The hydrothermal features of Ye l l owstone are magnifi c e n tevidence of Earth’s volcanic activ i t y. Amazingly, they arealso habitats in which microscopic organisms called thermophiles—“thermo” for heat, “phile” for love r —s u r v ive and thrive. Grand Prismatic Spring at Midwa yG eyser Basin ( a b ov e ) is an outstanding example of thisdual characteristic. Visitors are awed by its size and admireits brilliant colors. How eve r, the boardwalk they follow(lower right corner of photo) spans a vast habitat for a variety of thermophiles. Drawing on energy and chemicalbuilding blocks available in the hot springs, microbes construct substantial communities throughout the park.

All thermophiles require hot wa t e rbut differ in other habitat needs.Some thrive in only acidic wa t e r,others require sulphur or calciumcarbonate, still others live in alka-line springs. Depending on theseother characteristics, some aredescribed more specifically withterms such as thermoacidophile(heat and acid lover) or extremo- or hyperthermophile (extreme heatl ove r ) .

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T h e rmophile community from Geyser Hill, Upper Geyser Basin

G rand Prismatic Spri n g(both photos)

N ew ch a p t e r

A b o u tM i c ro b e s

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When you look into Ye l l ow s t o n e ’s colorful hydrothermal pools, imagine you are looking through a window into Earth’s past to theb eginnings of life itself. The thermophiles that thrive in these poolsand their runoff channels are heat-loving microorganisms (also calledmicrobes), some of which are descendants of the earliest lifeforms onE a r t h .Scientists think that during the first three billion years of Earth’s h i s t o r y, microorganisms transformed the original, anoxic (withouto x y g e n ) atmosphere into something that could support complex formsof life. Microbes harnessed energy stored in chemicals such as ironand hydrogen sulfide in a process called c h e m o s y n t h e s i s . And theydid this in environments that are lethal to humans—in boiling acidic oralkaline hot springs like the hot springs found in Ye l l owstone NationalPark.

M i c r o o rganisms were the first lifeforms capable of photosynthesis—using sunlight to convert carbon dioxide to oxygen and other byproducts. These lifeforms, called cyanobacteria, began to create anatmosphere that would eventually support human life. Cyanobacteriaare found in some of the colorful mats and streamers of Ye l l ow s t o n e ’shot springs. In the last few decades, scientists discovered that cyanobacteria andother microbes comprise the majority of species in the world—yet lessthan one percent of them have been studied.

Microbial research has also led to a revised tree of life, far diff e r e n tfrom the one taught for decades (see next page ) . The “old” tree’sbranches—animal, plant, fungi—are now combined in one branch ofthe tree. The other two branches consist solely of microorga n i s m s ,including an entire branch of microorganisms not known until the1 9 7 0 s — A r c h a e a .

Ye l l ow s t o n e ’s thermophilic communities include species in all threebranches. These microbes and their environments provide a living laboratory studied by a variety of scientists. Their research fi n d i n g sconnect Ye l l owstone to other ancient lifeforms on Earth, and to thepossibilities of life elsewhere in our solar system (see last section).

M i c ro b e :A m i nute life fo rm ; a microorganism.

M i c ro o rg a n i s m :An organism of microscopic or submicroscopic size.Both from American Heritage Dictionary, 4th edition

M i c robes in Ye l l ow s t o n eIn addition to the thermophilic microorganisms, millions of other microbes thri ve in Ye l l ow s t o n e ’s soils,s t r e a m s, ri ve r s, lake s, vegetation, and animals. S o m eof them are discussed in other chapters of this book.

T h e rmophiles as seen througha microscope. Images court e s yof the T h e rmal Biology Instituteof Montana State Unive r s i t y.

Hot spring in Upper Geyser Basin

T h e r m o p h i l e sin the

Tree of Life

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The tree shows the divergence of va rious groups of organisms from the beginning of life on Earth, about four billionyears ago. It was originated by Carl Woese in the 1970s. D r. Woese also proposed the new center branch, Archaea,which includes many microorganisms fo rm e rly considered bacteri a . The red line links the earliest oganisms thatevo l ved from a common ancestor.

