13C- and 15N-labeled algal exudates (isotope tracing & nanoSIMS)

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

---

Bacterial DOM assimilation (algal exudate tracing, nanoSIMS)
Approach: 13C/15N-labelled algal exudates + nanoSIMS single-cell analysis
Context: lab incubations
Spatial scale: single cell
Temporal scale: hours to days
Units: µmol C or N L-1 d-1; fmol cell-1 d-1 (single cell)
Community captured: heterotrophic bacteria
Co-measurements: temperature, DOC, bacterial abundance

Method Overview

Phytoplankton are labelled with ¹³C-bicarbonate and ¹⁵N-ammonium or -nitrate, and allowed to exude isotopically enriched dissolved organic matter (DOM) into the medium. This labelled algal exudate is then added to seawater containing heterotrophic bacteria, and the incorporation of ¹³C and ¹⁵N from algal-derived DOM into bacterial biomass is tracked over time. At the single-cell level, nanoSIMS measures the ¹³C:¹²C and ¹⁵N:¹⁴N ratios in individual bacterial cells, revealing which bacteria in the community are actively assimilating algal exudates and at what rate. This approach links bacterial metabolic activity directly to algal DOM production at the single-cell level.

Scale of measurement

Single-cell resolution on incubations of hours to days. Lab-based method that requires controlled co-cultures or mesocosm incubations.

Data generated

Per-cell carbon and nitrogen assimilation rates from algal exudates (fmol cell^-1 d^-1). The community-level transfer rate is obtained by multiplying per-cell rates by bacterial abundance. Identifies functional guilds of bacteria specialized in algal DOM assimilation.

Units & currency

Units are µmol L^-1 d^-1 (community) or fmol cell^-1 d^-1 (single cell). The currency is carbon and nitrogen.

Sample size

Typical samples are < 1 L in volume.

Limitations

The labelled exudate pool is a proxy for in situ algal DOM but may differ in composition from natural exudates. Low throughput of nanoSIMS analysis limits the number of cells measured. Isotope dilution from unlabelled background DOM reduces sensitivity, particularly over longer incubations.

Example Applications & Protocols

Classic examples

• Mayali et al. (2023) Single-cell isotope tracing reveals functional guilds of bacteria associated with the diatom Phaeodactylum tricornutum

Common calculations/conversions

• Per-cell C assimilation rate = [(¹³C/¹²C)_cell − (¹³C/¹²C)_natural] / [(¹³C/¹²C)_exudate − (¹³C/¹²C)_natural] × C_cell / incubation time.