Desert Varnish & Lichen Crust

Microscopic examination reveals that lichens are really fungi containing photosynthetic algal cells. In fact, they are called "lichenized fungi" by mycologists (people who study fungi). These fungi are usually members of the fungal class Ascomycotes; however, some species belong to the Basidiomycotes. The lichen body is called a thallus, and is composed of intertwined fungal filaments. Lichens are conveniently grouped into 3 characteristic growth forms. Foliose lichens have a leaf-like or lobed thallus, loosely attached to the substrate by root-like rhizines or by a central stalk-like umbilicus. Fruticose lichens have an intricately branched upright or pendulous thallus. Crustose lichens are low-growing, with the entire thallus firmly attached to the rock or other substrate. The thallus of crustose lichens may be squamulose (composed of crowded, overlapping scales) or areolate (cracked into numerous angular sections like a dried lake bed).

A remarkable biochemical phenomenon resulting from the alga-fungus union in lichens is the production of lichen acids, unique phenolic compounds produced by no other organisms. In fact, they are not even produced by the separate fungal or algal symbionts, only by the combined lichen. Lichen acids have the unusual property of changing colors under different pH conditions. In fact, the litmus dye that turns red in acid solutions and blue in alkaline solutions was originally derived from the fruticose lichen Roccella. The color change under different pH conditions is caused by changes in the electron configuration (resonance) of the molecule. This affects absorption and reflection of light; hence, the vivid color changes.

T he presence of certain lichen acids is used to identify some species. A drop of sodium hydroxide or lye (NaOH) and a drop of sodium hypochlorite or household bleach (NaClO) is added to the exposed lichen tissue (cortex and medulla) to see if it turns yellow, purple or red. [The cortex is the outermost layer of the thallus composed of compressed hyphal cells. The medulla is the innermost layer of the thallus composed of loosely interwoven hyphae and algal cells.] Since potassium hydroxide (KOH) rather than NaOH has been commonly used on lichens, the hydroxide test is abbreviated as K. The hypochlorite test is abbreviated as C. K+ yellow indicates a positive reaction for atranorin (cortex), alectorialic acid (medulla) and stictic acid (medulla). K+ yellow turning red indicates a positive reaction for norstictic acid and salazinic acid (medulla). C+ red indicates a positive reaction for gyrophoric acid, lecanoric acid and olivetoric acid (medulla). The orange cortical pigment parietin in Caloplaca, Teloschistes and Xanthoria has a K+ reaction of deep purple. Of course, K- and C- indicates a negative reaction with no color change.

Lichen acids were the source of important dyes for cotton and wool in medieval Europe. Two purple and red dyes, orchil (sometimes spelled archil) and cudbear, were obtained from the lichens Roccella and Ochrolechia. Lichen dyes were dissolved in human urine, and the yarns were immersed in this mixture. Ammonia salts in the urine functioned as mordants to make the dyes permanent. Although most of the lichen dye industry was replaced by cheaper aniline dyes from coal tar, some lichen dyes are still used today. A brownish dye from the foliose lichen Parmelia omphalodes is used on hand-woven Harris tweeds from the Outer Hebrides. Orcein, a purple-red chromosomal stain found in every microbiology laboratory is derived from Roccella tinctoria. Pine lichen (Letharia vulpina), a beautiful chartreuse fruticose lichen that grows on the bark of pines and fir throughout the mountains of the Pacific United States, contains a mildly toxic yellow dye called vulpinic acid. The striking canary-yellow porcupine quills woven into the baskets of Klamoth and Yurok Indians were dyed with this lichen.

Some lichens contain various phenolic acids and essential oils that produce fragrant odors in scented soaps and help fix the aroma of fine perfumes. For centuries a lovely fruticose lichen called oak moss (Evernia prunastri) has been collected in Europe for making perfume.Through a complex process of solvent extraction and distillation, oak moss has become an important ingredient in the manufacture of perfumes and high-quality cosmetics. This remarkable lichen occurs in California, but air pollution has eliminated it throughout most of its former range in southern California. Oak moss still clings to the branches of ponderosa pines on Palomar Mountain in San Diego County.

