Desert Varnish & Lichen Crust

• Introduction: Desert Varnish & Lichen Crust
  
• Desert Varnish Coating On Rocks & Boulders
  
• Lichen Crust On Surface Of Rocks & Boulders
  
• Crustose Marine Lichens Of The Pacific Coast
  
• Crustose Terrestrial Lichens Of North America
  
• Lichen Succession On Bark Rock Surfaces
  
• Concluding Remarks About Rock Lichens
  
• The Remarkable Kingdom Of True Fungi
  
• References About Desert Varnish & Lichens

• Varnish-Coated Boulder In Colorado Desert
  
• Modern Petroglyph Carved In Desert Varnish
  
• Desert Varnish In Alabama Hills Of California
  
• See Colorful Rock Lichens On Basalt Outcrop
  
• Candelariella On Basalt With Twisted Filaree
  
• See Minute Tardigrades That Live On Lichens
  
• See Illustration Of A Foliose Rock Lichen
  
• See Illustration Of A Crustose Rock Lichen
  
• Marine Rock Lichens Of The Pacific Coast
  
• See Panoramic View Of Tuolumne Meadows
  
• Close-Up View Of Sierra Nevada Rock Lichen
  
• See Some Spectacular Alpine Rock Lichens
  
• See Article About Lichen Dyes And Perfumes
  
• See Some Interesting Lichens In Fungus Article
  
• Additional Photos Of Crustose Rock Lichens
  
• Photographs Of Fruticose Lichens
  
• Photographs Of Foliose Lichens
  
• Photographs Of Soil & Bark Lichens
  
• Photographs Of Rock & Bark Lichens
  
• Lichen Images From Wyoming & Montana

Long-term stable symbiotic relationships between two or more organisms may lead to evolutionary change. This phenomenon, known as symbiogenesis, is described in the book entitled Acquiring Genomes: A Theory of the Origins of Species by L. Margulis and D. Sagan (2002). According to the authors, genomic mergers are a major source of genetic variability leading to the evolution of species. Instead of relying on the hit-or-miss method of random mutations, symbiogenesis provides advantageous genetic combinations through the fusion of entire genomes from two or more organisms. This phenomenon may have been a major factor in the evolution of land plants from lichen-like ancestors.

The arise of photosynthetic organelles called chloroplasts represented a major step in the evolution and diversification of plant life on earth. This event had a significant effect on the evolution of animal life that depended on the plants for food, either directly in the case of herbivores, or indirectly in the case of carnivores. Chloroplasts exhibit remarkable similarities with cells of cyanobacteria, and may have shared a common prokaryotic ancestry. Indeed, the outer membrane structure and circular DNA molecules of chloroplasts and mitochondria are very similar to individual prokaroytic cells. According to the endosymbiont hypothesis (or endosymbiont theory for those who are less skeptical), ancient photosynthetic prokaryotic cells became incorporated within the cells of algae or ancestral plants, forming stable mutualistic symbionts known as chloroplasts. Mitochondria may have had a similar origin. Without chloroplasts and oxygen-producing photosynthesis, the amazing diversity of today's plants and animals would not have evolved.

According to Margulis and Sagan (2002), there is even a green photosynthetic animal named Elysia viridis, a minute slug (saccoglossan opisthobranch) that never feeds throughout its adult life. Instead, it obtains carbohydrate-rich molecules by bathing in the sunlight. This slug evolved from algae-eating ancestors, only the algal cells entered the slug's tissue and remained their as photobionts (photosynthetic symbionts). According to some invertebrate zoology textbooks, the chloroplasts from algae cells are sucked into the slug's gut and incorporated within digestive gland cells where photosynthesis occurs.

There are numerous examples of plants and animals that contain microbial symbionts within their tissues, including bacteria, cyanobacteria, protozoans and algal cells. Cycads, water ferns (Azolla), legumes and the tropical flowering plant Gunnera contain nitrogen-fixing bacteria in their tissues; sea anemones and corals contain photosynthertic unicellular algae (zooanthellae and zoochlorellae); the rumen of cattle contain cellulose-digesting bacteria; termite guts contain flagellated protists which contain wood-digesting bacteria; and human intestines contain bacteria that produce essential B vitamins.