B r a n ches of the tree• Domains Bacteria and Archaea include single-celled organisms that have simple cell architectures.• Domain Eukarya includes all organisms comprised of cells containing a nucleus and energy-generating organelles

such as mitochrondria and chloroplasts. A n i m a l s, plants, fungi, algae, and protozoa are members of Eukarya .

Underestanding the tree• Mutations (changes in the sequence of DNA) accompany the evolution of living organisms.• Closely related organisms have fewer mutations in their DNA sequences than more distantly related organisms.• Closely related organisms are located close to each other on the branches of the tree.• The earliest organisms are near the tree’s root, while the modern organisms are at the ends of the bra n c h e s.• Analysis of microorganism DNA shows Bacteria and Archaea are as different from each other as each is diffe r e n t

from Eukarya, even though they share a simple cell design.• A n i m a l s, plants, and fungi are late-comers, consistent with their late appearance in the fossil record.R e l evance to Ye l l ow s t o n eThe earliest organisms to evo l ve on Earth were likely microorganisms whose descendants are found today in ex t r e m eh i g h – t e m p e ra t u r e, and in some cases acidic, env i r o n m e n t s, such as those in Ye l l ow s t o n e. Their history exhibits p rinciples of ecology and the connections between geology, geochemistry, and biology.

The Tree of Life Continues to Evo l v eThree decades of microbial research have occurred since Dr. Woese first proposed this tree of life. Changes to the treereflect new knowledge and the settling of some controve r s i e s. R e f i n e m e n t s, changes, and controversies will continu eas our understanding of microbes and microbial ecosystems evo l ve s.

Ye l l ow s t o n e ’s hot springs contain species from these groups on the Tree of Life

about 4 billion ye a rs ag o

This tree of life byJa ck Farmer fi rs ta p p e a red in GSA To d a y, Ju l y2000. Used withp e r m i s s i o n .

T h e r m o p h i l eC o m mu n i t i e s

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T h e rmophiles gr ow in communities nu m b e ring billions of individu-als and often dozens of species. Some communities fo rm a coatingon sinter around the rims of hot springs and gey s e r s, such as atW h i rligig in Norris Geyser Basin, above. Others connect as ri bb o n sor “ s t r e a m e r s ” in runoff channels and other moving hot wa t e r( ri g h t ) .

Some thermophile communities gr ow incolumns or pedestals (above)—each seeminglyfree-standing fo rmation a thriving community ofits own connected to the surrounding fo rm a-t i o n s. Still other communities gr ow into thickmats (ri g h t ) . Within those mats, therm o p h i l especies may migrate up or down depending onthe air and water temperatures and other condi-t i o n s, demonstrating that these communities aredynamic and eve r - c h a n g i n g .

Thermophiles appear ina variety of shapes andc o l o rs , as shown on thisp age. I n formation aboutthe different thermophilehabitats begins on then ext page.

S t r e a m e rsBetween 163°F(73°C) and 198°F(92°C), fi l a m e n t o u sthermophiles formlong, flexible struc-tures called streamersin fa s t - f l owing wa t e rof runoff channels.Depending on the thermophilic species and minerals in the wa t e r, theymay be pink, yellow, orange, white, gray, or black (photo abov e ) . T h e

thermophilic speciesin these streamers aredirect descendants ofearly bacteria.

M a t sThermophiles formmats in water below167°F (75°C ).Species of four genera dominate themats listed in the boxat right. M a ny otherbacteria and Archaeaalso occur, eachadapted to diff e r e n ttemperatures andlight conditions

within the mat. They are fueled by the photosynthetic species and arei nvo l ved in decomposition of the mats. The interactions of the speciesform a mat that is laminated and seems solid (photo abov e ) .

The thermophilic mat community can be compared to a forestc o m m u n i t y. Its canopy species either need or can withstand abu n d a n tlight, and its understory species live with less or no light and maymetabolize chemicals such as hydrogen and iron.

