The role of nitrogen in the nutrition, function and distribution of Mucoromycotina ‘fine root endophyte’ fungi

Mycorrhizal symbioses are fundamental components of terrestrial ecosystems, forming in most plants and underpinning critical ecological processes. Central to these associations is carbon for nutrient exchange, in which fungi supply macronutrients to the host in return for carbon-based photosynthates. In nitrogen limited habitats, facultative saprotrophic mycorrhizal fungi can alleviate limitations on plant growth by directly accessing organic nitrogen pools. There is growing evidence that a group of mycorrhiza-forming fungi, Mucoromycotina 'fine root endophytes' (MFRE), share this capacity by transferring substantial nitrogen to host plants and acquire nitrogen from amino acids and algal necromass. However, the range of soil organic nitrogen substrates accessible to MFRE, and their responses to specific nitrogenous substrates has primarily been tested in vitro. In this thesis I aim to investigate whether MFRE’s saprotrophic traits may confer nutritional benefits to plant communities in nature.
I establish that two MFRE isolates can grow on a broader range of organic nitrogenous substrates that are present in the soil than previously thought. Using a field-relevant nitrogen gradient, I also show that MFRE transfer greater quantities of nitrogen and phosphorus to a host when inorganic nitrogen is more scarce.
Using previously resolved phylogenies, I place MFRE isolates within the family Densosporaceae. I used this placement to target MFRE associated with Ranunculus repens roots in different habitats across the Peak District National Park to determine whether nitrogen and underlying abiotic factors influence their distribution, composition and abundance. I found that their distribution was likely not driven by soil nitrogen but were context-dependant.
My work demonstrates MFRE function similarly to facultatively saprotrophic fungal lineages. Importantly, my work contextualises these findings in natural systems. My findings highlight that even though nitrogen plays an important role in their symbiosis, its influence in natural systems remains one of many factors interacting and influencing their function.