Studying trophic interactions between a plant pathogen and two different antagonistic microorganisms using a 13C-labeled compound and isotope ratio mass spectrometry

A microorganism labeled with stable isotope can be monitored in the environment and, if it is directly parasitized or its metabolites are used by other microorganisms, the label can be detected in that organism using isotope ratio mass spectrometry (IRMS). In this study, we labeled cells and DNA of Armillaria mellea (fungal plant pathogen), Trichoderma atroviride (fungus that acts as a mycoparasite of A. mellea) and Pseudomonas fluorescens (bacterium that has a toxic effect on A. mellea) with the stable isotope 13C. The kinetics of the uptake of 13C by the microorganisms were observed via the variation in the 13C/12C ratio (δ13C) over time. A. mellea absorbed this isotope more slowly (plateau at 21 days) than T. atroviride SC1 and P. fluorescens Pf-5 (plateaus at 3 days and 1 day, respectively). A. mellea and T. atroviride had higher δ13C values than P. fluorescens. In A. mellea and T. atroviride, the 13C values of the DNA samples were lower than those for the whole cells. However, this trend was not observed in P. fluorescens. IRMS allowed the detection of the active degradation and assimilation (via metabolites) of the labeled A. mellea by T. atroviride. The T. atroviride that was in contact with labeled A. mellea had higher δ13C values. On the other hand, little assimilation of 13C was detected in P. fluorescens Pf-5. This bacterium produces substances that can kill A. mellea. For this reason, only secreted or leached metabolites are thought to be assimilated by P. fluorescens Pf-5.