T h e r m o p h i l eH a b i t a t s :A l k a l i n e

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Where to see Most hot springs in the Firehole Va l l ey and West Thumb Geyser Basin

C h a r a c t e r i s t i c s• pH 7–11• u n d e rlain by rhyolite rock• water rich in silica, which fo rms sinter and

g ey s e rite deposits

T h e r m o p h i l e s• A b ove 167°F (75°C ), bacteria and Archaea

reside in boiling pools and as streamers in ru n o f fc h a n n e l s.

• A b ove 167°F (75°C ), streamers may be pink, ye l l ow, ora n g e, or gray. T h ey are comprised ofm a ny thermophile species, including T h e rm o c ri n i s,descended from an ancient bacteria that metabo-l i zes hydrogen and ox y g e n .

• B e l ow 167°F (75°C), thermophiles fo rm mats thatline cooler hot springs and runoff channels. T h emain species in these mats are S y n e c h o c o c c u s ( ac ya n o b a c t e ria), C h l o r o f l ex u s (filamentous gr e e nb a c t e ria), and two filamentous cya n o b a c t e ri a —P h o rm i d i u m and C a l o t h ri x.

Interesting facts• At this elevation, the surface boiling point is

1 9 9 ° F / 9 3 ° C.• The microorganisms in a 3x3 ̋chunk of

t h e rmophile mat outnumber the people on Eart h .

Researchers collected this sample of a thermophile mat, whichs h ows the multiple layers of commu n i t i e s. Photo courtesy T h e rm a lBiology Institute, Montana State Unive r s i t y.

A b ove : Hot springs along the Firehole Rive r, Upper Geyser Basin; B e l ow : t h e rmophilic streamers

Thermophiles that live in these acidic hot springs are consideredextremophiles because they live in boiling water that is highly acidic.T h ey are sometimes referred to as thermoacidophiles. S u l f o l o bus a c i d o c a l d a r i u s , an Archaeum that abounds in such springs, is wellnamed. It is a sulfur-eating ( S u l f o - ), lobe-shaped ( - l o bu s ) m i c r o o rga n-ism adapted to life in acidic ( a c i d o - ) hot ( - c a l d a r i u s ) places. OtherArchaea such as T h e r m o p ro t e u s and A c i d i a n u s also live in thesesprings.

S t r e a m e rs and MatsYe l l owish streamers and mats grow in the hottest acidic runoff channels, between 140°F (60°C) and 181°F (83°C). One of these genera, H y d roge n o b a c u l u m , may metabolize hydrogen and sulfurcompounds.

B e l ow 140°F (60°C), filamentous bacteria—including T h i o m o n a s ,A c i d i m i c robium, Desulfure l l a—and the Archaeum M e t a l l o s p h a e raform red-brown mats (see photo below). The color comes in part fromiron oxide, metabolized from iron by the thermophiles. High levels ofarsenic also contribute to the color.B e l ow 131°F (55°C), C y a n i d i u m and G a l d i e r i a form mats in acidicr u n o ff channels. Both species are algae, in the domain eukarya ( s e e

the tree of life, page 51). T h ey contain anucleus and chloroplasts for harvesting lighte n e rgy and generating oxygen as a byproduct.These mats are not as well laminated ascyanobacterial mats in alkaline springs, possibly because filamentous bacteria—an important “thread” in the alkaline mat— is absent. Instead, the acidic mats may be held together by fungi that consume alga lproducts. Many bacteria and Archaea alsoinhabit the mat and are invo l ved in its decom-position. At lower temperatures, C h l o re l l a , agreen alga, dominates the mat; Z y g og o n i u m ,a filamentous alga, thrives at even lower tem-peratures and is recognized by its dark purplec o l o r.

T h e r m o p h i l eH a b i t a t s :

A c i d i c

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Where to see • Mud Volcano (photo at ri g h t )• N o r ris Geyser Basin

C h a r a c t e r i s t i c s• pH 0–5• u n d e rlain by rhyolite rock

Thermophiles present• A b ove140°F (60°C), filamentous bacteria fo rm

ye l l owish streamers and mats.• B e l ow 140°F (60°C), filamentous bacteria and

Archaea fo rm red brown mats (see below ) .• B e l ow 131°F (55°C), algae and fungi fo rm mats in

runoff channels.• Sulfur-consuming microbes such as S u l fo l o bu s, a n

Archaeum, produces sulfuric acid, which breaksd own rocks into clay mu d .

