Is lichen symbiont mutualism a myth?

Abstract

AbstractLichens are classic examples of symbiosis, but some biologists have questioned whether the algal partner benefits from the relationship. Among the diverse lichen symbioses, the carbon transfer systems show remarkable convergences. When a compatible fungus is encountered, the alga proactively releases large amounts of carbohydrate, suggesting active participation rather than victimhood. Some lichen-related fungus–alga symbioses appear obligatory for the algal partner. Within true lichens, algal symbionts can persist at microsites where they might not otherwise be competitive, because of improved stress tolerance, reduced photoinhibition, protection from herbivores, and the more efficient moisture management and positioning for light interception that fungal structures provide. Algal clones continually disperse from the lichen thallus by diverse means, allowing the genotype to pioneer aposymbiotic colonies from a stable refuge. Because lichen-forming fungi conserve rather than consume their algal symbionts, the mutual self-interests of both partners substantially align in the stressful microhabitats where lichens are successful.