oceans of archaea - université laval · novel cultivation strategies, in situ biochemical and...

Oceans of ArchaeaAbundant oceanic Crenarchaeota appear to derive from thermophilicancestors that invaded low-temperature marine environments

Edward F. DeLong

Earth’s microbiota is remarkably per-vasive, thriving at extremely hightemperature, low and high pH, highsalinity, and low water availability.One lineage of microbial life in par-

ticular, the Archaea, is especially adept at ex-ploiting environmental extremes. Despite theirsuccess in these challenging habitats, the Ar-chaea may now also be viewed as acosmopolitan lot. These microbesexist in a wide variety of terres-trial, freshwater, and marine habi-tats, sometimes in very high abun-dance. The oceanic Marine GroupI Crenarchaeota, for example, ri-val total bacterial biomass in wa-ters below 100 m. These wide-spread Archaea appear to derivefrom thermophilic ancestors thatinvaded diverse low-temperatureenvironments. Novel cultivationstrategies, in situ biochemical andgeochemical methods, and environmental genom-ics promise to further our understanding of theseand other ubiquitous and abundant Archaea.

Brief Overview of Archaea

In the late 1970s, Carl Woese at the Universityof Illinois, Urbana-Champaign, and his collabo-rators, through their ribosomal RNA analyses,first recognized Archaea as one of three majormonophyletic lineages on Earth. Their analysesof several microbial groups, including extremehalophiles and methanogens, revealed a uniqueprokaryotic lineage that diverges from most fa-miliar bacterial groups at the deepest phyloge-netic levels. In recognizing the Archaea as aseparate evolutionary unit, Woese reclassifiedorganisms into three domains: Eucarya (all eu-

karyotes), Archaea, and Bacteria. Although al-ternative taxonomic schemes have been recentlyproposed, whole-genome and other analysestend to support Woese’s three-domain concept.

Well-known and cultivated archaea generallyfall into several major phenotypic groupings:these include extreme halophiles, methanogens,and extreme thermophiles and thermoacido-

philes. Early on, extremely halo-philic archaea (haloarchaea) werefirst noticed as bright-red coloniesgrowing on salted fish or hides.For many years, halophilic isolatesfrom salterns, salt deposits, andlandlocked seas provided excellentmodel systems for studying adap-tations to high salinity. It was onlymuch later, however, that it wasrealized that these salt-loving“bacteria” are actually membersof the domain Archaea.

Another major archaeal group,the strictly anaerobic methanogenic archaea,produce most of the methane in our atmosphere.Methanogens are one of the most cosmopolitanof cultivated archaeal groups, occupying diversehabitats that include anoxic sediments and ricepaddies, the rumen of cattle, the guts of termites,hydrothermal vents, and deep subterraneanhabitats. A third phenotypic group of culturedArchaea are the heat- and acid-loving thermo-philes and thermoacidophiles. These unusualArchaea dwell at hydrothermal vents, hotsprings, deep subterranean environments, oilreservoirs, and even on burning coal refuse piles.

Cultivated members within one kingdom ofthe domain Archaea, the Crenarchaeota (Fig. 1),consist entirely of thermophilic species. Re-markably, a recent report by Derek Lovely at theUniversity of Massachusetts, Amherst, suggests

Archaea exist ina wide varietyof terrestrial,

freshwater, andmarine habitats,

sometimes invery high

abundance

Edward F. DeLongis a senior scientistat the MontereyBay Aquarium Re-search Institute,Moss Landing,Calif., [email protected].

Volume 69, Number 10, 2003 / ASM News Y 503

that some Crenarchaeota can grow at tempera-tures exceeding 120°C (see p. 484). In additionto high-temperature growth, some archaealthermophiles like Picrophilus oshimae alsothrive at extremely low pH, growing best athydrogen ion concentrations approaching 1 M.

Archaeal Ubiquity?