Interesting facts• Acid pools in Norris Geyser Basin often appear

turbid due, in part, to the high concentrations of microorganisms in the wa t e r.

• Some of these hot springs have a pH near ze r o ;their water will bu rn holes in shoes and clothing.

• Archaea living in near-boiling acid hot springs aresome of the toughest known life fo rm s.

• V i ruses have been discovered in some near-boilingacidic hot spri n g s.

• R o a ring Mountain is an acidic thermophile c o m mu n i t y ; the Archaeum S u l fo l o bu s produces s u l f u ric acid, which accelerates erosion of them o u n t a i n s i d e.

Iron and arsenic contri bute dark orange colors to some t h e rmophiles in Norris Geyser Basin

Mud Volcano area

T h e r m o p h i l eH a b i t a t s :

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Underneath Mammoth Hot Springs, the dominant rock is limestonedeposited by ancient seas. Calcium carbonate from the limestone andsulfur from an underground source are brought to the surface by circu-lating hot wa t e r. Thus, the hot springs are rich with the sulfur and car-bonate. Sulfur, in the form of hydrogen sulfide, is toxic to cy a n o b a c t e r i aat high temperatures but nutritious for purple and green photosyntheticbacteria. Calcium carbonate precipitates from the hot spring wa t e r s ,building up the terrace structures and entombing microbial communitieswithin the newly forming rock matrix (see page 57 for more about this).

S t r e a m e rsWhen source pools are above 151°F (66°C), their runoff supports thecream-colored streamers of filamentous bacteria ( b e l o w ) . The creamcolor comes from calcium carbonate minerals and sulfur deposited onfilamentous thermophiles. These bacteria are descended from the earliest bacteria and metabolize sulfide in combination with oxygen.

M a t sThe carbonate- ands u l fide-rich springs ofMammoth containrare examples of laminated microbialmats formed by green bacteria in theabsence of cy a n o b a c-teria. These bacteriause hydrogen sulfi d ein chemosyntheticreactions, producingsulfur instead of oxygen as a by-product. Cream-colored streamersmay form above these mats where oxygen m i xes in from the air.

Where to see Mammoth Hot Spri n g s

C h a r a c t e r i s t i c s• pH 6–8 (neutral to slightly acidic)• u n d e rlain by ancient limestone deposits• water rich in calcium carbonate and sulfur, a

combination unique to Mammoth Hot Spri n g s

T h e r m o p h i l e s• 151°F (66°C) to 167°F (75°C), bacteria fo rm

streamers in runoff channels.• B e l ow 151°F (66°C), C h l o r o f l ex u s ( b a c t e ri a )

fo rms green mats and filamentous bacteria fo rmcream-colored streamers.

• B e l ow 136°F (58°C), C h r o m a t i u m ( b a c t e ria) fo rmp u rple mats.

• 77–129F (25–54C), C h l o r o b i u m ( b a c t e ria) fo rmgreen mats.

Interesting facts• The combination of hydrogen sulfide and high

t e m p e rature is toxic to cya n o b a c t e ri a . D ow n -stream, after hydrogen sulfide has been remove dby other organisms, cya n o b a c t e ria thri ves andc o n t ri butes to the orange color of active terra c e s.

• Other thermophiles can obtain energy from thehydrogen sulfide.

• Scientists are studying the role that therm o p h i l e smight play in calcium carbonate deposition.

See Chapters 3 and 10 for more about the geology ofMammoth Hot Springs.

Cream-colored streaming thermophilic communities at Mammoth

Trave rtine terraces at Mammoth Hot Spri n g s

T h e r m o p h i l e sIn Time

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W h a t ’s the Connection?• Ye l l ow s t o n e ’s hy d r o t h e rmal features contain

m o d e rn examples of Eart h ’s earliest life fo rm s,both chemo- and photosynthetic, and thus p r ovide a window into Eart h ’s ancient past.