Even without magnification, one of the most visible fungal characteristics of a lichen is the cup-shaped, spore-bearing structure called an apothecium. This structure is found in many related fungi belonging to the Class Ascomycetes, and is produced through a complex sexual cycle. The cup-shaped apothecia of some lichens resemble those of non-lichenized fungi, with a rim or margin of fungal hyphae (called a proper exciple) that is usually the same color as the cup. Many lichens develop a second (outer) rim the same color as the thallus. This uniquely lichen structure (called a thalline exciple) contains a layer of algal cells. Usually only one type of rim is visible under a hand lens, but some lichens have both types. These terms are important when keying out certain lichens, especially crustose species.

The thallus of some lichens is dotted with black pits called pycnidia, which are lined with minute, asexual conidia. These bacilliform spores (also called pycnidiospores) are strikingly similar in appearance to the male spermatia produced by certain non-lichenized fungi such as wheat rust. The following photo shows the common foliose bark lichen Hypogymnia imshaugii that grows on conifer branches in the mountains of San Diego County.

Presumably the fungal spores produced in apothecia may encounter algal cells on a rock or branch and begin a new career as conjugal partners, in a union observed in the laboratory but rarely seen in nature. However, lichens have evolved a much more effective method of asexual reproduction and dispersal through the production of soredia and isidia. Soredia are microscopic particles containing several algal cells enveloped by fungal hyphae. They are produced in pustules on the surface of the lichen called soralia. Like dust particles, the soredia become airborne and are probably responsible for the worldwide distribution of some lichens. Isidia are pimplelike protuberances containing both algae and fungi that readily disperse to new locations.

The following illustration shows a typical foliose lichen in cross section. The lower surface has root-like extensions called rhizines. The upper side has a cup-shaped apothecium lined on the inside with numerous sac-like structures called asci. Each ascus produces 8 ascopores which escape from the cup by wind and rain. The upper side also has a blister-like pustule called a soralium bearing numerous particles called soredia. Each soredium is composed of several green algal cells enveloped by fungal filaments (hyphae). It is interesting to note that some lichen characteristics, such as soredia, are not produced by typical fungi. They are only produced by the algal-fungus association through some unknown genetic mechanism.

A dozen or more species of lichens may grow on a single boulder, often completely covering the rock surface. Crustose lichens form such a tight interface with the crystalline surface of some rocks that they are practically impossible to scape off. They will even grow on shiny black obsidian. Many crustose rock lichens spend most of their lives in a desiccated "dormant" state and have extremely slow annual growth rates. Lichens dry out very rapidly and may loose up to 98 percent of their water content. When a lichen is wetted by rain or morning dew, it quickly imbibes water like blotting paper, and photosynthesis within its little algal partner is revived for a while. Since lichens are among the first plants to grow on bare rock, they play a role in soil formation by slowly etching the rock surface. Microscopic rock fragments intermeshed with the lichen become loosened by expansion and contraction, as the lichen is alternately moistened and dried.

On rugged granite outcrops, large colonies of lime-green map lichen (Rhizocarpon geographicum), ashy-gray Aspicilia cinerea and orange Caloplaca saxicola may be thousands of years old. In fact, the colorful chartreuse rock lichen Acarospora chlorophana may grow only a few millimeters per century. One has only to gaze at the spectacular panoramas of glacier-carved granite throughout the Sierra Nevada to appreciate the magnitude of some crustose lichens. The blackened surface of massive domes of Yosemite National Park is actually crustose species of Buellia, Verrucaria and Lecidea atrobrunnea. Growth rates from rock lichens on glacial moraines have been used to approximate the time interval between the advance and retreat of glaciers.

Rock lichens have played an important role in the survival of native people and explorers. In addition to providing food for their animals, Indians, Eskimos and Laplanders eat certain lichens. Leafy lichens called rock tripes (Umbilicaria) are eaten raw and are boiled in soups. They are also added to salads or deep fried, and are considered a delicacy in Japan. Throughout history, peasants of Persia have avoided mass starvation by eating the abundant crustose rock lichen Lecanora esculenta. This lichen readily becomes detached in small patches and is blown off the rocks by wind, often accumulating in crevices and under shrubs. It is mixed with meal and made into a kind of bread in Turkey and northern Iran. In fact, some biblical scholars think this lichen may have been the "manna" which saved the starving Israelites during their exodus from Egypt.