Despite expanding information on cultivatedArchaea, only a small fraction of naturally oc-curring archaeal diversity is currently repre-sented in culture collections. Cultivation-inde-pendent survey approaches pioneered byNorman Pace of the University of Colorado,Boulder, have greatly expanded our current ap-preciation of natural microbial diversity. Suchsurveys indicate that a significant amount ofmicrobial diversity found in natural habitats hasso far eluded cultivation in the laboratory. Char-acterizing these elusive and understudied butubiquitous microorganisms remains a criticalgoal for gaining a deeper and truer appreciationof microbial evolution, physiology, and ecology.

Although Archaea were thought to preferen-tially occupy environments that are inhospitableto Eucarya and Bacteria, their ecological andphysiological diversity turns out to be far greaterthan previously supposed. Some early hintscame from PCR-based, ribosomal RNA-gene

surveys of mixed microbial popula-tions from open ocean and coastal ma-rine waters.

From such studies, Jed Fuhrman ofUniversity of Southern California inLos Angeles presented evidence in1992 for one new type of archaeal ri-bosomal RNA sequence, recoveredfrom planktonic microbes at depths of500 m in the Pacific Ocean. At aboutthe same time, I reported the discoveryof two archaeal groups in coastal sea-water, one identical to that found byFuhrman (the Group I planktonic Cre-narchaeota, Fig. 1), and an additionalgroup (Group II planktonic Eur-yarchaeota, Fig. 1) distantly related tohalophiles and methanogens (Fig. 1).We found that these Archaea contrib-ute significantly to the total extractablerRNA in marine surface waters.

The surprise at that time was thatany Archaea could be found in cold,

aerobic habitats of coastal and open ocean wa-ters. No cultivated, characterized Archaea werethen known to grow at the salinities, tempera-tures, and oxygen concentrations found in tem-perate oceanic waters, shallow or deep. Soon,researchers were reporting finding rRNAs be-longing to divergent archaeal groups in all sortsof habitats, including soils, freshwater and ma-rine sediments, and anaerobic digestors. Manyof the new, nonmarine archaeal types in soilsand freshwater sediments were related to themarine Group I Crenarchaeota. Recently, a sur-vey of the Great Lakes by Randall Hicks andcolleagues at the University of Minnesota, Du-luth, has also revealed the prevalence of similartypes of Crenarchaeota. It has been surprising torealize that Archaea, initially thought to be rel-egated to extreme environments, actually thriveall over our planet, including the vast worldoceans, the Great Lakes, and the very soil be-neath our feet.

Among the four major groups of planktonicmarine Archaea, the most abundant appear tobe the first groups recognized, Group I Crenar-chaeota and Group II Euryarchaeota (Fig. 1).Two other planktonic archaeal groups have alsobeen detected, but appear to be lower in abun-dance, according to recent surveys. The GroupIII planktonic Archaea, found in waters belowthe photic zone, are peripherally related to the

F I G U R E 1

Schematic tree showing the relationships of the four major groups of planktonic marineArchaea relative to cultivated groups.

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order Thermoplasmatales. Members of the pe-lagic Group IV Archaea, discovered by Fran-cisco Rodriguez-Valera and collaborators of theUniversidad Miguel Hernandez in Spain, appearto be related to haloarchaea, and apparentlyinhabit deep ocean waters. Currently, hundredsof ribosomal RNA sequences derived from ma-rine planktonic Archaea exist in the public da-tabases. These data indicate that substantial ge-netic variability (e.g., microheterogeneity) existsamong very closely related sequence types, re-flecting unexplored evolutionary, genetic, andecological complexities.

Planktonic Archaeal Abundance,

Distribution, and Variability

Archaeal Groups I and II appear to be the mostabundant in the ocean and so far have been theeasiest to study in situ. Early reports using ra-diolabeled oligonucleotide probes to quantify

archaeal rRNA in marine plankton yielded someunexpected surprises. Perhaps most startling wasthe high relative abundance of Archaea in wintersurface waters off Antarctica, where planktonicCrenarchaeota sometimes comprise as much as20% of total microbial rRNA. Those coastalwaters are �1.8oC, a far cry from the 80oC orhigher growth temperature optima of cultivatedCrenarchaeota. Similar surveys in temperatewaters off the coast of California show that theplanktonic Crenarchaeota tend to be mostabundant in depths below about 100 m or so.

