2025 Volume 59 Issue 2 Pages 27-57
This paper is an attempt to generalize from my own perspective the information held by nitrogen isotope ratios of major cellular components such as amino acids and chlorophyll, based on results with which I have been deeply involved. Particular attention is paid to compounds that are essential for vital activities as primary metabolites. Cells have an autonomous mechanism, sometimes called homeostasis, that controls the flow of substances within the cell. The nitrogen isotopic ratio of the compound i constituting the cell can be generalized as δ15Ni=δ15Nplant +Δi(TP-1)+γi, where TP represents the trophic position, and Δi and γi are the trophic discrimination factor and the intracellular 15N distribution of compound i, respectively. Food web analysis is one of the rapidly expanding applications of the above equation. Such studies are related to such diverse topics as the relationship between environment and ecosystem, the assessment of nitrogen resources, the tracking of fish migrations, and the reconstruction of ancient human diets. The nitrogen isotopic ratio of chlorophyll, which is synthesized from amino acids and has a light-harvesting function in photosynthesis, is useful for studying the nitrogen cycle associated with primary production in the modern and recent ocean. The nitrogen isotopic ratio of porphyrins, the geological derivatives of chlorophyll, provides accurate and precise information on the nitrogen cycle through geologic time. This information may provide constraints not only on the dynamics of nitrogen at the Earth’s surface, but also on the degassing of nitrogen from the Earth’s interior. The importance of isotopic ratios of molecules deeply involved in life from the complex mixtures of nature has been reaffirmed, and they are becoming increasingly important in Earth environmental research.
