Arctic Life/Arctic Lichens

From Arctic Bioscan Wiki

What are Lichens?

Yellow lichen, Cetraria tilesii.

Lichens are organisms that arise through a symbiotic relationship between a species of fungus and a species of photosynthetic bacterium or alga. The fungus plays a key role in acquiring mineral nutrients through enzymatic digestion of rock or soil substrate, but lacks the chlorophyll needed to produce its own food. The photosynthetic bacterium or alga lives embedded in the fungus and converts sunlight to energy to feed both itself and its fungal host. The arrangement is a close-knit one of sharing food and living quarters.

Although lichens lack roots, they possess hair-like strands, called rhizines, that anchor the lichen to the surface of rocks, trees, other plants, and soils. There are approximately 275 lichen species in the Arctic – with roughly 108 species on Ellesmere Island alone – and all show striking architectural sophistication. Lichens can be grouped according to three major growth forms: crustose lichens form thin films on rocky surfaces; foliose lichens produce leaf-like structures; and fruticose lichens furnish elaborate, branching tubes.

Lichens in the Arctic

There are approximately 968 lichen species in the Arctic – roughly 331 species alone grow on Baffin Island. Lichen diversity is high in the Arctic because they are one of the few groups of organisms that can survive the extreme conditions of the region. Lichens are unique in that they can attach themselves directly to rock, deriving mineral nutrients from it. However, they are quick to take advantage of nutrient supplementation and a particularly rich assemblage of lichen species is often present on rocks that have been used as bird perches. Lichens are also numerous in soils with abundant humus. Conditions in the Arctic cause plant material to decompose slowly, resulting in a build up of this substrate. Humus provides a good medium for many lichens, particularly fruticose and foliose types. A few lichen species grow directly on rock surfaces, and some even grow on caribou antlers or animal bones.

There is a general shift in the colour of lichens from the south to north. In the southern portions of the Arctic, the most common lichens are yellow, but the dominant lichens are darker in the North. The yellow colour of the southern lichens reflects the presence of usnic acid, which absorbs ultraviolet (UV) light and protects the delicate algal cells within the lichen during the long summer days. The lichens may even have a competitive edge on the vascular plants that are not as resistant to UV light. In the North, UV radiation passes through a greater amount of atmosphere, thus significantly less strikes the ground. Usnic acid is not as important, but warmth is critical. By being darkly coloured, northern lichens are able to absorb more heat when the sun is shining.

Types of Lichens

Lichens are organisms that arise through a symbiotic relationship between a species of fungus and a species of photosynthetic bacterium or alga. The fungus plays a key role in acquiring mineral nutrients through enzymatic digestion of rock or soil substrate, but lacks the chlorophyll needed to produce its own food. The photosynthetic bacterium or alga lives embedded in the fungus and converts sunlight to energy to feed both itself and its fungal host. The arrangement is a close-knit one of sharing food and living quarters. Despite their uniform construction of algal and fungal cells, lichens show striking variation in their architecture. Crustose lichens adhere closely to the substrate and look very much like a "crust". Foliose lichens are leaf-shaped and loosely attached to the substrate, often in several places. By comparison, fruticose lichens are usually attached at only one point and are either upright, hollow stalks or shrubby and hair-like..

Uses for Lichens

Lichens are hardy plants, which have the ability to "bounce back" from prolonged periods of desiccation. They simply absorb water when it is available, wither into a state of dormancy when it is not, and come back to life once they are moistened again. In a twenty-four hour period, the water content of lichen can vary wildly, depending on atmospheric conditions. Lichens are able to absorb water from fog, rain, submergence, or directly from moisture in the air.

Scientists have studied the growth rates of lichens – in particular, map lichen, Rhizocarpon geographicum, and jewel lichen, Xanthoria elegans – and have discovered that individual lichens can live more than 4500 years! The constant growth rates of some lichens have been put to good use, as a tool to estimate the age of landforms, such as moraines. The larger the lichen patch on the exposed rock surfaces, the older the moraine.

Environmental conditions in the Arctic are harsh at the best of times. Low temperatures dominate the year and darkness extends for up to 4 months. In the summer, when the sun is shining, it may become too intense for the growth of lichen. However, lichens are resistant to physical extremes. Scientists have frozen them at -200oC for up to 18 hours and noted that the lichen were unharmed. One species, Xanthoria parietina, withstood temperatures close to absolute zero (-273oC) in a complete vacuum! High temperatures are also tolerated. Some lichens which grow in Utah's Death Valley regularly experience daytime temperatures of 70oC or more. Lichens have also been kept in drying agents, such as phosphorus pentoxide, for over a year, but still revived when wetted. However, lichens do have an Achilles heel – they are intolerant of atmospheric pollution. Lichens are so highly sensitive to sulphur dioxide that it has been possible to calculate the amount of this pollutant in the atmosphere by mapping the occurrence or disappearance of certain lichens in an area. They are also affected by nuclear radiation and can similarly be used to monitor radioactive contamination.

Throughout history, lichens have been used in dyes, perfumes, and medications – some still serve these functions. In Europe, one type of lichen is combined with an antibiotic to treat tuberculosis, while others are used in salves for treating cuts and skin diseases. The fashion industry even derives benefit from these tiny plants: Scottish tweeds and Indian cotton fabrics are dyed using lichen extracts. As well, generations of chemists have benefited from litmus paper – an acid-alkaline indicator – which was created using lichens.

