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Background  The use of enriched stable isotopes is of outstanding importance in chemical metrology as it allows the application of isotope dilution mass spectrometry (IDMS). Primary methods based on IDMS ensure the quality of the analytical measurements and traceability of the results to the international system of units. However, the synthesis of isotopically labelled molecules from enriched stable isotopes is an expensive and a difficult task. Either chemical and biochemical methods to produce labelled molecules have been proposed, but so far, few cost-effective methods have been described.    Results  The aim of this study was to use the microalgae  Chlamydomonas reinhardtii  to produce, at laboratory scale,  15 N-labelled amino acids with a high isotopic enrichment. To do that, a culture media containing  15 NH 4 Cl was used. No kinetic isotope effect (KIE) was observed. The labelled proteins biosynthesized by the microorganism were extracted from the biomass and the  15 N-labelled amino acids were obtained after a protein hydrolysis with HCl. The use of the wall deficient strain CC503 cw92 mt+ is fit for purpose, as it only assimilates ammonia as nitrogen source, avoiding isotope contamination with nitrogen from the atmosphere or the reagents used in the culture medium, and enhancing the protein extraction efficiency compared to cell-walled wild type  Chlamydomonas . The isotopic enrichment of the labelled amino acids was calculated from their isotopic composition measured by gas chromatography mass spectrometry (GC–MS). The average isotopic enrichment for the 16 amino acids characterized was 99.56 ± 0.05% and the concentration of the amino acids in the hydrolysate ranged from 18 to 90 µg/mL.    Conclusions  Previously reported biochemical methods to produce isotopically labelled proteins have been applied in the fields of proteomics and fluxomics. For these approaches, low amounts of products are required and the isotopic enrichment of the molecules has never been properly determined. So far, only  13 C-labelled fatty acids have been isolated from labelled microalga biomass as valuable industrial products. In this study, we propose  Chlamydomonas reinhardtii  CC503 as a feasible microorganism and strain to produce labelled biomass from which a standard containing sixteen  15 N-labelled amino acids could be obtained.    Electronic supplementary material  The online version of this article (doi:10.1186/s12934-017-0759-9) contains supplementary material, which is available to authorized users.

A cost-effective approach to produce 15N-labelled amino acids employing Chlamydomonas reinhardtii CC503