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

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. 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.
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. Synthesis: Our finding enlarges 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.