[5] It was here he met the flamboyant Professor of Plant Ecology and lichen taxonomist Gustaf Einar Du Rietz, with whom he undertook studies on crustose lichens found on shoreline rocks of Swedish lakes.
[6] That same year, joined by zoologist Christian Olrog,[7] Santesson embarked on what was supposed to be an eight-month research excursion in Patagonia (southern South America).
Olrog wrote a book in Swedish about their trip called Destination Eldslandet (1943), which tracked the adventures of the expedition.
[7] After his return Santesson worked on taxonomic treatments of the material he collected from South America, which resulted in the revision of several genera, including Menegazzia,[8] Cladina,[9] Dolichocarpus, Xanthopeltis,[10] and calicioid lichens.
[11][12] His work on Menegazzia has been called "an important and basic landmark in South American lichenology".
From 1973 to 1981, Santesson was professor and director of the Botanical Department at Stockholm's Swedish Museum of Natural History.
[15] Santesson undertook major lichen-collecting expeditions throughout the world, including Europe (British Isles, France, Madeira, Portugal, Switzerland, Spain); North America (United States; Mexico) Tenerife; Iceland; Africa (Kenya and Tanzania); South America (Patagonia; Peru); and Asia (China; Far East of Russia).
As Robert Lücking has noted, "he was the first to predict that species with different ascospores but otherwise similar characters, so-called sporomorphs, form closely related series."
Although this concept had been largely neglected by other taxonomists, his theory was validated decades later with the use of molecular phylogenetics in the genus Thelotrema.