Lichens are one of the best examples of symbiogenesis involving the fusion of algal and fungal genomes (kingdoms Protista and Fungi). Some lichens include the genome of a third kingdom Monera because they contain prokaryotic cells of cyanobacteria. In the case of lichens, this genomic merger has enabled these organisms to survive in some of the most inhospitable environments on earth, where neither symbiont could survive on its own. In fact, lichens are an excellent example of synergism because the whole is truly greater than the sum of its parts. The algal and fungal components develop into a unique body form with morphological features quite different from either symbiont.

Lichenization, the process by which fungal hyphae and algal cells literally grow together to form a mutualistic association, may help to explain the remarkable evolution of vascular plants on earth. Most textbooks of general botany suggest that land plants evolved from ancestral green algae. However, some authorities believe that vascular plants are far more than simple extensions of green algae. They are comparatively too complex, diversified too quickly, and contain numerous fungus-like cells. In his fascinating article "Are Vascular Plants Inside-Out Lichens?" (Ecology 69 (1): 17-23, 1988), Peter Atsatt of the University of California, Irvine discusses some of the evidence supporting a lichenized ancestor to vascular plants. According to Dr. Atsatt, the ancestral lichenization resulted in a "reverse-phase lichen" with a dominant algal component containing an endophytic, mineral scavenging fungus similar to extant mycorrhizal associations. Nuclear fusions between the fungal and algal cells resulted in hybrid nuclei containing the traits of both parents. In true synergistic fashion, this dual genome gave rise to a plant body composed of a mosaic of alga-like photosynthetic cells interspersed with specialized fungus-like cells.

Probably the most difficult concept for skeptical botanists to accept is the fungal ancestry in today's vascular plants. Dr. Atsatt discusses several types of cells and tissues in vascular plants which resemble fungal hyphae, including pollen tubes, vascular (xylem) tissue, laticifers, and haustoria. Pollen tubes not only resemble the growth of fungal hyphae, but in Pinus, cycads, and Ginkgo they are branched and actually absorb nutrients from the "host's" megasporangium. The latex-producing laticifers found in many members of the Euphorbiaceae, Asclepiadaceae and other dicotyledonous families are very similar to fungal hyphae. Nonarticulated laticifers are elongate, multinucleate cellular tubes that grow throughout the plant body of these families. Some endophytic parasitic flowering plants, such as certain dwarf mistletoes and the remarkable Pilostyles thurberi of the Colorado Desert, live completely within the host tissues and only emerge from their host to produce flowers. The vascular tissue of these endoparasites literally permeate the host tissues and truly resemble fungal hyphae. The absorptive haustorial organs of many parasitic flowering plants which penetrate the host tissue are also very reminiscent of fungal hyphae.

Additional evidence of fungal ancestry in land plants are minute bodies called "mycosomes." Dr. Atsatt has discovered these minute fungal propagules inside the plastids of algae, primitive and higher plants, including mosses (Polytrichum), club-mosses (Lycopodium), cycads (Cycas revoluta), Ginkgo biloba, conifers (Sequoia sempervirens) and mormon tea (Ephedra), parasitic flowering plants (Phoradendron,Cuscuta, Cassytha and Pilostyles), marine flowering plants (Zostera) and Indian paintbrush (Castilleja stenantha). Mycosomes are apparently widespread in plant and algal cells, and are produced by both ascomycete and basidiomycete fungi. [See Atsatt, P.R. 2003. "Fungus Propagules in Plastids: The Mycosome Hypthesis." International Microbiology 6: 17-27.] Dr. Atsatt defines mycosomes as "wall-deficient reproductive propagules (diameter ca 0.1-1.0 μm) that develop into larger forms: wall-less cells (protoplasts), yeast cells, or membrane-bounded thylakoid-containing spherules in which mycosomes reproduce."

