The microscopic life in soils is responsible for a large macroscopic flow of matter on a planetary scale. But the complexity of ecosystems and the organisms that make them up makes it difficult to move from one scale to another. The thesis entitled ‘Linking scales in arbuscular mycorrhizal symbiosis’ weaves together objects ranging from the size of a lipid droplet ( ~ 500 nm) to the total length of a fungal hypha on a planetary scale (~ 10^20m).

The first chapter introduces a framework for analysing fungal colonies using ‘propagation waves’, linking the microscopic scale of individual hyphal behaviour to macroscopic variables such as density and propagation velocity. These results constitute the first analysis of experimental data with a high degree of spatiotemporal resolution of the morphogenesis dynamics of a branched organism.

Chapter 2 describes in detail the ecological consequences of this macroscopic propagation by quantifying the exchange of resources involved. This result sheds new light on the science of mycorrhizae by making it possible to link phenotypic traits of fungal colonies to their consequences for plant growth in different environments.

The third chapter focuses on the link between matter flows in the fungal colony and hyphal elongation and thickening, highlighting the importance of lipid transport in these fungi, which are responsible for significant carbon flows on a global scale.

Finally, in a separate chapter, the results are projected onto the scale of ecosystems. In particular, the work on hyphal radius makes it possible to reduce the uncertainty in calculating the total biomass of mycorrhizal fungi in an ecosystem and in Arbuscule on a global scale.