Lichens grow on a variety of plants and hard surfaces and sometimes need to be controlled or removed by those managing amenity areas writes Graham Paul.
Lichens are not single organisms but a partnership or association between a fungus, an algae and/or a cyanobacteria. The relationship between these different organisms is known as symbiosis, where all partners receive some form of benefit.
The fungal partner, referred to as the mycobiont, needs a supply of sugars and carbohydrates for energy and growth, which it is unable to manufacture, and these are provided by single-celled green algae or in some cases by cyanobacteria (blue-green algae) – in a process called photosynthesis. The algal species or cyanobacteria are referred to as the photobionts. In return for basic food, the fungus provides its photosynthetic partners with moisture, minerals, protection from the elements and harmful UV radiation as well as physical support.
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The resulting conglomeration of these symbiotic partners that we call lichens are diverse and can be difficult to classify, as they contain two or more different organisms. The accepted strategy is to name lichens after the fungal partner.
The self-sufficiency of lichens allows them to grow in inhospitable places such as on bare rock and in extreme conditions; from arctic tundra to hot deserts, on rocky coasts battered by salt-laden winds and even on toxic slag heaps.They are also abundant on leaves and branches in woodland, on footpaths, walls and roofs of buildings. In harsh environments they survive on just what can be extracted from the air, with warmth and light from the sun.
Lichens have been around for a very long time; almost since dry land was first colonised from life in the sea. They have gained the status of ‘pioneer’ species, a reflection of their extreme survival abilities. They can be very slow growing; often increasing in size by as little as 1mm per year. However, they compensate for this by their long life span.
The study of these fascinating organisms is known as lichenology, a subject that has occupied lichenologists for thousands of years. Originally, lichens were thought to be autonomous organisms and it was not until 1867 that the Swiss botanist Simon Schwendener put forward his ‘dual’ theory, that their true nature began to emerge. His theory was based on observing specimens with a crude microscope and many of his peers rejected this postulation due to lack of conclusive experimental evidence. After many years of study, the final proof of Schwendener’s theory came in 1939 when Eugene Thomas published the results of a laboratory experiment in which he artificially re-synthesised the lichen Cladonia pyxidata from its component mycobiont and photobiont parts.
Some 28,000 lichens have been discovered worldwide. The fungal species that form such associations are said to have been ‘lichenised’ and can be found in all of the fungal phyla. However, 98% of lichens are formed from associations with the ‘cup fungi’ (from the phylum Ascomycota), with members of the Basidiomycota accounting for most of the remaining 2%. The cup fungi are characterised by disk or cup-shaped structures called apothecia, often resembling ‘jam tarts’, which bear sacks (asci) containing ascospores produced by sexual reproduction.
In contrast to the large number of different fungi that have been found in lichens, these form associations with only a very small number of species of photobiont partners (approximately 100), which belong to just 30 genera. Two thirds are green algae and the remaining third are cyanobacteria. These statistics are made more complex by the fact that some lichens contain both green algae and cyanobacteria. It is estimated that unicellular green algae are found in 60% of all lichens, the most common being Trebouxia spp. – occurring in half of all lichen species. Approximately 30% of lichens contain the orange-pigmented green algae Trentepohlia spp., whilst 10% contain cyanobacteria, half of which belong to the genus Nostoc.
The Fungus forms the major part of the lichen body (thallus) and serves to protect the photobiont from environmental extremes. The photobiont accounts for less than 20% of the total mass and is usually contained just below the upper surface of the structure where it can receive controlled exposure to the sun. Many species of mycobionts synthesise a chemical sunscreen to protect the photobiont from harmful UV radiation.
Most lichens anchor themselves to the substrate using superficial root-like structures called rhizines. These do not function like plant roots; being solely used for the purpose of holding the lichen still. The basic nutrients required for growth are provided by the photobiont in the form of carbohydrates manufactured from carbon dioxide and moisture obtained from the air.
Some species have been found to be sensitive to air pollutants and will not survive in atmospheres with high concentrations of heavy metals, sulphur, nitrogen or their oxides; sulphur dioxide and nitrous oxide e.g. from vehicle exhaust fumes. Other lichens, however, thrive in such conditions and can be found growing close to motorways, trunk routes and industrial areas. These pollutants can affect the lichen population directly by their presence in the air or by dissolving in moisture and falling as acid rain. This sensitivity has provided researchers with a method of monitoring air pollution by surveying areas for the presence or absence of indicator Lichens.
Reproduction
Lichens reproduce asexually in several ways; the simplest being by fragmentation of the thallus, whereby small pieces break off the main body and grow to form new lichen. Some species produce small packages called ‘soredia’ composed of both the fungal mycelium and a few algal cells – when dispersed from the parent, these handy ‘lichen kits’ contain everything needed to grow a new individual. Soredia are usually grouped together in clusters known as ‘soralia’, which can be easily seen with a hand lens as a powdery texture on the upper surface of some lichens.
Sexual reproduction occurs only in the mycobiont partner and is slightly different in Basidiomycete lichen but since the majority are derived from Ascomycete fungi, I shall concentrate on these. The male and female gametes fuse to form ascospores, which are contained in asci stored in the cup or disk structures called apothecia. Once released the ascospores will germinate under suitable conditions and capture an algal cell (or cyanobacteria) to start the life cycle again. The shape and colour of these apothecia is varied and is useful in identification of the lichen.
There are several types of lichen growth forms that are also useful in identification.
- Crustose lichens have the appearance of ‘paint splashes’ and grow as a crust closely attached to the substrate
- Foliose lichens are leaf-like, having a distinct upper and lower surface and are often only loosely attached to the substrate
- Fruticose lichens look like tiny shrubs or tufts of vegetation, with branches that are either hair-like, strap-like or scaly
- Squamulose lichens are composed of small scales or plate-like structures, often anchored along one edge.
Uses
Lichens have been used by many cultures for dying wool and textiles. In Scotland, brown dyes (known as crottle) and red dyes (corkir) were both obtained from lichens and used to colour the wool for weaving tartans. Some purple dyes were extracted from Roccella spp lichens, which is also the source of the water soluble pH indicator dye ‘Litmus’. Lichens were also widely used in traditional medicine and we now know that extracts from many species contain antibiotics; one of the most potent being usnic acid, obtained from Usnea spp.
Reindeer lichen (Cladina spp.) is a staple food of reindeer and caribou in the Arctic and lichens have been eaten by humans as a delicacy in some regions of the world but for the most part, consumption has tended to be only in times of famine, since the carbohydrates they contain are mostly indigestible. In addition, some lichens contain toxic substances that we can only tolerate in small amounts. Indeed, a few of the yellow lichens contain vulpinic acid, which was used as a poison for killing wolves and by certain indigenous American Indians for poisoning arrowheads.
Evidence suggests that the ancient Egyptians of the eighteenth dynasty (1550 BC) were using lichens for embalming, some 200 years before the reign of the pharaoh Tutankhamun. One such lichen Pseudevernia furfuracea, known for its preservative and aromatic qualities, is believed to have been used for preserving bread as well as human tissue.
Control
There are few opportunities for cultural control of lichens due to their strong survival abilities. Some of the leafy species that are loosely attached can be removed by scrubbing and this is often made easier when the target is thoroughly wet. There are no chemicals approved for the control of lichen, however, some of the proprietary biocides, surface or patio cleaners may be useful in removing them e.g. ‘Safor’ which contains organic acids and ‘Qualgex’ containing citric acid.
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