• Ye l l owstone hy d r o t h e rmal communities reveal theextremes that life can endure, providing clues toe nvironments that might harbor life on otherwo rl d s.

• Ye l l owstone environments show how minera l i z a-tion preserves biosignatures of thermophilic c o m munities in the fossil record, which could helpscientists recognize similar signatures elsew h e r e.

• Based on the history of life on Earth, the searchfor life on other planets seems more likely toencounter evidence of microorganisms than ofmore complex life.

Life began very early in Earth’s history (see timeline, below), p e r h a p sbefore 3.8 billion years ago. By the close of the Archaean Eon, some2.5 billion years ago, microorganisms had evo l ved to remarkable l evels of metabolic sophistication. Thermophiles in Ye l l ow s t o n e ’s hotsprings are living connections to the primal Earth of billions of yearsago. They are also studied by scientists searching for life on otherplanets, where extreme environmental conditions may support similarl i f e f o r m s .

C h e m o s y n t h e s i s : An Ancient Process Studies suggest that the common ancestor of all modern orga n i s m smay have lived in a high-temperature environment like a Ye l l ow s t o n ehot spring. Descendents of these early organisms currently inhabitYe l l ow s t o n e ’s hot springs, where they live by chemosynthesis—combining inorganic chemicals to liberate energ y, which is then usedfor growth. Such energy sources likely fueled Earth’s earliest life-forms, and remain a mainstay for organisms living in hy d r o t h e r m a le nvironments where sunlight is unava i l a b l e .

P h o t o s y n t h e s i s : Key to the Present Photosynthesis was key to creating an atmosphere that would eve n t u-ally support plants and animals. All types of photosynthesis are repre-sented in Ye l l ow s t o n e ’s thermophile mat communities. The simplestand earliest type of photosynthesis, anoxygenic photosynthesis, wa sprobably conducted by green and purple bacteria by splitting hy d r o g e ns u l fide and producing sulfur. To d a y, such communities exist inMammoth Hot Springs.

Oxygenic photosynthesis—generating oxygen by splitting wa t e r — i sconducted by microbes such as cyanobacteria, which form mats in

Photo above from NASA/JPL; time line provided by the T h e rmal Biology Institute, Montana State Unive r s i t y

springs wherever sulfide is low or has beenr e m oved by other organisms, such as inNorris Geyser Basin. Algae also conductoxygenic photosynthesis and are found inacidic hot springs such as at Norris.

S t ro m a t o l i t e s : Signatures of LifeStromatolites are sediments laminated bymicrobial activ i t y. Found in ancient rocks,stromatolites are perhaps the most abu n d a n tand widespread evidence of early microbialecosystems.

Stromatolites also form in Ye l l ow s t o n e ’shydrothermal features as thermophiles areentombed within travertine and sinterdeposits. Thermophile communities leavebehind evidence of their shapes as biological“ s i g n a t u r e s .” Scientists compare the signa-tures of these modern and recent stromato-lites to those of ancient deposits elsew h e r e(e.g., 350-million-year-old Australian sinterdeposits) to better understand the env i r o n-ment and evolution on early Earth. MammothHot Springs is a particularly good locationfor these studies because of rapid depositionrates and abundant thermophile communities.

F rom Earth to Mars—and Beyo n d ?Ye l l ow s t o n e ’s hydrothermal features and their associated communities of thermophilesare studied by scientists who are searchingfor evidence of life on other planets. The connection is extreme environments. If lifeoriginated in the extreme conditions thoughtto have been widespread on ancient Earth, itmay well have developed on other planets—or even exist today.

