Winfrenatia reticulata

Three-component symbiosis

A closer look at thin sections from the Rhynie chert has uncovered a more intricate structure in Winfrenatia than the typical two-partner lichen model suggests. The ridged thallus consists mostly of empty, mucilage-lined sheaths that originally held a filamentous cyanobacterium similar to modern Nostocales; these sheaths create the three-dimensional pockets once thought to be fungal tissue. On the floor of each pocket sit clusters of spherical cyanobacterial cells, while true fungal hyphae are mostly found at the base, winding around and occasionally enveloping these cells. This setup shows the fungus primarily interacts with the coccoid partner, while the filamentous cyanobacterium acts as the thallus's structural framework. With two different cyanobacteria sharing a single fungus, Winfrenatia is an Early Devonian example of a three-membered symbiosis, not a conventional lichen. No existing lichen pairs a glomeromycete-like fungus with both filamentous and coccoid cyanobacteria, making this fossil arrangement unique. The researchers propose this collaboration might have supported early soil formation by linking photosynthesis, nitrogen fixation, and fungal nutrient uptake, helping pioneer communities endure the periodic drought and temperature fluctuations thought to occur in the Rhynie hot spring environment.