What Is a Lichen? Not One Organism but a Working Partnership

The partnership was identified in 1867 by Simon Schwendener, and the claim that a lichen was two organisms was rejected furiously by lichenologists for years before being accepted, which is a useful reminder about how disruptive a correct idea can be. The fungal partner, called the mycobiont, makes up most of the body and gives the lichen its shape and its name, since lichens are classified as fungi. The photosynthetic partner, the photobiont, is a green alga in most cases and a nitrogen-fixing cyanobacterium in others, with some lichens hosting both. Research published in 2016 complicated the picture considerably by identifying a second fungus, a basidiomycete yeast, embedded in the outer layer of many lichens that had been studied for a century, which helped explain why some lichens with apparently identical partners look and behave differently. The current view treats a lichen as a small ecosystem including bacteria rather than as a tidy two-party arrangement.

Each partner supplies what the other cannot obtain:

• •The fungus builds a protective outer layer that shields the photobiont from excess light and from drying, and houses it in a defined layer within the body
• •The photobiont photosynthesises and transfers a large proportion of the sugar it produces to the fungus, in the form of a sugar alcohol or glucose depending on the partner
• •Cyanobacterial partners additionally fix atmospheric nitrogen, which is why lichens are significant nitrogen sources in nutrient-poor ecosystems
• •The fungus absorbs water and minerals directly from rain, dew and dust across its whole surface, since a lichen has no roots and no waxy cuticle
• •Lichens produce hundreds of distinctive chemical compounds found nowhere else, which deter grazers, screen ultraviolet light and chemically weather rock
• •Whether the relationship is mutualism or controlled parasitism has been argued for a century, since the fungus benefits more clearly and can in some cases be cultured only with difficulty while the alga grows perfectly well alone

The defining physiological trick is poikilohydry, meaning a lichen does not regulate its water content at all and simply equilibrates with the surroundings. When dry it becomes brittle, metabolism stops entirely and it can survive conditions that would kill almost anything, including extreme heat, extreme cold, vacuum and high radiation, with lichens having been exposed to open space on the exterior of a spacecraft and resumed growth afterwards. When wetted it swells and resumes photosynthesis within minutes. The cost of that strategy is that it can only grow while wet and adequately lit, which in many habitats is a small fraction of the time, and growth is correspondingly slow, with crustose lichens on rock extending by a fraction of a millimetre a year. That slowness makes some lichen patches extraordinarily old, and the technique of lichenometry uses the diameter of the largest lichen on a rock surface to estimate how long ago it was exposed, which has been used to date glacial retreat and rockfalls.

Lichens colonise bare surfaces and begin soil formation, physically prising apart rock with expanding and contracting bodies and chemically dissolving it with the acids they produce, which is a first step in primary succession after ice retreat or lava flow. They are a critical winter food for reindeer and caribou, which are among the few animals able to digest them, and that dependence made lichen contamination the mechanism by which radioactive fallout from the Chernobyl accident reached Sami herding communities, since lichens absorb directly from the atmosphere and concentrate what they take. That same property makes them exceptional pollution indicators: because they have no roots and take everything from the air, and because they cannot shed contaminated tissue, the species present at a site record air quality over years, and sensitivity to sulphur dioxide in particular is well enough characterised that lichen surveys were used to map industrial pollution across Europe and to document its decline after emissions controls.

A lichen is a fungus housing photosynthetic algae or cyanobacteria, with the fungus supplying structure, water capture and protection and the photobiont supplying sugar, and recent work has added yeasts and bacteria to the picture. It does not regulate its water content, so it shuts down completely when dry and survives extremes including vacuum, at the cost of growing only a fraction of a millimetre a year. Taking everything from the air makes lichens precise long-term indicators of pollution.