The chemosynthetic microbes that thrive insome of Ye l l ow s t o n e ’s hot springs do so bymetabolizing inorganic chemicals, a source ofe n e rgy that does not require sunlight. Suche n e rgy sources provide the most likely habitable niches for life on Mars or on themoons of Jupiter—Ganymede, Europa, andCallisto—where uninhabitable surface condi-tions preclude photosynthesis. Chemicale n e rgy sources, along with ex t e n s ive ground-water systems (such as on Mars) or below -crust oceans (such as on Jupiter’s moons)could provide habitats for life.The study of stromatolites on Earth may also be applied to the search for life on otherplanets. If stromatolites are eventually found

in the rocks of Mars or on other planets, wewould have unequivocal evidence that lifeonce existed elsewhere in the unive r s e .Ye l l owstone National Park will continue tobe an important site for studies at the phy s i c a land chemical limits of survival. These studieswill give scientists a better understanding ofthe conditions that give rise to and supportlife, and of how to recognize lifeforms inancient rocks and on distant planets.

T h e r m o p h i l e sIn Time

and Space

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O p p o rt u n i t y, the Mars Exploration Rove r, took this image on March 1, 2004. Data indicate the layer rock is made up of types ofs u l fate that could have only been created by interaction betwe e nwater and martian rock over extended amounts of time. L i fe onE a rth is sustained by extended interaction between water and thee nv i r o n m e n t . Evidence of a similar relationship between water andr o ck on Mars does not necessarily mean that life did develop onM a r s, but it does bring the possibility one step closer to reality.Photo and text adapted from www. n a s a . g ov, image by Nasa/JPL

For additional info rm a t i o nabout the scientific value oft h e rm o p h i l e s, see Chapter 9,“ B i o p r o s p e c t i n g .”

For MoreI n fo r m a t i o n

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B a rn s, Susan et al. 1 9 9 6 . Pe r s p e c t i ves on archaeal dive r s i t y,t h e rm o p h i l y, and monophyly from environmental rRNAs e q u e n c e s. Proceedings Natl Acad. S c i . 9 3 : 9 1 8 8 – 9 1 9 3

B o o m e r, Sarah et al. 2 0 0 2 . Molecular chara c t e rization of nove lred green nonsulfur bacteria from five distinct hot spri n gc o m munities in Ye l l owstone National Pa rk . Applied andE nvironmental Microbiology. Ja nu a ry : 3 4 6 – 3 5 5 .

B r o ck, T. D. 1 9 7 8 . T h e rmophilic microorganisms and life at hight e m p e ra t u r e s. S p ri n g e r - Ve rlag, New Yo rk .

B r o ck, T. D. 1 9 9 8 . E a rly days in Ye l l owstone Microbiology.ASM New s. 6 4 : 1 3 7 - 1 4 0

Carsten, Laura . 2 0 0 1 . L i fe in extreme env i r o n m e n t s. N o rt h we s tScience and Te c h n o l o g y. S p ri n g : 1 4 – 2 1 .

Fa rm e r, Ja ck D. 2 0 0 0 . H y d r o t h e rmal Systems: D o o r ways toE a rly Biosphere Evo l u t i o n . GSA To d ay 1 0 ( 7 ) : 1 – 9 .

Fa rm e r, J. D., and D. J. Des Mara i s.1 9 9 9 . E x p l o ring for a recordof ancient Martian life. J. G e o p hy s. R e s. 1 0 4 .

Fo u ke, Bruce et al. 2 0 0 0 . Depositional facies and aqueous-solidg e o c h e m i s t ry of trave rtine-depositing hot spri n g s.J Sedimentary Research 7 0 ( 3 ) : 5 6 5 – 5 8 5 .

G i h ring, Thomas et al. 2 0 0 1 . Rapid arsenite oxidation by T.aquatis and T. t h e rm o p h i l u s: Field and labora t o ry inve s t i g a-t i o n s. E nv i r o n . S c i . Te c h n o l . 3 5 : 3 8 5 7 – 3 8 6 2 .

Madigan, Michael and Aharon Oren. 1 9 9 9 . T h e rmophilic andhalophilic ex t r e m o p h i l e s. Current Opinion in Microbiology2 : 2 6 5 – 2 6 9 .

Pierson, Beve rly and Mary Pa r e n t e a u . 2 0 0 0 . Phototrophs inhigh iron microbial mats: m i c r o s t ructure of mats in iron-depositing hot spri n g s. Microbiology Ecology 3 2 : 1 8 1 – 1 9 6 .

