Thread by @metalichen

New #preprint alert! What are the core symbionts of lichens? TLDR: we analyzed >400 lichen metagenomes from around the world and found that 4 bacterial groups are as frequent in lichens as the alga, one of the two main partners in the symbiosis. 🧵1/11 biorxiv.org/content/10.110…

We aimed to create a global census of lichen-associated organisms based on shotgun metagenomics, and independent from biases associated with culturing and PCR amplification. We used nearly every publicly available lichen metagenome plus de novo data 2/11

Originally, we expected lichen bacteria to be a more or less random ‘soup’ of Proteobacteria. But we were wrong. In reality, the majority of lichen bacteria came from just four families. What’s more, a single bacterial * genus * occurred in ~1/3 of all lichens sampled 3/11

This super-frequent bacterium, Lichenihabitans aka LAR1, was known from ~25 lichens before, but now we know it is ubiquitous. Which begs the question - what does it do in the context of lichen symbiosis? And that brings me to another big result 4/11

Lichenihabitans is in Rhizobiales, and many hypothesized that it and other lichen bacteria fix nitrogen and supply it to eukaryotic symbionts. Based on our genome annotations, we found NO evidence that lichen bacteria (other than cyanobacterial photobionts) fix nitrogen 5/11

So what do lichen bacteria do? The most frequent bacteria appear to synthesize essential vitamins and participate in cycling lichen biomass. Some of these bacteria might be methylotrophs, but that’s not even the most interesting part 6/11

We predict that many lichen bacteria - including Lichenihabitans - are photosynthetic heterotrophs, which use light as an additional energy source. This was known before from one lichen bacterium, but turned out to be a major theme 7/11

We also found that basidiomycete fungi found in previous studies could be detected in around 2/3 of metagenomes. Their detectability, like that of the bacteria, declines with decreasing sequencing depth, but there are ways to ferret them out. 8/11

So, what did we learn about lichens? A model is emerging of three-way metabolic complementarity, one that will need lab testing, now with very specific hypotheses. 9/11

Huge thanks to everyone involved in the paper, especially Paul Saary, @ellenscameron, and @robdfinn, as well as @theironark, @ddiazescandon, @veera_tuovinen, @OPasso1, @LisaYStein1, Spencer Goyette, Helmut Mayrhofer, Håkon Holien, Tor Tønsberg, and, of course @TobySpribille 10/11

Finally, thanks to @TobySpribille (microscopy) and @Abelosokh (false coloring) for the beautiful lichen image in the first tweet 11/11