Modified fluorescence in situ hybridizationassays have led to better estimates of archaealcell concentrations throughout the oceanic wa-ter column. Off the California coast, we showedthat planktonic Crenarchaeota represent morethan 20% of the total picoplanton cells in waterfrom 80 m to 3,000 m. Our later studies withDavid M. Karl and colleagues of the University

Breaking the Ice, Finding New Habitats for ArchaeaMarine biologist Edward DeLongreceived an unforgettable lessonabout eight years ago during afield trip to Antarctica, when theship he was on became lodged inice. What ordinarily should havebeen a four-day journey fromSouth America to Antarcticastretched into two weeks with norescue in sight. “When you’redown in Antarctica, nature callsthe shots and you learn to beadaptable,” says DeLong, a seniorscientist with the Monterey BayAquarium Research Institute inMoss Landing, Calif.

DeLong and his colleagueswere en route to the AntarcticaPeninsula in August 1995, theaustral winter, to study Archaea,then newly discovered to inhabitcold-ocean waters. They weretraveling by icebreaker, but thatdidn’t prevent the ship from get-ting trapped in the ice. Ice formed

around the boat, and nothingbudged. Fortunately, the situationwasn’t dire, but it was unlikelythat another icebreaker would beavailable to dislodge them. Sothey waited, wondering whetherthey would have to remain thereuntil the summer thaw.

When a shift in the winds ledthe ice to break up, they were freebut also low on fuel. They had toturn back, refuel, and start again.“It wasn’t scary, just an interest-ing experience in sociology, to seehow you and your colleaguesadapt to being stuck on a boat fortwo weeks with nothing to do,”Delong says. “Actually, it wasfun.”

DeLong has been having funfor his entire career. It beganduring his childhood with a loveof the sea. Delong, now 45,grew up in Sonoma, Calif., wherehe learned to skin dive for aba-

lone, scubadive, and pad-dle a kayak.“I fell in lovew i t h t h eocean as akid. I reallydo love thesea,” he says.“I always tell people that it’samazing I get paid to do what Ido.”

After finishing high school, De-Long moved to Alaska for a year“to find myself,” then returned toCalifornia to start college. Hestudied biology at Santa RosaJunior College, Calif., in 1980,before obtaining a B.S. degree inbacteriology from the Universityof California, Davis (UCD) in1982, and a Ph.D. in marine biol-ogy from the Scripps Institute ofOceanography, San Diego, Calif.,in 1986. In 1989, he received an

Continued


of Hawaii, Manoa, show a similar trend in theNorth Pacific Ocean Gyre, with pelagic Crenar-chaeota comprising more than 30% of the totalmicrobial cells from 200 m down to 5,000 m(Fig. 2).

Estimates from these cell counts indicate thatplanktonic Crenarchaeota account for a largefraction of the total microbial biomass through-out the world’s oceans—consisting of about 1 �1028 cells, or about 20% or so of all the bacterialand archaeal cells found in the sea. Recent esti-mates based on the abundance of archaeal lipidsin seawater are consistent with these cell counts.In some locations, archaeal abundance appearsto vary with the seasons. For instance, off theAntarctic Peninsula, the pelagic Crenarchaeota(Group I) appear more abundant in surface win-ter waters and subsurface summer waters. Intemperate seas, Marine Group II Euryarcha-eota, unlike Marine Group I Crenarchaeota,

seem to be more abundant at the surface. Thisgroup also shows evidence of seasonal fluctua-tions, reaching highest abundance in summer inthe North Sea. Marine Group II Archaea spring“blooms” have been observed off the coast ofCalifornia that are restricted to the photic zone.