Although there is ample fossil evidence suggesting that algae and fungi lived over 500 million years ago, there is little fossil evidence of true ancestral lichens from that era. However, a startling new hypothesis from Gregory Retallack of the University of Oregon may shed some light on the existence of Precambrian lichens. Since their discovery in southern Australia in the late 1950s, the fossil remains of the Ediacaran biota have puzzled paleobiologists. These strange, flattened creatures lived about 600 million years ago presumedly at the bottom of shallow coastal seas. They have been classified in several primitive animal phyla, from jellyfish, echinoderms and worm-like animals to large alga-like protists, and may have been ancestral to other animal phyla. A number of paleontologists refer to these organisms as "Vendobionta," and regard them as an extinct early experiment in the evolution of life. By about 530 million years ago they were all replaced by shelled Cambrian animals. But according to Dr. Retallack, these bizarre creatures may have been ancient lichens. In his fascinating article "Were the Ediacaran Fossils Lichens?" (Paleobiology 20 (4): 523-544, 1994), Dr. Retallack eloquently discusses the evidence supporting his lichen hypothesis. Comparing their thickness to that of much younger tree-trunk fossils, he concludes that the fossils resisted compaction after burial almost as well as sturdy logs. Their sturdiness, large size (up to one meter across), lack of any mouth, digestive cavity or musculature, and evidence from their microscopic examination all suggest to Retallack that Ediacaran fossils were lichens. The presumed marine habitats of Ediacaran fossils is not crucial to their interpretation as lichens, because rock lichens live in the sea and on land. If one can hypothesize that at least some of the Ediacarans may be ancestral to certain animal groups, then perhaps a lichen-like Ediacaran ancestor gave rise to vascular plants!

According to Blair Hedges of Pennylvania State University (Science, August 2001), aquatic fungi evolved into a terrestrial form about 1.3 billion years ago. These early fungal forms were actually lichens because they formed a symbiotic relationship with primitive aquatic green algae. The early land surface of the Earth at this time contained numerous colorful rock lichens. The bright pigments served to reduce the harmful effects of ultraviolet radiation in a primitive atmosphere. Evidence from mutation rates in 119 genes common to living fungi and plants, indicates that ancient moss-like land plants appeared about 700 million years ago.

Lichens are far more than mere biological curiosities. They are very successful and unique life forms that may hold the secrets to complex evolutionary processes, cell differentiation and gene expression in vascular plants.

One of the most remarkable biogeochemical phenomena in arid desert regions of the world is desert varnish. Although it may be only a hundredth of a millimeter in thickness, desert varnish often colors entire desert mountain ranges black or reddish brown. Desert varnish is a thin coating (patina) of manganese, iron and clays on the surface of sun-baked boulders. According to Ronald I. Dorn and Theodore M. Oberlander (Science Volume 213, 1981), desert varnish is formed by colonies of microscopic bacteria living on the rock surface for thousands of years. The bacteria absorb trace amounts of manganese and iron from the atmosphere and precipitate it as a black layer of manganese oxide or reddish iron oxide on the rock surfaces. This thin layer also includes cemented clay particles which help to shield the bacteria against desiccation, extreme heat and intense solar radiation.

Several genera of bacteria are known to produce desert varnish, including Metallogenium and Pedomicrobium. They consist of minute spherical, rod- shaped or pear-shaped cells only 0.4 to 2 micrometers long, with peculiar cellular extensions. In fact, the individual cells are smaller than human red blood cells which are about 7.5 micrometers in diameter. Because of the radiating filaments from individual cells and colonies, they are called appendaged bacteria. All living systems require the vital energy molecule ATP (adenosine triphosphate) in order to function. In our cells ATP is constantly produced within minute bodies called mitochondria. As electrons flow along the membranes of our mitochondria, molecules of ATP are generated. The electrons come from the breakdown (oxidation) of glucose from our diet. Although varnish bacteria do not have mitochondria, they do have a similar inner membrane structure through which electrons flow to generate ATP. However, in varnish bacteria the electrons come from the oxidation of manganese and iron rather than glucose. Herein lies the marvelous adaptive advantage for producing a layer of black and red varnish on desert boulders.