Reindeer lichen, Cladonia rangiferina, is the most important food item for caribou in the Arctic. Other notable, and easily recognized, species include black hair lichen, Alectoria nigricans, which looks like a tangled mess of hair on the ground. Mane lichen, A. ochroleuca, is similar to black hair lichen except that its "branches" are yellowish-tan at the base, become blue-green along their length, and are blue-black at the tips. Yellow lichen, Cetraria tilesii, provides a welcome splash of colour on an otherwise barren, grey field of rocks. Bloodspot lichen, Haematoma lapponicum, is a pale grey crustose lichen that grows on acid rocks and is dotted with red spots resembling droplets of dried blood. Birds, such as ptramigans, plovers, and sandpipers, make use of the soft, hollow, white branches of worm lichen, Thamnolia subuliformis and T. vermicularis, as a nest-building material. The sunburst lichen, Arctoparmelia centrifuga, forms a number of bright yellow concentric rings. These rings represent successive waves of growth from a central point, each new bout of growth awaiting the decay and nutrient release of the preceding round.

Lichen Biology

Basic Morphology

While their gross appearance differs, the inner structures of all three lichen groups are similar. Each lichen is composed of a number of layers. The outer layer (farthest from the substrate) is known as the cortex and is composed of fungal cells. The next layer – the algal layer – is dominated by algal cells in a matrix of fungal threads that hold them in place, close to the surface. The third layer is the medulla, which is largely made up of fungal cells. In crustose lichens, this layer is directly connected to the substrate, but the other two groups differ: foliose lichens have another layer, the lower cortex, and are attached to the substrate by means of rhizinae, hair-like strands; in fruticose lichens, the cortex and algal layer are repeated below the medulla. The entire structure of any lichen is known as a lichen body, or thallus. To learn more about the different types of lichens, and what makes them unique, please explore the diagrams below.

Internal structure of a crustose lichen.
Internal structure of a fruticose lichen.
Internal structure of a foliose lichen.

Lichen reproduction remains a bit of a mystery as their composite nature complicates things. Vegetative propagation is the simplest method of reproduction and it occurs through the fracturing of small pieces, or soredia, that contain both fungal and algal components, resulting in a clone of the parent. However, most lichens also show a commitment to sexual reproduction since they produce fruiting bodies. These structures consist of a number of bulbous sacs that contain reproductive spores of the fungal component, but lack algal cells. These spores can create a new fungal organism, but only after it has become united with compatible algal cells can it be considered a lichen. Exactly how the algal cells connect with the fungus is unknown, but it is likely through wind dispersal.

Crustose Lichens
Jewel lichen.

The most abundant type of lichen that occurs on rock surfaces throughout the world at large and in the Arctic is crustose lichens. The large surface area of these lichens is an adaptation for obtaining nutrients by breaking down the rock face. Crustose lichens more tolerant of acidic environments can be found on granitic or acidic rocks, while less tolerant species are found on limestones and other sedimentary rocks. There is a wide variety of these lichens in the Arctic due to the prevalence of rock surfaces, on which few vascular plants can survive.

Jewel lichen (Xanthoria elegans): This arctic species is intolerant of acidic environments and can be found growing on limestone, bird-perch rocks, old bones, and occasionally wood. Nesting areas are often identified from afar by the splashes of orange colour. Jewel lichen is circumpolar in distribution.

Foliose Lichens

Foliose lichens are leaf-like and are more loosely attached to the substrate than crustose lichens. They can be found on most substrates including rock, soil, and wood. Soils that are high in humus – the partially broken down remains of plant material – are a favoured environment. Due to the vast areas of humus-covered tundra in the Arctic, foliose lichens have become extremely varied and abundant.

Reindeer lichen (Cladonia rangiferina): This arctic species is abundant throughout the Arctic and is the single most important food for the vast herds of caribou that range across Canada's North. It grows in soils that are rich in humus, as well as in the thin layer that tops rocky outcroppings. It is less common in the High Arctic, but is widespread and abundant throughout the tundra and boreal forest regions.

Yellow lichen (Cetraria tilesii): This species grows quite well on calcareous (calcium rich) soils and gravel. It is common throughout the Arctic, but ranges as far south as New Mexico. It is more brittle than other lichens due to its thin outer layer and its loose inner layers.

Rock tripe (Umbilicari spp.): These species are known collectively as rock tripe and are often found growing on exposed rock in the Arctic. They are circumpolar in distribution and have been used by explorers as food in emergencies and resemble the lining of a cow's stomach, hence their common name.

Reindeer lichen.
Yellow lichen.
Rock tripe.
Fruticose Lichens
Worm lichen.

Fruticose lichens are highly branched and brush-like in appearance, and are attached loosely to the substrate. They are more common on soils and other soft surfaces than on rock. Humus soils that contain a large proportion of decaying plant matter support the widest variety of fruticose lichens in the Arctic, but there are a few species that take advantage of the shelter of cracks in large boulders.

Worm lichen (Thamnolia subuliformis): This arctic species is named for its striking appearance. It is common throughout the Arctic and grows in many locations, including frost boils, mossy thickets, and among the willows and heaths. Worm lichen prefers sheltered localities and is commonly used by the golden plover as nesting material.