Another Surprise—an Archaeal Symbiont

of Sponges, Cenarchaeum symbiosum

While a graduate student at the University ofCalifornia, Santa Barbara, Christina Prestonmade an unexpected discovery. She found that asingle species of cold-loving crenarchaeote liveswithin the tissues of the coastal marine spongeAxinella mexicana (Fig. 3). Dubbed Cenar-chaeum symbiosum, this symbiotic crenarchae-ote can thrive at low temperatures—in this case,about 10oC. This sponge-archaeal association isspecific, with C. symbiosum detectable onlywithin Axinella mexicana, and not in surround-

independent study award fromWoods Hole Oceanographic In-stitute, Woods Hole, Mass.,where he met his wife, KathleenLedyard, a biogeochemist. Theyhave a six-year-old son.

Earlier, DeLong thought hewould end up becoming a medicaltechnologist, specializing in mi-crobiology. But at UCD, he metPaul Baumann, a professor andmicrobiologist who is an expert inmarine bacterial taxonomy andbiochemistry. “Paul got me intothis whole research world of ma-rine microbiology, and I graduallyfigured out you could make a ca-reer doing this sort of thing,”DeLong says.

After finishing his thesis workwith Art Yayanos at Scripps, De-Long did his postdoctoral workwith Norman Pace, then at Indi-ana University, Bloomington,who introduced him to the tech-niques of using DNA sequences toidentify microbes. These ap-

proaches allow scientists to sur-vey entire microbial communitieswithout the requirement of culti-vation in the lab.

“There was a real big impactwhen these techniques becameavailable to study naturally occur-ring microbial communities enmasse, rather than studying mi-crobial cultures one by one,”DeLong says. “Science jumps for-ward a lot of times in lockstepwith new technologies, and thisopens up new world views for us.”

In the late 1970s, microbiolo-gist Carl Woese had begun tocompare microorganisms by us-ing purely genetic information—and learned that Archaea are notordinary bacteria, as originallythought. Later, scientists learnedthat these and related microor-ganisms do not live only in ex-treme environments, such as boil-ing water, acid-laden streams, orsuper-salty brine. “We didn’t evenknow that they lived in cold-

ocean water, but it turns out theyare major ocean inhabitants,” hesays. “If you would have askedany microbiologist 10 years agowhether Archaea were abundantin the seawater, they would havejustifiably said ‘no—it’s not anextreme environment.’ In actualfact, oceanic Archaea representone of the major groups of Ar-chaea on Earth. What we are upto now is trying to learn moreabout their biology and ecology,”he adds.

The goal of that nearly ill-fated,ice-breaking field trip in 1995 wasto gather samples of cold-sea wa-ter, hoping to find Archaea. Whenthey arrived on the continent,they crossed the sea ice on skis,drilling holes to extract seawater.“We found, as predicted, that thefrigid Antarctic waters werechock-full of Archaea,” he says.

Marlene Cimons

Marlene Cimons is a freelance writerin Bethesda, Md.


ing seawater or other sponges. This sym-biotic archaeon grows well at temper-atures of 10°C, more than 60°C belowthe growth temperature optimum of itsclosest cultivated relatives in the Cren-archaeota (Fig. 1). Since C. symbiosumcan be successfully maintained in lab-oratory aquaria within the tissues of itshost, it provides a useful source ofcold-dwelling crenarchaeotal biomass.Because C. symbiosum is the only ar-chaeon present, archaeal cell prepara-tions from the sponge are useful forbiochemical studies—for example, todefine the lipid composition of cold-dwelling Crenarchaeota. This ar-chaeon should also be useful to helpfurther define the physiological, bio-chemical, and genomic characteristicsof cold-dwelling Crenarchaeota. Simi-lar species from other habitats, includ-ing Mediterranean sponges of the ge-nus Axinella, now also have beenreported to specifically harbor archaeaclosely related to C. symbiosum.

Physiological Ecology of Oceanic

Archaea

Relatively little is known about thephysiology of oceanic Archaea species,due in part to the unavailability of purecultures. Creative applications of bio-chemical, geochemical, and genomictechniques, however, are now provid-ing valuable insights into planktonicCrenarchaeota, including informationabout specific enzymes, membranelipid composition, and potential car-bon sources.

