15N/13C-labelled DOM/cells

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

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Osmotrophic mixotrophy (13C/15N DOM assimilation)
Approach: 13C/15N-labelled dissolved or particulate organic matter; EA-IRMS or nanoSIMS
Context: incubation, lab
Spatial scale: single cell to community
Temporal scale: 24 h
Units: µmol C or N L-1 d-1; fmol cell-1 d-1 (nanoSIMS)
Community captured: osmotrophic mixotrophs (e.g., specific flagellate species or picophytoplankton)
Co-measurements: temperature, PAR, bulk POC and PON, cell abundance

Method Overview

¹³C- and ¹⁵N-labelled dissolved organic matter (DOM) or particulate organic cells are added to incubation bottles containing natural seawater communities. Osmotrophic mixotrophs — organisms capable of both photosynthesis and uptake of dissolved or particulate organic compounds — incorporate the labelled substrates into their biomass. After incubation, the bulk ¹³C and ¹⁵N enrichment of the particulate fraction is measured by EA-IRMS, providing community-level assimilation rates. For single-cell resolution, cells are fixed, filtered, and analysed by nanoSIMS, revealing which individual cells (identified by FISH or flow cytometry) assimilated the labelled substrate and at what per-cell rate.

Scale of measurement

Discrete samples are collected and incubated for 24 h. NanoSIMS provides single-cell resolution with ~50 nm spatial precision.

Data generated

Community-level DOM assimilation rates (µmol C or N L^-1 d^-1) from EA-IRMS, or per-cell assimilation rates (fmol C or N cell^-1 d^-1) from nanoSIMS. Identifies which populations in the community are actively assimilating organic substrates.

Units & currency

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

Sample size

Typical samples are > 1 L in volume for bulk EA-IRMS; mL for nanoSIMS.

Limitations

There is a trade-off between incubation time (needed for detectable ¹³C/¹⁵N enrichment) and confidence that the label has not cycled through multiple steps (e.g., DOM mineralized and re-fixed by autotrophs). Bottle effects apply. For nanoSIMS, the low throughput (tens to hundreds of cells per session) limits statistical power.

Example Applications & Protocols

Recent applications

• Millette et al. (2023) Mixoplankton and mixotrophy: future research priorities

Common calculations/conversions

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