Cobalamin uptake with 57Cobalt-B12

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

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Cobalamin uptake (57Co-B12)
Approach: radiotracer (57Co-labelled cobalamin) incubation
Context: incubation, lab
Spatial scale: point sample
Temporal scale: 4–24 h
Units: mol cell-1 d-1; mol L-1 d-1; mol mol C-1 d-1
Community captured: all (usually > 0.2 or 0.7 µm)
Co-measurements: biomass (Chl, cells, POC, cell volume)

Method Overview

Cobalamin (vitamin B₁₂) uptake rates are measured using ⁵⁷Co-labelled cyanocobalamin (⁵⁷Co-B12; half-life ~271 days, gamma emitter). The radiolabelled cobalamin is added to seawater at tracer concentrations under trace-metal-clean conditions and equilibrated with the sample. After incubation (4–24 h), cells are collected on filters, rinsed to remove surface-adsorbed cobalamin, and counted by gamma spectrometry. Uptake rates are calculated from the fraction of ⁵⁷Co-B12 incorporated relative to total dissolved ⁵⁷Co-B12.

Cobalamin is an essential cofactor that many phytoplankton cannot synthesize and must obtain from cobalamin-producing bacteria, making uptake rate measurements central to understanding phytoplankton–bacteria vitamin interactions.

Scale of measurement

Point sample after 4–24 h of incubation under trace-metal-clean conditions.

Data generated

Cobalamin uptake rates (mol cell^-1 d^-1 or mol L^-1 d^-1). Combined with cellular B₁₂ quotas, the community's vitamin B₁₂ demand and turnover can be characterized.

Units & currency

Units are mol cell^-1 d^-1, mol L^-1 d^-1, or mol mol C^-1 d^-1. The currency is Co (as cobalamin).

Sample size

Typical samples are < 1 L in volume.

Limitations

The method assumes that ⁵⁷Co remains associated with the intact cobalamin molecule throughout the incubation (no degradation of cobalamin to other Co-containing compounds). In practice, some B₁₂ degradation can occur, leading to uptake of ⁵⁷Co in non-cobalamin forms. Natural cobalamin-binding proteins in seawater can compete with cellular uptake, and their effect must be considered when estimating bioavailable cobalamin.

Example Applications & Protocols

Classic examples

• Bertrand et al. (2011) Vitamin B₁₂ and iron colimitation of phytoplankton growth in the Ross Sea

Common calculations/conversions

• B₁₂ uptake rate = (cpm_cells / cpm_dissolved) × [B_12,dissolved] / incubation time.