Cold marine sediments contain highlevels of tetraether lipids (Fig. 4), de-rived from the membrane lipids ofplanktonic Archaea, according to JaapS. Sinninghe-Damste of the RoyalNetherlands Institute for Sea Research(NIOZ), Texel, the Netherlands. Freshlycollected marine plankton samplescontaining abundant planktonic Cren-archaeota also contain high levels ofsimilar tetraether lipid sediments. Fur-ther, Cenarchaeum symbiosum, a mem-ber of the Marine Group I archaeal

F I G U R E 2

Abundance of planktonic Archaea at the Hawaii Ocean Time series station as a functionof depth, determined by fluorescence in situ hybridization counts. Percentage of MarineGroup I Archaea relative to total microbial counts in 2A is indicated by color; red, 40% ofcell total; dark blue, 0% of cell total. Figure 2B shows the distribution of cell typesdetermined by FISH, averaged over one year. Crenarchaeota refers specifically to MarineGroup I Archaea, and Euryarchaeota refers specifically to Marine Group II Archaea. FromKarner, M., E. F. DeLong, and D. M. Karl. 2001. Archaeal dominance in the mesopelagiczone of the Pacific Ocean. Nature 409:507–510, with permission.


F I G U R E 3

The marine sponge Axinella mexicana and its archaeal symbiont, Cenarchaeum symbiosum. 3A., Axinella mexicana a bright reddemosponge found off the California coast. 3B., A. mexicana maintained in laboratory aquaria. 3C., FISH experiment showing C.symbiosum population present in the sponge tissues (in green). Many of the C. symbiosum cells are visibly dividing.


clade, contains the exact same tet-raether lipids as are found in plankton.Taken together, these data establishthat Marine Group I Archaea are amain source of the tetraether lipids, al-though additional archaeal sources(e.g., Group II) for these lipids in plank-ton cannot be ruled out.

Stefan Schouten and colleagues ofNIOZ found structural differences be-tween the lipids of planktonic archaeaand those of archaeal thermophilessuch as Sulfolobus solfataricus. Glyc-erol dibiphytanyl glycerol tetraethers(GDGTs) are commonly found ar-chaeal lipid structures in marine sedi-ments and plankton, and they include anovel ether lipid, called crenarchaeol,containing a six-membered cyclohexylring (Fig. 4, structure e). Studies with C.symbiosum have confirmed that thesenovel lipids are definitely found inmembers of the Marine Group I Cren-archaeota. This uniquely characteristiccyclohexyl ring structure likely hasadaptive significance for the low-temperatureArchaea. The Netherlands group proposes thatthis change from the standard, five-memberedring (found in thermophilic archaeal lipids) addsa kink in the tetraether chains, rendering themembrane structures they form more fluid atlower temperatures.

If planktonic archaea are globally abundant,what niche do they occupy? The Netherlandsgroup analyzed stable isotopes in marine sedi-ment-derived archaeal lipids in attempting todetermine the carbon sources that support ar-chaeal growth. Their initial findings suggestedthat planktonic archaea use either algal carbo-hydrates, proteins, or dissolved bicarbonate andthus do not indicate whether these archaea areautotrophs or heteroptrophs. Other studies us-ing radiolabeled amino acids, microautoradiog-raphy, and flourescent in situ hybridization(FISH) have provided some evidence indicatingthat planktonic archaea might incorporate ex-ogenous amino acids.

In her thesis work Ann Pearson, working withTim Eglinton and John Hayes at the WoodsHole Oceanographic Institution, Woods Hole,Mass., purified large amounts of marine ar-chaeal lipids from the deep sea, presumablyderived from planktonic Crenarchaeota. Using

naturally occurring 14C in oceanic organic mat-ter and CO2 as a tracer, their results indicatedthat deep-water archaea are not using organicmatter derived from recent photoautotrophicprimary production. Instead, these deep-seaCrenarchaeota apparently are incorporatingdissolved CO2 as their main carbon source.Meanwhile, the Netherlands group has recentlyacquired additional data supporting this hy-pothesis, via 13CO2 isotope tracer-labeling ex-periments. This finding, if true, is quite a sur-prise, implying that oceanic Archaea fix CO2

and may represent an unexpected source of pri-mary productivity in the sea. If correct, what arethese Archaea using as an energy source to fuelcell growth and CO2 fixation? Answering thisquestion may provide quite a bit of new infor-mation about carbon and energy flux in the deepwaters of the world’s oceans.

