And unlike most teenage humans, lichen, you would be right.
Lichens are incredible organisms, and we understand so little about how they arise, exist, and respond to their surroundings. Lichens are the symbiosis between a fungus, a unicellular green or brown alga or cyanobacterium, and many, many colonizing bacteria. What looks like one colorful, fleshy, ruffled organism is actually thousands of photosynthesizing algae (called photobionts) cradled in webs of threadlike fungal mycelium (the mycobiont). What’s more, every new baby lichen must acquire its own photobionts! A baby mycobiont produced by adult lichen sexual reproduction has to fortuitously encounter and mesh with a new alga, or it will never become lichenized, grow, and reproduce itself.
There are many species of lichen (named after their fungal mycobiont), but recent work shows that there are actually many more species than we previously thought. These species are genetically very different, based on nuclear and mitochondrial DNA from the fungal mycobiont, but are morphologically cryptic: we just can’t visually or chemically tell them apart. That’s difficult enough, but it gets worse when you consider the algal photobiont. Shattering the peaceful idea that each individual lichen is a member of a certain species is the fact that one species of fungal mycobiont can form a lichenized thallus with one of many different species of algae. In some lichens, like the beautiful Ramalina menziesii, this results in clearly different physiology and morphology; however, in most lichens, this is just another source of wild and cryptic diversity within a set of individuals that look and behave nearly identically.
So what, in the name of all that is holy, is a species of lichen? How does a mycobiont choose its photobiont? Does the mycobiont have specific needs, selecting for a specific photobiont? Does the photobiont have specific needs, only surviving in certain mycobionts? Can a mycobiont accept nearly any photobiont, and their “choice” is equivalent to picking an available alga out of a hat? Finally, and most interestingly–who responds to the environment? Who evolves to prefer rock, soil, or leaf substrates? Who evolves to deal with dry, exposed environments? If we study lichens from different habitats, will different species of mycobiont be associated with different environmental conditions, or will the photobionts be the ones who change?
These are all things that we as biologists are working to understand. A recent paper by Leavitt et al. (2015) in the Journal of Molecular Ecology sequenced mycobiont and photobiont DNA from several important genera of lichens. They were searching for A) cryptic diversity (new species that we haven’t been able to identify using morphological features), B) To understand how specific the association was between each genus of mycobiont and their symbiotic photobiont, and C) To understand if lichens or photobionts could be mapped to certain environments, or “ecoregions”, as designated by the World Wildlife Fund (WWF). On this grand scale, they found that some lichen genera were incredibly specific, only living with one or two species of photobiont. Other lichen genera were the complete opposite, living with any of 10 or more photobiont species. The specificity of these mycobiont/photobiont relationships were not clearly explained by their residence in any particular ecoregion, or the broadness of their worldwide distribution.
A followup paper, Leavitt et al. (2016), in Molecular Phylogenetics and Evolution (fun name!) explored similar questions, but in one broad genus of lichen, Rhizoplaca. Within Rhizoplaca they again found that most species associated with one of several possible photobionts in the Trebouxia genus of green algae, and were generally showed no truly specific relationships. Again, on the scale of ecoregions, they found that ecosystem types could not explain Rhizoplaca distributions or their choice of Trebouxia photobiont.
While it could be that the fungal process of photobiont selection and subsequent lichenization is pretty much random, dependent only on the available pool of photobiont, the physiologist in me stubbornly continues to believe that the environment must play a role in the selection of lichens and their partners. I want to know the different UV and desiccation tolerance of the various Trebouxia species. I want to know where and when they are found in the wild, and how well they survive in microhabitats that differ in water availability and relative humidity. I want to know how association with different photobionts changes the growth rate or time to reproduction of the lichen mycobiont. I wonder if, when we study these traits, it will help us understand which lichen lives where, and why they associate with the photobionts that they do.
Mostly because I can’t take another lichen throwing my plates at the wall.