A Simple, High-Precision, High-Sensitivity Tracer Assay for N(inf2) Fixation.

Area of Science:

• Marine biology
• Biogeochemistry
• Environmental science

Background:

• Nitrogen fixation is crucial for marine ecosystems.
• Accurate measurement of nitrogen fixation is essential for understanding nutrient cycling.
• Existing methods like acetylene reduction assay have limitations.

Purpose of the Study:

• To present a simple, precise, and sensitive protocol for direct measurement of N2 fixation.
• To validate a new method using stable isotope tracing with 15N2.
• To compare the new method with the acetylene (C2H2) reduction assay.

Main Methods:

• Utilized a modern, multiple-collector isotope ratio mass spectrometer for high sensitivity.
• Employed 15N2 tracer additions to ambient N2 pools in plankton incubations.
• Conducted experiments in the Gotland Basin, Baltic Sea, comparing 15N2 fixation with C2H2 reduction.

Main Results:

• The 15N2 tracer method achieved a low limit of detection for N2 fixation.
• Sensitivity and precision were comparable to or better than the C2H2 reduction assay.
• Excellent agreement was observed between the two methods, with a fixed ratio of C2H2 reduced to N2 fixed (4.68 ± 0.11).

Conclusions:

• The 15N2 tracer protocol is a valid and sensitive method for direct N2 fixation measurement.
• The study confirms the reliability of the C2H2 reduction method in the Baltic Sea system.
• The developed 15N2 protocols are recommended for broader application in other marine systems.