Genomics and Evolutionary

Origins of Oceanic Archaea

Modern genomic approaches are now provingextremely useful for studying microbes thathave resisted cultivation. In the early 1990s,working with Jeff Stein, then at the AgouronInstitute, La Jolla, Calif., we isolated large

F I G U R E 4

Archaeal lipid structures found in marine plankton and sediments. GDGT, glyceroldibiphytanyl glycerol tetraether. The most common forms found in marine plankton,incorporating sidechains a, or d and e, are GDGTs I and VIII. Modified after S. Schouten,E. C. Hopmans, R. D. Pancost, and J. S. S. Damste�. 2001. Widespread occurrence ofstructurally diverse tetraether membrane lipids: Evidence for the ubiquitous presence oflow-temperature relatives of hyperthermophiles. Proc. Natl. Acad. Sci. USA 97:14421–14426, Copyright 2001, National Academy of Sciences, U.S.A.


genomic fragments from the planktonic MarineGroup I Crenarchaeota. Thus began our explo-rations in environmental genomics. Later inmy lab, Christa Schleper, now at the TechnicalUniversity of Darmstadt in Germany, devel-oped high-quality genomic libraries from thesymbiotic archaeon Cenarchaeum symbiosum.Schleper next identified C. symbiosum DNApolymerase in those genomic libraries, and ex-pressed, purified, and characterized it biochem-ically. Although the DNA polymerase aminoacid sequence most closely resembles thermo-stable DNA polymerases from other hyperther-mophilic Crenarchaeota, the enzyme itself is notthermostable. It is inactivated at about 40oC,consistent with the lower temperature originsand ecological niche of C. symbiosum. We latercollaborated with a group at Diversa Corpora-tion, San Diego, Calif., to sequence 100 kbp ofthe C. symbiosum genome and learn more aboutthe genome of this archaeon.

More recently, support from the NationalScience Foundation has allowed us to embarkon full-genome sequencing of C. symbiosum.With a third of the genome now in hand, newclues about the physiological and genetic archi-tecture of marine Crenarchaeota are alreadybeginning to emerge. These new genomic datapromise to teach us much about the naturalhistory, genetics, evolution, and physiology ofthe ubiquitous marine Crenarchaeota.

What extraordinary events led to the widedistribution of Crenarchaeota in habitats rang-ing from boiling hot springs of 100oC to frigidpolar waters of –1.8oC ? There are several cluesto the natural history of this fascinating groupthat come from a variety of disciplines, includ-ing molecular evolution, biochemistry, and geo-chemistry. All of these perspectives tend to sug-gest that the ancestors of modern-day, cold-adapted marine Archaea once lived in anoxic,

high-temperature habitats. It also seems likely,based on phylogenetic analyses, that differentlineages within the Crenarchaeota adapted tolow-temperature habitats on multiple occasions.The journey from hot to cold environmentsseems to have been a common trek in the courseof evolution within different archaeal groups.

There are a number of outstanding puzzlesconcerning the marine Archaea that remain tobe solved. For instance, the marine Group ICrenarchaeota are eurybathyl, that is, they aredistributed throughout the ocean depths fromnear-surface waters to the bottom of the abyss.Studies of fine-scale genetic and physiologicaldifferences in the group are likely to tell us quitea bit more about deep-sea adaptations. Deter-mining the specific carbon and energy sources ofGroup I Crenarchaeota remains another out-standing problem concerning this ubiquitousmicrobial group. Resolving this mystery willhelp us better interpret the ecological roles andsignificance of Archaea in oceanic ecosystems.

