Radioactively labeled prey surrogates

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

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Bacterivory (radiolabelled prey surrogates)
Approach: 3H or 14C radiolabelled bacteria; filtration and scintillation counting
Context: incubation
Spatial scale: point sample
Temporal scale: minutes to hours
Units: prey cell-1 h-1
Community captured: bulk protistan grazers
Co-measurements: abundances of predator and prey; carbon content

Method Overview

Bacteria are radiolabelled by growing them in the presence of ³H-thymidine or ¹⁴C-substrates, so that their DNA or biomass carries a radioactive label. These labelled bacteria are added to seawater samples at near-natural densities and allowed to be consumed by protistan grazers during a short incubation. After incubation, samples are filtered through two successive filters: larger pores (e.g., 1 µm) retain the protistan grazers while allowing unlabelled and labelled bacterial prey to pass; the filtrate is then passed through a smaller filter retaining bacteria. Radioactivity on the large-pore filter (protists with ingested labelled bacteria) is measured by liquid scintillation counting and converted to ingestion rates.

Scale of measurement

Discrete samples collected after short incubation (minutes to one hour).

Data generated

Bulk ingestion rate of radiolabelled bacteria by the protistan community (cells L^-1 h^-1 or ng C L^-1 h^-1).

Units & currency

Units are prey cell^-1 h^-1. The currency is cells.

Sample size

Typical samples are < 1 L in volume.

Limitations

The method assumes no selective grazing, i.e., that the radiolabelled prey are consumed at the same rate as natural bacteria. Good separation of labelled bacterial prey (passing through the filter) and protistan predators (retained on the filter) must be verified; any labelled bacteria retained on the larger-pore filter inflate apparent grazing. The approach provides community-level rates and cannot resolve which individual predator taxa are grazing.

Example Applications & Protocols

Classic examples

• Taylor & Sullivan (1984) The use of ¹⁴C-labeled bacteria as a tracer of ingestion and metabolism of bacterial biomass by microbial grazers

Common calculations/conversions

• Ingestion rate = (cpm_{grazer filter} / cpm_{labeled bacteria per cell}) / (predator abundance × incubation time).