From Plant Press, Vol. 19, No. 4, October 2016.
By Manuela Dal Forno
Do you remember the question in Introductory Biology 101, “What are lichens?” According to traditional concepts, a lichen is the resulting structure (known as a thallus) from the symbiosis between a fungal partner (the mycobiont) and an algal-like partner (the photobiont), either a green alga and/or a cyanobacterium (“blue-green alga”). Lichens play important roles in the environments they live in, participating in nutrient and water cycles and particularly nitrogen fixation, forming biological soil crusts, and serving for animals in many ways, such as camouflage, shelter, nests, and food, among many other ecological functions lichens have. Humans use lichens as traditional sources for medicine and in the pharmaceutical and perfume industry. They also play an outstanding role as biological indicators of environmental health.
You probably learned this as graduate or undergraduate student, or if you were lucky, while taking biology in high school. However, times have changed and the definition of lichens also needs a bit of an update. We now know that lichens harbor a diverse and complex community of bacteria, archaea (single-cell organisms), and fungi, as stable components of the symbiosis. These microorganisms together make up the lichen microbiome. And more recently, Toby Spribille and colleagues (Science 353: 488–492. 2016) found that additional fungal partners may also play important roles for the association aside from the mycobiont.
The earliest studies of lichen microbiomes revealed the existence of diverse communities of bacteria in addition to the two dominant partners (Gonzáles et al. 2005 FEMS Microbiol. Ecol. 54: 401–415; Cardinale et al. 2006 FEMS Microbiol. Ecol. 57: 484–495, Cardinale et al. 2008 FEMS Microbiol. Ecol. 66: 63–71). Most of these studies have focused on bacterial diversity and their potential roles in the lichenization process (Grube et al. 2009 ISME J. 3: 1105–1115; Hodkinson & Lutzoni 2009 Symbiosis 49: 163–180; Bates et al. 2011 Appl. Environ. Microbiol. 77: 1309–1314; Hodkinson et al. 2012 Environ. Microbiol. 14: 147–161; Cernava et al. 2015 Front. Microbiol. 6: 620; Grube et al. 2015 ISME J. 9: 412–424; Erlacher et al. 2015 Front. Microbiol. 6: 53). Others have also explored some specific questions, such as bacterial community shifts related to lichen parasitism (Grube et al. 2012 FEMS Microbiol. Ecol. 82: 472–481; Wedin et al. 2015 Environ. Microbiol. 18: 1428–1439). In general, these studies have shown that lichen-associated microorganisms are not randomly distributed in lichens, but instead reflect the systematic affinities of the lichen partners and/or the associated habitat conditions. It is hypothesized that, just as the human microbiome has important functional roles in human health, the lichen microbiome may be involved in processes such as nutrient supply, resistance against biotic and abiotic factors, and production of hormones (Grube et al. 2015 ISME J. 9: 412–424).
To lichenologists, these are key discoveries since they wave a flag for lichenology as a whole, and we do need this “advertisement,” as lichenology has not always been a highly visible field and people are not generally aware that lichens are a significant part of the ecosystem.
In September, a recent paper about “plant blindness” (Balding & Williams 2016 Conserv. Biol.) and follow-up commentary article (Dasgupta 2016 https://news.mongabay.com/2016/09/can-plant-blindness-be-cured/) was circulated among coworkers in the Smithsonian’s Department of Botany. Lichens, along with other lesser-known organisms living on our diverse planet, suffer from the same phenomenon of blindness, and I have done what I could to combat “lichen blindness” throughout my career. Lichens are not plants, but historically and currently, they have been studied under the cryptogamic umbrella offered through botany departments around the world. If animals are the most charismatic component of global biodiversity noticed by people, very distantly followed by plants, then lichens are indisputably at the lowest levels of what the public sees.
How can we raise awareness for lichens? Well, there are several ongoing efforts around the world to combat this lichen blindness. The Field Museum in Chicago, for example, is currently hosting an exhibit called “Lichens: The Coolest Things You’ve Never Heard Of”, and has a Ford Bronco door completely covered by lichens collected in Puerto Rico as the centerpiece. A full assessment of all species found in this car is available in the FUNGI magazine’s special issue on lichens. One of my favorite quotes from the exhibition is by Robert Lücking, who says that a person studying birds, for example, never gets asked, “what is a bird,” but when one says, “I study lichens,” the follow up question is usually, “What is a lichen?” This is exactly what I aim to change if I can, one person at a time.
Major efforts in the lichenological community are also making knowledge accessible online through portals, such as the Consortium of North American Lichen Herbaria (http://lichenportal.org/), with the United States National Herbarium (US) at the Smithsonian being a participant institution. The US collection is amongst the top ten largest lichen collections in the world, with approximately 250,000 specimens, including an estimated 2,500 types. It is particularly rich in Parmeliaceae, Graphidaceae, and Cladoniaceae specimens, thanks to the work of the late Mason E. Hale and Paula T. DePriest, who both served as curators of the collection.
The title of this article pays homage to a paper my colleague Adriano A. Spielmann (UFMS, Brazil) sent me a few years ago entitled “A symbiotic view of life: We have never been individuals” by S.F. Gilbert, J. Sapp, and A.I. Tauber (Q. Rev. Biol. 87(4): 325–341. 2012). In the last sentence of the paper the authors proclaim that “we are all lichens.” Recent excitement about the lichen microbiome and additional partners reminds me that this sentence makes a powerful, yet charismatic point about how we all depend on other organisms to become who we are.
Our understanding of microbiome diversity and functionality in organisms and systems has rapidly advanced as new technologies have been developed (and have become more accessible) and greater access to bioinformatics pipelines permits investigators to deal with large amounts of data. Lichens have very rarely been the subject of large-scale microbiome studies, but this is happily beginning to change.
These are exciting times for lichen studies. I thank all my colleagues, through their publications and social media posts that expose some of the basic questions regarding lichen symbiosis, diversity, and evolution. I hope this moment catches the attention of many students who will contribute to the exciting future ahead in lichenology.
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