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).