R eysenbach, A-L. et al, eds. 2 0 0 1 . T h e rm o p h i l e s : B i o d i ve r s i t y,E c o l o g y, and Evo l u t i o n . K l u wer Academic Press/Plenu m ,N ew Yo rk .

R eysenbach, A-L and Sherry Cady. 2 0 0 1 . Microbiology ofancient and modern hy d r o t h e rmal systems. Trends inM i c r o b i o l o g y 9 ( 2 ) : 8 0 – 8 6 .

R eysenbach, A-L, et al. 2 0 0 0 . Microbial diversity at 83°C inCalcite Spri n g s, Ye l l owstone National Pa rk . E x t r e m o p h i l e s4 : 6 1 – 6 7 .

Sheehan, K.B. et al. 2 0 0 4 .Ye l l owstone Under the Microscope.The Globe Pequot Press/Gilford, CT.

Wa l t e r, M. R . and D. J. Des Mara i s. 1 9 9 3 . P r e s e rvation of biological info rmation in thermal spring deposits: d eve l o p i n ga strategy for the search for fossil life on Mars. I c a rus 101:1 2 9 – 1 4 3 .

Ward, D. M . 1 9 9 8 . Microbiology in Ye l l owstone National Pa rk .ASM New s 6 4 : 1 4 1 – 1 4 6 .

Ward, D. M . et al. 1 9 9 2 . M o d e rn phototrophic microbial mats:a n ox y g e n i c, intermittently ox y g e n i c / a n ox y g e n i c, therm a l ,e u c a ryotic and terrestri a l . In The Proterozoic Biosphere: aM u l t i d i s c i p l i n a ry Study, J. W. Schopf and C. Klein, eds.C a m b ridge Univ. P r e s s / C a m b ri d g e.

Ward, D. M . and R.W. C a s t e n h o l z . 2 0 0 0 . C ya n o b a c t e ria in g e o t h e rmal habitats. In Ecology of Cya n o b a c t e ri a . K l u we rAcademic Publishers

Ward, D. M . et al. 2 0 0 2 . N a t u ral history of microorganisms inhabiting hot spring microbial mat commu n i t i e s : clues to theo rigin of microbial diversity and implications for micro- andm a c r o - b i o l o g y. In B i o d i versity of Microbial Life : Foundation ofE a rt h ’s Biosphere, J. T. S t a l ey and A.-L. R eysenbach, eds.John W i l ey and Sons/New Yo rk .

Ward, D. M . et al. 1 9 8 9 . Hot spring microbial mats: a n ox y g e n i cand oxygenic mats of possible evo l u t i o n a ry significance. I nMicrobial Mats: P hysiological Ecology of Benthic MicrobialC o m mu n i t i e s, Y. Cohen and E. R o s e n burg, eds. A m . S o c .M i c r o b i o l / Washington, D. C.

Additional info rmation ava i l a ble on numerous we b s i t e s. S e a r c htopics include therm o p h i l e s, extreme life, and astrobiology.

Additional Info r m a t i o nfrom Ye l l owstone National Pa r kYe l l owstone National Pa rk we b s i t e, www. n p s. g ov / yell, includes

an array of park info rmation about resources, science, recreation, and issues.

Ye l l owstone Science, published quart e rl y, reports on researchand includes articles on natural and cultural resources. Fr e e ;ava i l a ble from the Ye l l owstone Center for Resources, in theYe l l owstone Research Libra ry, or online at www. n p s. g ov / ye l l .

Ye l l owstone To d ay, p u blished seasonally and distri buted ate n t rance gates and visitor centers, includes features on parkresources such as hy d r o t h e rmal fe a t u r e s.

Area trail guides detail geology of major areas of the park .Ava i l a ble for a modest donation at Canyon, Fountain Pa i n tPot, Mammoth, Norri s, Old Faithful, and West Thumb areas.

Site Bulletins, published as needed, provide more detailed info r-mation on park topics. Fr e e ; ava i l a ble upon request from visi-tor centers.