32-P incorporation

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

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Phosphate uptake (32P)
Approach: radiotracer (32P-phosphate) incubation
Context: incubation, lab
Spatial scale: point sample
Temporal scale: hours to days
Units: mol P L-1 h-1
Community captured: microalgae and bacteria
Co-measurements: cell abundance

Method Overview

Phosphate uptake rates are measured by adding ³²P-labelled inorganic phosphate (H₃³²PO₄) at tracer concentrations to seawater samples. Organisms take up the radiolabelled phosphate, and the amount incorporated into particulate material is measured after filtration onto polycarbonate membranes (0.2 or 0.7 µm) by liquid scintillation counting. The uptake rate is calculated from the fraction of added ³²P incorporated relative to the ambient + added phosphate concentration.

Scale of measurement

Sampling after hours to days incubation. Short incubations are preferred to minimize isotope dilution and bottle effects.

Data generated

Phosphate uptake rates (mol P L^-1 h^-1), reflecting the rate at which the community assimilates inorganic phosphate. At saturating substrate, V_max is obtained. Under near-ambient conditions, the in situ uptake rate is approximated.

Units & currency

Units are mol P L^-1 h^-1. The currency is phosphorus.

Sample size

Typical samples are < 1 L in volume.

Limitations

Rates represent V_max when substrate is added at saturating concentrations. Isotope dilution from ongoing regeneration of unlabelled Pi during the incubation can cause underestimation. ³²P is a high-energy beta emitter requiring appropriate radiation safety protocols. Bottle effects apply.

Example Applications & Protocols

Classic examples

• Duhamel et al. (2017) Phosphorus dynamics in biogeochemically distinct regions of the southeast subtropical Pacific Ocean

Common calculations/conversions

• P uptake rate (nmol P L^-1 h^-1) = (cpm_filter / cpm_{total added}) × [P_i,total] / incubation time; where [P_i,total] = [ambient Pi] + [³²P added].