Lichens: You scratch my back, I'll feed your hyphae

Letharia columbiana from California.

Winter in the Pacific Northwest is the season for lichens. Deciduous trees have lost their foliage, revealing a colorful “understory” of a diverse lichen flora that clings to the branches and trunks of trees. For these organisms, resources for growth seem to be abundant at this time of year: there is no shortage of moisture from rain and snow, and despite shorter days and abundant cloud cover, the absence of leaves probably means there is a fair amount of light that penetrates tree canopies.

Lichens are composite organisms. The bulk of the tissue consists of the body of a fungal host (the “mycobiont”). Fungi are composed of chains of cells known as hyphae. The other half of the lichen partnership is a photosynthetic alga or cyanobacterium (the “phycobiont” or “photobiont”). The cells of the photosynthetic partner are embedded within the body of the fungus. The fungus is responsible for the structure (shape) of the lichen body. About 90% of lichens are host to green algae, including the genera Trebouxia and Trentepohlia (Purvis 2000). The rest have cyanobacterial partners (or on rare occasions, both green algae and cyanobacteria). Cyanobacteria are oxygen-producing photosynthetic bacteria. Worldwide, there are about 14,000 species of lichens (Brodo et al. 2001).



In this fruticose lichen, it appears that small greenish areas where the photobiont may be present can be seen through the fungal tissues on the main axis of this specimen.

Lichens represent a classic example of a symbiosis: two organisms that live in close association, often in inter-dependency. In ecology, symbiosis is a broad term that encompasses a range of relationships between two or more partners – everything from parasitism (one partner benefits at the expense of another) to mutualism (both partners are positively affected by the association). One view of the lichen symbiosis is that it is a mutualism. For instance, a potential benefit to the algal partner by living with its fungal host may be amelioration of desiccation stress. Just as the fungal partner could allow the alga to thrive in very dry places (e.g., the surface of desert rocks) by providing a home for growth, the alga might allow the fungus to live in environments with less organic matter (again, think barren rocks) than would otherwise be tolerable because it provides food (Purvis 2000). Sugars produced by photosynthesis in the photobiont are incorporated into the fungal tissue (Smith et al. 1969). By making dry places hospitable for the alga or carbon-poor places hospitable for the fungus, the lichen symbioses is an interesting example of one organism expanding the realized niche space of another (Purvis 2000). 

However, the lichen lifestyle may not always be beneficial for the algae. Photobionts like the cyanobacterium Nostoc can make it just fine outside of the lichen association. Brodo et al. (2001) note that the lichen symbiosis actually represents a range of associations from ones in which the algal partner may not be appreciably harmed by the fungus to ones that might be better characterized as a prison for the phycobiont. In fact, as early as 1869, the Swiss botanist Schwendener suggested that lichen fungi may be parasitic on their fungal hosts (Purvis 2000).

I still have much to learn about lichen biology, but here are a few interesting points about the lichen symbiosis that I picked up scanning some research:

- Lichen symbioses have evolved multiple times during the course of fungal evolution (Gargas et al. 1995). So, picking up a photosynthetic partner seems to be an advantageous evolutionary strategy. Most lichen associations are formed with ascomycete fungi, but a few basidiomycetes (mushroom-forming fungi) form lichens as well (Lawrey et al. 2009).

- Lichen symbioses are sometimes more of an extended family gathering than merely a two member partnership. Some fungi have both green algae and cyanobacteria as symbionts, sometimes living in separate places within the fungal tissue, sometimes living in closer proximity (Brodo et al. 2001, Henskens et al. 2012). Moreover, diverse bacteria and even other endophytic fungi can be associated with the lichen microcosm (Arnold et al. 2009, Grube et al. 2009, Bates et al. 2011, Hodkinson et al. 2012). “Hey you, move over! It is getting crowded in here!”

- The presence of one partner may influence the physiology of the other. In a study of the lichen Cladonia, the lichen association caused up-regulation of genes involved in photoprotection and antioxidation pathways in the alga and fungus respectively (Kranner et al. 2005).

- In lichens where the algal partner as been identified down to the species level (only a small percentage thus far), individual fungal species generally only associate with a specific algal species. The alga, on the other hand, shows less fidelity – one species may appear in many fungal hosts (Brodo et al. 2001, Yahr et al. 2004).

- Reproduction of lichens is perhaps a little more complex than in other species because two organisms are involved (Brodo et al. 2001). One way that maintenance of the lichen association from generation to generation is achieved is by vertical “transmission”. In vertical transmission, the propagules of the mycobiont and phycobiont can be produced vegetatively (so parents and daughters are the same genetically) and they disperse together to new living quarters (DalGrande et al. 2012). However, a paper by Wornik and Grube (2010) suggests that young lichen fungi can pick up algae anew from the environment, even if they originally dispersed with a photobiont to begin with. 

In summary, these points emphasize just how complex and diverse the lichen symbiosis can be. There is tremendous variation in the expression of the symbiosis and in the identity and arrangement of the partners involved. I’d bet there is a lot of fascinating biology and ecological insight yet to be gained from studying lichens. If species numbers, geographic distribution and lifestyle diversity are valid measures of evolutionary “success”, then the lichen association has been successful indeed!

References
Arnold et al. 2009. Systematic Biology 58:283.
Bates et al. 2011 Applied and Environmental Microbiology 77:1309.
Brodo et al. 2001. Lichens of North America. Yale University Press.
DalGrande et al. 2012. Molecular Ecology 21:3159.
Gargas et al. 1995. Science 268:1492.
Grube et al. 2009. ISMEJ 3:1105.
Henskens et al. 2012 Annals of Botany 110:555.
Hodkinson et al. 2012. Environmental Microbiology 14:147.
Lawrey et al. 2009. Mycological Research 113:1154.
Kranner et al. 2005 Proc Natl Acad Sci USA
Purvis, W. 2000. Lichens. Smithsonian Institution Press.
Smith et al. 1969. Biological Reviews 44:17.
Wornik and Grube. 2010. Micobial Ecology 59:150.
Yahr et al. 2004. Molecular Ecology 13:3367.

Two crustose lichens. In the bottom photograph, the large disk-like structures are apothecia, sites where fungal spores are produced.