One of the biggest plant-related stories of recent times concerns one of the smallest entities and makes the point that you shouldn’t just look so far and no further. It concerns those shape-shifting compound organisms, lichens, that can appear like leaves, tiny bushes and all sorts of morphotypes in between.
Traditionally, these communities* have been held up as great examples of a mutualistic symbiosis involving two partners – an ascomycete fungus, the mycobiont, and a photosynthetic ‘tenant’ lodged within and between the threads of the host fungus. The tenant, or photobiont, is usually either a unicellular green alga or a cyanobacterium (or blue-green alga, if you prefer). The mutualism results from the fungus providing the ‘home’ for the autotrophic photosynthesiser, which in its turn pays for this privilege by handing over some of its photosynthate – as a sort of rent – to the heterotrophic ‘landlord’. However, peering a little more closely into this cosy twosome (whose bipartite nature was first recognised about 150 years ago by such 19th century botanical ‘greats’ as de Bary and Schwendener – see Fink 1913**), Toby Spribille et al. have found a third partner.
This additional lichen-dweller is another fungus but this time a basidiomycete yeast, Cyphobasidium (previously known only as a gall-forming lichenicolous fungus). As a unicellular fungus, buried amongst the rest of the tissue (the ‘cortex’), it would be hard to spot – unless you were looking for it, which was the case for those who reported this astonishing discovery. Although initially discovered in the macrolichens Bryoria fremontii (‘edible horsehair’ lichen) and B. tortuosa (‘tortured horsehair’ lichen), this finding is not just a ‘one-off’. This Cyphobasidium association has since been found in > 170 other lichen spp., from countries and continents across the globe – USA, Canada, Europe, Africa, South America, Asia, and Antarctica. Thus, it would now appear that “the structurally important lichen cortex, long treated as a zone of differentiated ascomycete cells, appears to consistently contain two unrelated fungi”.
So, the question now is what does the basidiomycete add to the relationship?*** Is a lichen still a mutualistic symbiosis, or something else? And, if one probes deeper at ever-decreasing microscopic resolution, what other organisms/entities might also be found within a lichen? If all of this begins to sound a little fictional, it’s worth bearing in mind that Beatrix Potter, well-known English author of books for children – and older readers – was a mycologist of some note before she turned her hand to writing fiction. ****
* Despite their dual – now plural – nature, lichens are given a single binomial and for taxonomic purposes are included within the Kingdom Fungi.
** Even if the nature of the relationship was considered one where the fungus parasitised the alga.
*** Isn’t it obvious though? Well, Mr Cuttings thinks so. Yeasts are famous for making alcohol. So, this new three-player arrangement has all the necessary ingredients – venue (courtesy of the ascomycete fungus), fermentable sugars (from the photobiont), and drink (basidiomycete’s contribution) – for a good old-fashioned ‘knees-up’. So, let’s get this party started!
**** One might mischievously suggest that, anticipating the discovery of this mycological tri-trophic ‘manger à trois’, the pursuit of fungology was becoming far too steamy for genteel Victorian Ms Potter who thereafter eschewed such unseemly pursuits in favour of much more moral tales about produce-pilfering rabbits and the like that talk and act as humans.
[Ed. – Notwithstanding these scientific advances, at least Jane Hirshfield’s lichen poem is still accurate since it is sufficiently future-proofed in referring simply to “marriage of fungi and algae” without mentioning how ‘bigamous’ that relationship might be. For more musings on lichenisation and lichen-forming fungi – published before the Cyphobasidium revelation – see David Hawksworth. But, the really big question is: How long before this fairly fascinating, fundamental phyco-fungal fact finds its way in to the textbooks?]