Frankia alni is a Gram-positive species of actinomycete filamentous bacterium that lives in symbiosis with actinorhizal plants in the genus Alnus.

Alnus species grow in a wide range of habitats that include glacial till, sand hills, the banks of water courses, bogs, dry volcanic lava flows and ash alluvium.

[4] The first symptom of infection by Frankia alni is a branching and curling of the root hairs of the alder as the bacterium moves in.

These are roughly spherical cellular structures that measure two to six millimetres in diameter and have a laminated lipid envelope.

The vesicles serve to limit the diffusion of oxygen, thus assisting the reduction process that is catalysed by the enzyme nitrogenase.

The final product of the reactions is glutamate, which is thus normally the most abundant free amino acid in the cell cytoplasm.