Manganese varnish formation. Modified from Dorn & Oberlander, 1982. Reddish iron varnish develops in a similar oxidation reaction.

Varnish bacteria thrive on smooth rock surfaces in arid climates. According to Ronald Dorn, perhaps 10,000 years are required for a complete varnish coating to form in the deserts of the southwestern United States. In fact, dating of varnished surfaces is of enormous importance to the study of desert landforms and to the study of early humans in America, since many artifacts lying on the ground become coated with desert varnish. Boulders of the Anza-Borrego Desert region are covered with a reddish-brown iron oxide, while boulders in parts of Owens Valley are blackened by a manganese oxide varnish.

The thin layer of reddish iron oxide varnish on this rock surface has been etched to reveal the lighter granodiorite beneath. Several Indian tribes utilized desert varnish to create their marvelous petroglyphs.

In the Alabama Hills near Lone Pine, the rocks are so black that they resemble basalt; however, if you scratch through the varnish layer, the light-colored granite is exposed. For thousands of years native Indians have used desert varnish for their rock carvings (called petroglyphs). Throughout northern Owens Valley, there are acres of elaborate petroglyphs carved into desert varnish and Bishop tuff, including spirals, circles, wavy lines, footprints, men, deer and desert bighorn sheep. It is fascinating to speculate on the origin and meaning of all these carvings.

This varnish-coated rock (with black layer of manganese oxide) in the Alabama Hills near Lone Pine, California has broken away revealing the lighter granitic core.

See Additional Photos Of Desert Varnish

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Lichen Crust On Rocks And Boulders

Without a doubt, the most colorful coatings on rocks are produced by lichens, a remarkable symbiotic relationship between microscopic algal cells and fungal filaments. Although lichens can also withstand extreme environmental conditions, they generally cannot survive as well on the dry, sun-baked boulders where desert varnish microbes flourish. Rock lichens come in a variety of bright colors, from red, orange and yellow to brilliant shades of green. There are hundreds of species in the southwestern United States, including leafy forms and low-growing crustose species resembling a thick layer of paint.

This basalt outcrop on the Santa Rosa Plateau of southern California is covered by a dense crust of four colorful lichen species, including orange Caloplaca, yellow Candelaria & Candelariella and gray Xanthoparmelia.

The lichen body is composed of algal cells living inside a compact mass of fungal tissue. The algae are photosynthetic and provide the fungus with carbohydrate nutrients. Three genera of unicellular green algae (Trebouxia, Pseudotrebouxia and Trentepohlia) are involved in the majority of lichens. Two genera of filamentous cyanobacteria (Nostoc and Scytonema) are also found in many species of lichens. In fact, some remarkable lichens contain two symbionts, an alga and a cyanobacterium.

Placopsis gelida: A striking North American crustose lichen containing two autotrophic symbionts (algae and cyanobacteria). The pinkish bodies in the center (called cephalodia) contain nitrogen-fixing cyanobacteria.

The delicate algal and bacterial cells also gain mechanical protection from hostile climatic conditions by being tightly enveloped in a dense meshwork of fungal filaments. This is especially true on arid boulders where neither partner could survive on its on. Indeed the relationship is a kind of marriage where each member depends on the other for its survival. It wasn't until 1867 that the dual symbiotic nature of lichens was described by the Swiss botanist Simon Schwendener. One of the staunch proponents of the alga/fungus marriage hypothesis was the excellent naturalist and scientific illustrator of this time by the name of Beatrix Potter--who latter became a famous author of children's stories.

A granite boulder in the Sierra Nevada of California covered with colonies of map lichen (Rhizocarpon geographicum). This species of crustose lichen is appropriately named because the colonies resemble continents on the earth.

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). 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.