Fluorescently labeled prey surrogates

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

---

Bacterivory (fluorescently labelled prey surrogates)
Approach: fluorescently labelled bacteria (FLB) or beads added to seawater; ingestion counted by epifluorescence microscopy or FCM
Context: incubation
Spatial scale: point sample
Temporal scale: minutes to hours
Units: prey cell-1 h-1
Community captured: bulk or size-fractionated protistan grazers
Co-measurements: abundances of predator and prey; carbon content of prey and predator

Method Overview

Fluorescently labelled bacteria (FLB) or fluorescent latex microspheres (FLA, fluorescently labelled algae) are prepared to mimic the size and surface properties of natural bacterial prey. FLB are prepared by heat-killing natural bacteria or cultures, staining with fluorescent dyes (e.g., DTAF, DAPI, or SYBR Gold), and resuspending in filtered seawater. These surrogates are added to seawater at concentrations similar to natural bacteria (~5–10% of natural bacterial abundance). After a short incubation (typically 20–60 min), samples are fixed and filtered onto black polycarbonate membranes. Protistan grazers on the filters are examined by epifluorescence microscopy; the number of FLB or beads ingested per predator cell is counted and divided by incubation time to give the ingestion rate.

Scale of measurement

Short incubations (minutes to one hour) to minimize artifacts from bottle confinement and secondary ingestion of labelled particles by non-target grazers.

Data generated

Per-cell ingestion rates (FLB prey cell^-1 h^-1) for individual protistan taxa or for the bulk protist community. Multiplication by predator abundance gives volumetric bacterivory rates.

Units & currency

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

Sample size

Typical samples are < 1 L in volume.

Limitations

FLB are assumed to be consumed at the same rate as natural bacteria, but prey selectivity means some protists preferentially ingest live over dead or heat-killed prey, leading to underestimation of actual bacterivory. The selection of surrogate type (beads vs. bacteria; cultured vs. enriched from natural community; heat-killed vs. live) substantially affects measured ingestion rates and must be considered when comparing results across studies.

Example Applications & Protocols

Classic examples

• Sherr et al. (1987) Use of monodispersed, fluorescently labeled bacteria to estimate in situ protozoan bacterivory

Common calculations/conversions

• Ingestion rate (cells predator^-1 h^-1) = (mean FLB per predator cell) / incubation time; corrected for the fraction of the total bacterial pool that FLB represent.
• Community bacterivory (cells L^-1 h^-1) = ingestion rate × predator abundance.