Two distinct nutritional syndromes have been described in temperate green orchids. Most orchids form mycorrhizas with rhizoctonia fungi and are considered autotrophic. Some orchids, however, associate with fungi that simultaneously form ectomycorrhizas with surrounding trees and derive their carbon from these fungi. This evolutionarily derived condition has been called mixotrophy or partial mycoheterotrophy and is characterized by 13C enrichment and high N content. Although it has been suggested that the two major nutritional syndromes are clearly distinct and tightly linked to the composition of mycorrhizal communities, recent studies have challenged this assumption. Here, we investigated whether mycorrhizal communities and nutritional syndromes differed between seven green orchid species that co-occur under similar ecological conditions (coastal dune slacks). Our results showed that mycorrhizal communities differed significantly between orchid species. Rhizoctonia fungi dominated in Dactylorhiza sp., Herminium monorchis, and Epipactis palustris, which were autotrophic based on 13C and N content. Conversely, Liparis loeselii and Epipactis neerlandica associated primarily with ectomycorrhizal fungi but surprisingly, 13C and N content supported mixotrophy only in E. neerlandica. This, together with the finding of some ectomycorrhizal fungi in rhizoctonia-associated orchids, suggests that there exists an ecological continuum between the two syndromes. The presence of a large number of indicator species associating with individual orchid species further confirms previous findings that mycorrhizal fungi may be important factors driving niche-partitioning in terrestrial orchids and therefore contribute to orchid coexistence.

The rise and diversification of land plants was accompanied by mycorrhizal symbiosis, from their emergence to their adaptation to various biomes and ecological situations (Selosse et al., 2015). In most mycorrhizal associations, fungi provide soil minerals to the plant, in exchange for sugars derived from photosynthesis (Smith & Read, 2008;van der Heijden et al., 2015). However, several plant species adapted to shaded forest conditions by secondarily reversing this exchange of carbohydrates: they became achlorophyllous thanks to carbon provided by the fungus. This so-called mycoheterotrophic nutrition is described in over 400 species and evolved at least 40 times independently (Merckx, 2013), raising the question of what predispositions underlie these convergences.