Mycoheterotrophic plants living on arbuscular mycorrhizal fungi are generally enriched in ¹³C, ¹⁵N and ²H isotopes

1. Fully mycoheterotrophic plants are thought to obtain carbon exclusively from their root-associated fungal partners. The general enrichment of these plants in the heavy isotopes ¹³C and ¹⁵N suggests that fungi are the main nutrient source for these plants. Yet, the majority of studies have targeted mycoheterotrophic plants associated with ectomycorrhizal, orchid mycorrhizal and saprotrophic fungi, while mycoheterotrophic plants living on arbuscular mycorrhizal fungi remain understudied.
2. Here, we sampled 13 species of arbuscular mycorrhizal fully mycoheterotrophic plants from five families and co-occurring autotrophic reference plants growing in forests of tropical South America, tropical South East Asia and temperate Australasia. We measured stable isotope natural abundances (δ¹³C, δ¹⁵N, δ²H and δ¹⁸O), determined total nitrogen concentrations and used high-throughput DNA sequencing to characterize the arbuscular mycorrhizal fungal communities associated with the sampled mycoheterotrophic plants.
3. We observed a general enrichment in ¹³C and ¹⁵N isotopes across mycoheterotrophic plant families and geographic regions. We confirm cases where no ¹⁵N enrichment is present, but we show that in general arbuscular mycoheterotrophic plants are enriched in ¹⁵N. Moreover, we demonstrate for the first time that these plants are significantly enriched in ²H but not in ¹⁸O in relation to their autotrophic references. The fungal communities targeted by the mycoheterotrophs mainly consist of Glomeraceae and show strong association with the isotopic signatures and geographic origin of the plants.
4. Synthesis. Our findings enlarge the limited knowledge on the multi-element stable isotopic signatures of mycoheterotrophic plants living on arbuscular mycorrhizal fungi. We show that these plants are enriched in ¹³C and ²H as expected due to their mycoheterotrophic nutrition, and that in general they are also enriched in ¹⁵N, despite some exceptions. Variation in stable isotope signatures is likely influenced by plant taxonomy, geography and fungal community composition.