Understanding how lichen reproduce is not simple, as Toby Spribille explains in his commentary Lichen symbionts outside of symbiosis: how do they find their match?. “While leading lichen walks for the public, I like to point out that although some of the most common lichens globally reproduce by vertical transmission, with “ready-to-go” bundles of fungal hyphae and algae, the vast majority of lichen species worldwide are formed by fungal partners that reproduce sexually, mostly via ascospores. These fungi, I explain, have to find a new partner every time they form a new lichen; in the language of symbiosis science, they acquire symbionts horizontally. This almost invariably leads to the question ‘how then do the fungus and alga find each other?'” This reproduction problem is what Cardós and colleagues examine, using the rare lichen Pectenia plumbea.
A lichen, in its simplest form, is a partnership of two organisms — a fungus, known as the mycobiont and an alga, known as the photobiont. The alga produces carbohydrates that the fungus takes. In turn, the fungus provides shelter and some nutrients to the alga. The mycobiont needs the photobiont for energy. So when the mycobiont sexually reproduces, it needs to associate with a suitable alga to germinate.
It’s a lot easier for asexually reproductive lichens. They can simply send out propagules, packages with both mycobionts and photobionts. But how much more difficult can sexual reproduction be for a lichen?
Cardós has been asking this question after looking at the genetic diversity of P. plumbea. In central Spain, there’s hardly any diversity at all, and the prevailing thought had been that it showed that P. plumbea had limited long-distance dispersal. Cardós and colleagues wondered if sexual reproduction could be the problem. If it’s difficult to find a new photobiont to partner with, then dispersal because wherever the fungal spores land, they fail to germinate.
What they found was that P. plumbea needs photobionts of the Nostoc lineage, a specific type of cyanobacteria. Looking at these cyanobacteria, Cardós and colleagues found that these bacteria were not very mobile and lacked the equipment to set up symbiosis with a fungus easily.
This discovery means that P. plumbea has a mycobiont that cannot germinate without help from a photobiont, and a photobiont that is not very good at setting up relationships with a mycobiont.
The rescuer is the lichen Dendriscocaulon umhausense. D. umhausense is a lichen that has the Nostoc bacteria that P. plumbea needs. D. umhausense reproduces asexually, so it has no trouble establishing itself. If P. plumbea mycobiont spores land on D. umhausense, the P. plumbea spores can scavenge the cyanbacteria for themselves.
“This constitutes a clear case of lichen facilitation,” write Cardós and colleagues in their article. “The ‘core species’ also benefits from the situation as a proportion of its photobionts that are dispersed onto sub-optimal substrates are ‘scavenged’ into the thalli of other guild members.”
Commenting on the paper, Spribille wrote: “Microbial cell interactions in nature are notoriously difficult to observe directly, but molecular approaches such as those adopted by Cardós et al. have helped us understand symbiont pairings and develop testable hypotheses about recruitment… Pairing approaches such as amplicon sequencing with identification of the cellular sources of lichen symbiont DNA that is regularly detected on and in natural surfaces may yield important clues to the lives of symbionts outside of the symbiosis.”