Nitrogen content

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

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Phytoplankton nitrogen content
Approach: elemental analysis (CHN), Redfield assumption, or 15N incorporation
Context: environmental, lab
Spatial scale: point sample; single cell
Temporal scale: snapshot
Units: µg N L-1; pg N cell-1
Community captured: group and bulk
Co-measurements: cell abundance

Method Overview

Particulate organic nitrogen (PON) is measured by CHN elemental analysis of GF/F-filtered seawater samples. Group- or species-specific nitrogen content is obtained by flow cytometric sorting followed by CHN analysis. Alternatively, PON can be estimated from particulate organic carbon (POC) by applying the Redfield C:N ratio (6.6 by atoms), or derived from ¹⁵N tracer incubations by tracking the enrichment of ¹⁵N in the particulate fraction. Cell-specific N content can also be estimated from cell biovolume combined with taxon-specific C:N:volume relationships.

Nitrogen content is used to normalize N uptake rates, to calculate cellular N:C ratios as an indicator of nutrient status, and as a co-measurement alongside carbon content and primary production rate data.

Scale of measurement

Bulk PON measurements provide a community-level point estimate. Single-cell measurements (sorted CHN or nanoSIMS) provide cellular resolution.

Data generated

Bulk PON (µg N L^-1) or per-cell nitrogen content (pg N cell^-1). N:C ratios characterize the nutrient status of the phytoplankton community (Redfield ratio = 0.15 mol N mol^-1 C).

Units & currency

Units are µg N L^-1 (bulk) or pg N cell^-1. The currency is nitrogen.

Sample size

Sample volumes range from single-cell to liters, depending on the approach.

Limitations

Bulk PON includes detrital and non-living nitrogen; phytoplankton-specific estimates require cell sorting. The Redfield ratio assumption introduces uncertainty in nutrient-stressed communities where N:C deviates strongly from the canonical 6.6. CHN analysis requires careful blank correction and GF/F pre-combustion. As with POC, detection limits can be a constraint in oligotrophic waters.

Example Applications & Protocols

Common calculations/conversions

• Redfield-based PON estimate: PON (µmol N L^-1) = POC (µmol C L^-1) / 6.6.
• Nutrient status index: N:C ratio vs. Redfield; N:C < 0.15 indicates nitrogen limitation.