Abstract
The nitrogen isotopic composition of amino acids is a valuable tool for elucidating organic matter cycling, with threonine (Thr) drawing particular attention due to its distinctive isotopic behavior. In this study, we examined nitrogen isotope fractionation of Thr through pure culture experiments involving four microorganisms: a bacterium (Escherichia coli), a yeast (Saccharomyces cerevisiae), and two archaeal species (Methanothermobacter thermautotrophicus, Halobacterium salinarum). When ammonia served as the nitrogen source, the isotopic offset between Thr and phenylalanine (Phe) aligned with previously reported values for phytoplankton. Conversely, when a mixture of free amino acids was supplied as the nitrogen source, both yeast and archaea exhibited negative nitrogen isotopic fractionation of Thr, consistent with the trend previously observed in animal studies. In contrast, E. coli demonstrated positive isotope fractionation, suggesting potential involvement of Thr biosynthesis. These findings offer critical insights for refining interpretations of the nitrogen isotopic composition of Thr in environmental organic matter studies.
