Abstract
Excerpt: Lichen-forming fungi and their algal symbionts together produce composite thalli that often have the appearance and properties of unitary organisms. Although the exact contours of the concept may vary according to viewpoint, lichens have been commonly distinguished from other fungal–algal symbioses by a useful and widely cited definition (Hawksworth, 1988; Hawksworth & Honegger, 1994), wherein the mycobiont comprises the external component (exhabitant) that houses the interior but extracellular algal symbiont(s). More recently, increased attention has been focused on additional microorganisms, particularly fungi and bacteria, that form part of the lichen thallus microbiome (Grube et al., 2015; Spribille et al., 2016; Cernava et al., 2017; Smith et al., 2020; Tzovaras et al., 2020; Cometto et al., 2022). This has revealed a new dimension to lichen biology, similar to that recognizable in plants and animals as the diverse microbial inhabitants of their internal and external surfaces are explored (Porras-Alfaro & Bayman, 2011; Ezenwa et al., 2012; Gilbert et al., 2018). Such research highlights the degree to which biological communities coexist at very different scales. A plant, an animal, and a lichen thallus are all components of a broader community of interacting macro-organisms, while at a finer level of organization, each may individually harbor its own microbial community or communities. Although microbiome research has changed how we think about plants and animals, it has not resulted in any effort to redefine them. By contrast, a number of lichen biologists have recently proposed expanding the definition of a lichen to include other microorganisms that colonize the thallus. These proposals, and indeed the question of whether a new definition is warranted or desirable, ought to be considered critically.
