55-Iron uptake

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

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Iron uptake (55Fe)
Approach: radiotracer (55Fe) incubation
Context: incubation, lab
Spatial scale: point sample
Temporal scale: 4–24 h
Units: (x)mol Fe biomass-1 d-1; (x)mol Fe L-1 d-1
Community captured: all (usually > 0.2 or 0.7 µm)
Co-measurements: biomass (Chl, cells, POC, cell volume)

Method Overview

Iron uptake rates are measured by adding ⁵⁵Fe (a low-energy gamma/X-ray emitter, half-life ~2.7 years) as a radiolabelled FeCl₃ tracer to seawater samples under trace-metal-clean conditions. The ⁵⁵Fe spike is added at concentrations comparable to ambient dissolved iron concentrations and allowed to equilibrate with the ambient iron-binding ligand pool before incubation. After a 4–24 h dark or light incubation, the particulate fraction is collected by filtration, washed with oxalate solution to remove adsorbed (extracellular) iron, and the filter radioactivity is measured by liquid scintillation counting. The uptake rate is calculated from the fraction of ⁵⁵Fe incorporated relative to the total dissolved ⁵⁵Fe in the incubation.

Scale of measurement

Sampling after 4–24 h incubation. Trace-metal-clean sampling and handling are essential throughout to avoid iron contamination.

Data generated

Iron uptake rates normalized to biomass (mol Fe cell^-1 d^-1, mol Fe mol C^-1 d^-1) or expressed as volumetric rates (mol Fe L^-1 d^-1). When combined with iron quotas (cellular Fe content), growth-rate-specific iron requirements (K_Fe) can be derived.

Units & currency

Units are (x)mol Fe biomass^-1 d^-1 or (x)mol Fe L^-1 d^-1. The currency is Fe.

Sample size

Typical samples are < 1 L in volume.

Limitations

The method assumes minimal isotopic fractionation between ⁵⁵Fe and ambient ^56/54Fe. In field incubations, bottle effects can alter iron speciation and uptake rates relative to in situ conditions. The distribution of iron between the organic-ligand-bound and inorganic forms (bioavailability) must equilibrate before incubation; inadequate equilibration leads to inaccurate estimates. Radiotracer recycling (released iron taken up again) can overestimate rates in longer incubations.

Example Applications & Protocols

Classic examples

• Hudson & Morel (1990) Iron transport in marine phytoplankton: kinetics of cellular and medium coordination reactions

Common calculations/conversions

• Fe uptake rate (mol Fe L^-1 d^-1) = (cpm_cells / cpm_{total dissolved}) × [Fe_dissolved] / incubation time.
• Biomass-specific rate = volumetric rate / [biomass]; requires cell counts or POC measurements.