Cobalamin uptake with 57Cobalt-B12

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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^[1].

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.

Repositories & databases

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 ^[1]

Recent applications

Common calculations/conversions

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

References

↑ ^1.0 ^1.1 Bertrand, E. M., Saito, M. A., Rose, J. M., Riesselman, C. R., Lohan, M. C., Noble, A. E., Lee, P. A., & DiTullio, G. R. (2011). Vitamin B₁₂ and iron colimitation of phytoplankton growth in the Ross Sea. Limnology and Oceanography, 56(4), 1079–1091. https://doi.org/10.3389/fmicb.2011.00160

[Bertrand2011-1] 1.0 ^1.1 Bertrand, E. M., Saito, M. A., Rose, J. M., Riesselman, C. R., Lohan, M. C., Noble, A. E., Lee, P. A., & DiTullio, G. R. (2011). Vitamin B₁₂ and iron colimitation of phytoplankton growth in the Ross Sea. Limnology and Oceanography, 56(4), 1079–1091. https://doi.org/10.3389/fmicb.2011.00160

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