Fractionation of nitrogen isotopes during amino acid metabolism in heterotrophic and chemolithoautotrophic microbes across Eukarya, Bacteria, and Archaea: Effects of nitrogen sources and metabolic pathways

The nitrogen isotopic composition of amino acids (delta N-15(AA)) is a promising tool for estimating the trophic position and nitrogen sources of organisms in grazing food webs. The utility of delta N-15(AA) analysis of microbial processes in the biogeochemistry remains uncertain, because information about the delta N-15(AA) values of chemotrophic microbes are still limited. In this study, we report the delta N-15(AA) values in five cultured heterotrophic or chemolithoautotrophic microbes (one fungus, one bacterium, and three archaea) with controlled nitrogen sources in synthetic media. When the microbes were grown on ammonium, the apparent nitrogen isotopic fractionation of amino acids relative to glutamic acid in the microbes (e.g., -3.0 +/- 1.0 parts per thousand for phenylalanine) was similar to those in algae. When the microbes were grown on a free amino acid mixture, the delta N-15 offsets of amino acids in the microbial cells relative to the amino acids in the culture media (e.g., + 8.2 +/- 0.8 parts per thousand for glutamic acid and +0.1 +/- 0.2 parts per thousand for phenylalanine) were close to the standard isotopic fractionation between animals and their diets. These results suggest that the delta(15)N(AA)d pattern can be used to distinguish two end-member metabolic modes in microbes (i.e., de novo synthesis of amino acids from inorganic nitrogen vs. assimilation of amino acids from environments). We also discussed the effects of metabolic pathways on the delta N-15(AA) values and their fractionation with respect to the diversity of amino acid metabolism in microbes. These findings will form an important baseline for interpreting the delta N-15(AA) values of organisms and detritus in natural environments. (C) 2017 Elsevier Ltd. All rights reserved.