Over 700 lichens have been found in Iceland, over 400 of which are crustose lichens (see below). Other Icelandic lichens are either fruticose or foliose. New species of lichen are constantly being identified in Iceland. In the spring of 2006, three new foliose lichens were added to the growing list, including tube lichen, Hypogymnia austerodes, first found on April 14th, 2006, in Eyjafjörður Fjord (see photograph by Hörður Kristinsson to the right).
Symbiosis of a fungus and an alga or bacterium
Although mosses and lichens can look quite similar, lichens are not, in fact, a single organism. Unlike mosses, which are simple non-vascular plants, lichens are a symbiosis of a fungus and a green alga and/or cyanobacterium. The fungus is typically the dominant or determining partner in the symbiosis. The name of the fungus is also the name of the lichen; the same green alga species may often be found in a number of lichens. The fungus provides favourable habitat conditions for the alga, such as moisture and protection from strong sunlight, in return for a large part of the alga's production. Lichens are extremely versatile and often thrive in places where other vegetation has difficulty surviving. Lichens thus account for an ever-greater share of mountain flora with increasing altitude, and lichens are often the first plants to colonise a freshly hardened lava field. Lichen-forming fungi often reproduce sexually, producing spores in sacs known as asci. The vast majority of lichen-forming fungi are sac fungi (ascomycetes). Germinating ascospores must find a suitable symbiont (green alga, cyanobacterium) in order to develop as a lichen. A number of lichens do reproduce asexually: special organs (soredia, isidia) containing both fungal filaments and algal cells break off from the thallus and are then dispersed, often by wind or animals. Many lichens produce chemical compounds that have not been found elsewhere in nature. These are often acids, referred to as lichen acids. Some lichens contain acids that make them useful as dyes or even natural medicines where a lichen acid has antibacterial properties.
Fruticose, foliose and crustose
Traditionally, lichens are divided into three main categories on the basis of their appearance. Fruticose lichens are branching and shrubby; they generally rise up from the substrate on which they grow. Examples of fruticose lichens include reindeer lichen (also known as reindeer moss) and cup lichens. Foliose lichens have a leafy appearance and tend to cling to their substrate. Iceland moss is a deceptively named foliose lichen; other foliose lichens found in Iceland include dog lichens and rock tripe. Crustose lichens form a crust on the substrate (typically cliffs, bark or soil); they are by far the most common type of lichen in Iceland. These categories are an informal means of classifying lichens and do not reflect actual relationships between lichens. Lichen-forming fungi are not a “natural” group in the sense that they have not descended from a single common ancestor. In other words, they are not monophyletic: multiple groups of fungi have independently initiated symbiosis with a green alga or cyanobacterium. The vast majority of lichen-forming fungi are sac fungi (ascomycetes). Around half of all known sac fungi are lichen-forming. In Iceland, lichen-forming sac fungi belong to a total of 17 different orders. The majority (just over 500 species) belong to order Lecanorales. Around 70 species belong to order Peltigerales, and another 70 or so species belong to order Verrucariales. Fewer than 50 species belonging to order Pertusariales are found in Iceland. Other orders are represented by 1-17 species. In addition to this, at least one Basidiomycota fungi species in Iceland forms lichen.