The plurality of photobionts within single lichen thalli

Photobiont plurality refers to the presence of multiple photobionts within a single lichen thallus. This phenomenon was described at the end of the last century but has received deeper attention in the last decade. The available literature on this topic is limited and studies address only a few groups of lichens. Here, the most recent and important findings on photobiont plurality reported to date are revisited. The most commonly reported photobionts coexisting in a lichen thallus are algae of the genus Trebouxia, though other algae and cyanobacteria more or less phylogenetically related may also coexist together. A thallus with multiple photobionts may be formed at the beginning by the association of hyphae with several photobionts at once or by the acquisition of additional photobionts during life. The most important factors affecting the cooccurrence of multiple photobionts in a single thallus are likely the specificity and selectivity of mycobionts and local environmental conditions. Photobiont plurality has been investigated so far in about fifty species of lichen-forming fungi among the approximately 20,000 mycobiont species described. Coexisting photobionts differ in responses to several environmental, mostly stress conditions, suggesting that the presence of multiple photobionts in a thallus may provide an advantage for colonizing new habitats and may be essential for survival in localities with harsh or frequently changing conditions.