PS 13-179 - Stable isotopic composition of chitin amendment affects composition of microbial communities in mycorrhizal hyphosphere

Abstract: Stable isotopes have frequently and successfully been used to quantify nutrient and carbon transformation pathways and organisms involved therein in both simplified and complex ecosystems. The simplified artificial experimental systems are providing an elegant tool to specifically target functional guilds involved is specific processes such as N₂ or CO₂ fixation, organic pollutant detoxification etc. One of the underlying assumptions is that the organisms and enzymes do not discriminate between isotopically labelled and unlabelled substrates, or that the discrimination is negligible compared with the studied processes. This proved to be incorrect at least in case of heavily labelled deuterium substrates, which slow down microorganism growth and metabolism (so called isotopic effect). Yet, for heavier elements like carbon (C) or nitrogen (N) it is still widely assumed that isotopic labelling has no measurable effect on the rates of transformation and no differential effect on various microbes possibly competing for the shared resources. Here we used dually labelled chitin (100% ¹⁵N, 50% ¹³C) and unlabelled chitin of fungal origin to investigate whether isotopic labelling of such organic supplement to the root-free zone of mycorrhizal and nonmycorrhizal plants affected composition of microbial communities (bacteria, fungi, protists). Organic patches were further compared with patches added with inorganic N. Microbial communities were strongly affected by organic vs. inorganic N supply (which is everything but surprising), but also by isotopic composition of the chitin and by mycorrhizal fungal presence, in this order. All three experimental factors (i.e., mycorrhizal fungal presence, form of N, and isotopic labelling of chitin) mostly affected all three microbial guilds. Except mycorrhiza, which only caused significant and consistent effects on bacterial and protistan communities, and not on the (saprotrophic) fungi. Our results indicate that stable isotope probing experiments may need careful checks as to fulfilling basic assumptions of the method (particularly when heavily labelled substrate are being used), and possibly also adaptations of the results evaluation/interpretation to take into account (merely quantitative) shifts in microbial community composition as affected by stable isotopes.