So, where to next with the marine Archaea?Steve Giovannoni at Oregon State University,Corvallis, has recently developed novel cultiva-tion strategies that have successfully recoveredelusive and difficult-to-cultivate planktonic bac-teria (see ASM News, August 2003, p. 370).Similar approaches may eventually lead to theisolation of pure cultures of planktonic Archaea.This would aid tremendously in our understand-ing of the physiology and ecology of theseocean-going microbes. In situ studies using ar-chaeal-specific biomarkers and geochemicaltracers also offer much promise for studyingindigenous marine Archaea on their own turf.Finally, genomic studies will provide us with notonly the parts list of marine Archaea, but alsoclues as to the physiological activities of thiswidespread, abundant, and fascinating compo-nent of microbial life on Earth.

ACKNOWLEDGMENTS

I am indebted to the intellectual and creative contributions (and camaraderie) of my laboratory cohorts and collaborators,especially: Oded Beja, Lynne Christianson, Matthew Church, John Hayes, David Karl, Jose de la Torre, Peter Girguis, StevenHallam, Ramon Massana, Alison Murray, Ann Pearson, Christina Preston, Christa Schleper, Jeff Stein, Trent Taylor, and KeYing Wu. Carl Woese and Norman Pace have generously provided me with ideas, advice, and inspiration over the past decade.Undergraduate research with Paul Baumann at the University of California, Davis, launched me on my career in science. Mythesis advisor Art Yayanos gave me just enough rope to hang myself, for which I will always be grateful. My work onplanktonic Archaea has been supported by the National Science Foundation and the David and Lucile Packard Foundation.

SUGGESTED READING

Church, M. J., E. F. DeLong, H. W. Ducklow, M. B. Karner, C. M. Preston, and D. M. Karl. 2003. Abundance anddistribution of planktonic Archaea and Bacteria in the Western Antarctic Peninsula. Limnol. Oceanogr. 48:1893–1902.


Damste, J. S., S. Schouten, E. C. Hopmans, A. C. van Duin, and J. A. Geenevasen. 2002. Crenarchaeol: the characteristic coreglycerol dibiphytanyl glycerol tetraether membrane lipid of cosmopolitan pelagic Crenarchaeota. J. Lipid Res. 43:1641–1651.DeLong, E. F., L. L. King, R. Massana, H. Cittone, A. Murray, C. Schleper, and S. G. Wakeham. 1998. Dibiphytanyl etherlipids in nonthermophilic crenarchaeotes. Appl. Environ. Microbiol. 64:1133–1138.DeLong, E. F. 1998. Everything in moderation: Archaea as “nonextremophiles.” Curr. Opin. Genet. Develop. 8:649–654.Fuhrman, J. A., K. McCallum, and A. A. Davis. 1992. Novel major archaebacterial group from marine plankton. Nature356:148–9.Karner, M., E. F. DeLong, and D. M. Karl. 2001. Archaeal dominance in the mesopelagic zone of the Pacific Ocean. Nature409:507–510.Pearson, A., A. McNichol, B. Benitez-Nelson, J. Hayes, and T. Eglinton. 2001. Origins of lipid biomarkers in Santa MonicaBasin surface sediment: a case study using compound-specific delta 14C analysis. Geochimica et Cosmochimica Acta65:3123–3137.Preston, C. M., K. Y. Wu, T. F. Molinski, and E. F. DeLong. 1996. A psychrophilic crenarchaeon inhabits a marine sponge;Cenarchaeum symbiosum, gen. nov. sp. nov. Proc. Natl. Acad. Sci. USA 93:6241–6246.Schleper, C., R. V. Swanson, E. J. Mathur, and E. F. DeLong. 1997. Characterization of a DNA polymerase from theuncultivated psychrophilic archaeon Cenarchaeum symbiosum. J. Bacteriol. 179:7803–7811.Stein, J., T. L. Marsh, K. Y. Wu, H. Shizuya, and E. F. DeLong. 1996. Characterization of uncultivated marine prokaryotes:isolation and analysis of a 40-kilobase genome fragment from a planktonic marine archaeon. J. Bacteriol. 178:591–599.


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