Dark septate endophytes and arbuscular mycorrhizal fungi (Paris-morphotype) affect the stable isotope composition of 'classically' non-mycorrhizal plants

The vast majority of terrestrial plants exchange nutrients with fungal partners forming different mycorrhizal types. The minority of plants considered as non-mycorrhizal, however, are not necessarily free of any fungi, but are frequently colonized by elusive fungal endophytes, such asdark septate endophytes(DSE) orfine root endophytes(FRE). While a functional role ofFREin the improvement of nutrient gain was recently elucidated, the function ofDSEis still in discussion and was here addressed for 36 plant species belonging to the families Equisetaceae, Cypereaceae and Caryophyllaceae. Molecular and microscopic staining approaches were conducted to verify the presence ofDSEin the investigated species. Stable isotope natural abundances of the elements carbon, nitrogen, hydrogen and oxygen and total nitrogen concentrations were analysed for the respective species of the target plant families and accompanying mycorrhizal and non-mycorrhizal (Brassicaceae) plant species. Staining approaches confirmed the presence ofDSEin all investigated species within the families Equisetaceae, Cyperaceae and Caryophyllaceae. A co-colonization withParis-type arbuscular mycorrhiza (AM) was occasionally found by staining and molecular approaches in species of the Equisetaceae. Species of the Equisetaceae, Cyperaceae and Caryophyllaceae were significantly(15)N-enriched in comparison to accompanying plants. In addition, a significant(13)C and(2)H enrichment and increased total nitrogen concentrations were found for representatives of the Equisetaceae. The(15)N enrichment found here for representatives of Equisetaceae, Cyperaceae and Caryophyllaceae provides evidence for a functional role of the ubiquitousDSEfungi.DSEfungi obviously provide access to(15)N-enriched soil organic compounds probably in exchange for organic carbon compounds from plant photosynthesis. As indicated by additional(13)C and(2)H enrichments, representatives of the Equisetaceae apparently gain simultaneously organic carbon compounds from their AM fungi of theParis-morphotype. Thus, species of the Equisetaceae have to be considered as partially, or in case of the achlorophyllous fertileEquisetum arvense, as fully mycoheterotrophic at least in some stages of their life cycle. So far, mostly underappreciated fungi classified asDSEare suggested to occupy an ecologically relevant role similar to mycorrhizae and the occurrence of simultaneous functions ofDSEand AM fungi in Equisetaceae is proposed. A freePlain Language Summarycan be found within the Supporting Information of this article.