In the natural environment, plants are constantly invading and colonizing new habitats--a phenomenon known as succession. When succession starts on bare rock or ground it is called xerarch succession. Since lichens are among the first plants to colonize bare rock, they play an important role in primary succession. After lichens have etched and crumbled the rock surface for centuries, mineral soil and organic matter begins to accumulate. Then other plants such as mosses and grasses begin to grow, followed by herbs, hardy shrubs, and finally trees. Although lichens produce weak organic (phenolic) acids, it is doubtful that these acids have much effect on the etching of rocks, unless they are calcareous. For most rock surfaces, the etching process is probably mechanical. Crustose rock lichens are able to grow on bare rock, sinking their spreading thallus into every minute nook and cranny. Microscopic rock fragments intermeshed with lichen thallus become loosened by expansion and contraction, as the thallus is alternately moistened and dried. The rock-breaking power of plants is greatly enhanced when seeds fall into cracks and then germinate. This is particularly true in the case of woody shrubs and conifers with powerful expanding root systems. Throughout the subalpine Sierra Nevada there are forests of pine, fir, and hemlock growing in relatively shallow soils and duff overlaying solid granite. This massive bedrock of granite was scoured and polished by glaciers as recently as 12,000 years ago.

In addition to desert varnish and lichens, other organisms coat the surfaces of boulders and tree trunks, including cyanobacteria and green algae. Cyanobacteria (formerly called blue-green algae) often form dense, feltlike meshworks of microscopic filaments. They cover the trunks of trees and boulders, particularly in wet regions or shady habitats. In Guatemala, the massive pyramids in the ancient Maya city of Tikal are covered with a thin black layer of cyanobacteria of the genus Scytonema. In forests of southern California, a related species of filamentous cyanobacteria grows on the trunks of young incense cedars. From a distance the trees look as though they were charred by a fire.

Through the remarkable marriage between algae and fungi, lichens have etched out an existence in places where no other life forms could possibly live. Although they have survived millions of years of evolution, many lichen species are now endangered by atmospheric pollution. This same fate may also affect some of the desert varnish microbes. Since lichens absorb most of their mineral nutrients from the air and rain water, they are especially vulnerable to toxic air-borne pollutants. Because they have no way to excrete the elements they absorb into their tissue, toxic compounds become even more concentrated. The toxins cause the photosynthetic algal cells to deteriorate and the subsequent death of the fungal spouse. Extensive off-road vehicle activity in some desert areas stirs up vast quantities of alkaline dust. This increased atmospheric alkalinity may affect manganese oxidation by varnish bacteria, thus slowing or inhibiting the development of desert varnish.

Radioactive fallout is one of the most insidious atmospheric toxins, affecting entire ecosystems through biological magnification. Soil lichens, including reindeer moss (Cladina rangiferina) and Iceland moss (Cetraria islandica), carpet thousands of square miles of North American and Eurasian tundra, providing a major food source for caribou, reindeer and musk ox. Laplanders absorb more radiation than other people on earth because they subsist largely on reindeer, which in turn feed on lichens. In fact, Lapps may concentrate up to one hundred times the level of radioactive isotopes found in tundra lichens. When radioactive clouds from the Chernobyl nuclear power plant meltdown rained cesium 137 on hundreds of miles of reindeer moss, more than 70,000 reindeer had to be destroyed because of dangerous levels of radiation.

These same tundra lichens may have played a major role in the migration of people from Siberia to North America across the Bering Land Bridge over 20,000 years ago. If these ancient explorers fed primarily on native animals, which in turn fed on lichens, then the importance of lichens to their remarkable journey is astonishing.

Like the proverbial canary used to detect invisible but deadly methane fumes in a coal mine, lichens are sensitive barometers of atmospheric pollution. In fact, their vulnerability has made them very effective air pollution monitoring stations for the U.S. Forest Service and National Park Service. Lichens and desert varnish are also some of the most fascinating and complex of all living organisms. We still have a lot to learn from them. In addition, they coat our mountains with a myriad of beautiful colors.

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