified fractions reflects incorporation into particulate inorganic carbon (PIC = CaCO₃), i.e., calcification rate. This approach avoids radioactive materials and can be coupled with ¹⁵N uptake measurements in the same incubation.

Scale of measurement

Point samples after hours of incubation. At the single-cell level, nanoSIMS can measure per-cell calcification rates in individually identified coccolithophores.

Data generated

Calcification rates (µmol PIC C L^-1 d^-1) and organic carbon fixation rates (µmol POC C L^-1 d^-1), enabling direct calculation of the PIC:POC ratio (coccolithophore calcification ratio).

Units & currency

Units are µmol C L^-1 d^-1. The currency is carbon.

Sample size

Typical samples range from 0.3 to 4 L for bulk; nanoSIMS requires mL volumes with cells sorted or identified by FISH.

Limitations

Natural variability in background δ¹³C of the DIC pool must be accurately measured. Isotopic changes in the DIC pool during the incubation (from CO₂ invasion or outgassing) can bias rates. At the single-cell level, initial cellular carbon content must be estimated from biovolume, introducing uncertainty for cells with variable carbon densities. Bottle effects from incubation can arise.

Example Applications & Protocols

Classic examples

• 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

• Calcification rate = [(¹³C_total − ¹³C_POC) / ¹³C_DIC] × DIC / incubation time; where ¹³C_total and ¹³C_POC are the enrichments in total and acid-treated fractions respectively.