Pii: S0169-5347(97)01230-5

15 Living on emerged land raises several problems: water may be rare or absent, implying (1) that mineral nutrition is more or less impaired, (2) radiation (mainly toxic UV-rays) is not filtered out and (3) strong fluctuations of temperature are not prevented. In spite of those adversities, Precambrian land was colonized by phototrophic microorganisms, probably prokaryotes1, and multicellular terrestrial phototrophs arose during the Silurian, about 450 million years ago2. How did phototrophs overcome land stresses? Nowadays, the pioneer colonizers are mainly lichens – not simple phototrophs, but a mutualistic association of a phototroph with a fungus. At closer inspection, almost all present-day phototrophs adapted to terrestrial ecosystems form mutualistic associations with fungi (Box 1), which are often lost in secondarily aquatic phototrophs, as documented for mycorrhizal symbiosis3. The fungi involved in these associations, the ‘mycobionts’, are non-septate Zygomycotina and septate Ascomycotina and Basidiomycotina. Fossil record4 and molecular clock data5 (Box 2) suggest that terrestrial non-septate fungi originated during the Cambrian, and septate fungi during the Devonian. They thus had opportunities to interact with the terrestrial flora during much of its evolution. Here, we review new evidence that (1) mutualism with fungi is widespread among extant phototrophs and ancient in land ecosystems and (2) that it allowed various critical adaptations during the evolution of land phototrophs.