D'Hondt_Science306-1 by wulcon


									   RESEARCH ARTICLE                                                                                         Fe [inferred to be Fe(II)] consistently exhibit
                       Distributions of                                                                     concentration maxima deep in the drilled sed-
                                                                                                            iment columns (e.g., Fig. 1). These con-
                                                                                                            centration profiles indicate that biologically
                  Microbial Activities in Deep                                                              catalyzed reactions consume and release me-
                                                                                                            tabolites deep in the sediment column at all
                    Subseafloor Sediments                                                                   of the sites. The microbial processes implicit
                                                                                                            in Fig. 1 include organic carbon oxidation,
               Steven D’Hondt,1* Bo Barker Jørgensen,1 D. Jay Miller,1                                      ammonification, methanogenesis, methanotro-
          Anja Batzke,2 Ruth Blake,1 Barry A. Cragg,1 Heribert Cypionka,1                                   phy, sulfate reduction, and manganese re-
            Gerald R. Dickens,1 Timothy Ferdelman,1 Kai-Uwe Hinrichs,1                                      duction. Other processes that occur in these
                                                                                                            sediments include iron reduction and the pro-
          Nils G. Holm,1 Richard Mitterer,1 Arthur Spivack,1 Guizhi Wang,3
                                                                                                            duction and consumption of formate, acetate,
           Barbara Bekins,1 Bert Engelen,2 Kathryn Ford,1 Glen Gettemy,1                                    lactate, hydrogen, ethane, and propane (6).
              Scott D. Rutherford,4 Henrik Sass,2 C. Gregory Skilbeck,1                                         These activities are unexpectedly diverse.
              Ivano W. Aiello,1 Gilles Guerin,1 Christopher H. House,1
                                          `                                                                 The interstitial water chemistry of shallow
                  Fumio Inagaki,1 Patrick Meister,1 Thomas Naehr,1                                          marine sediments generally exhibits a pre-
                 Sachiko Niitsuma,1 R. John Parkes,1 Axel Schippers,1                                       dictable zonation, with peak concentrations
                  David C. Smith,1 Andreas Teske,1 Juergen Wiegel,1                                         of dissolved products from different redox
                                                                                                            processes [Mn(II), Fe(II), SH2S, and CH4]
                  Christian Naranjo Padilla,1 Juana Luz Solis Acosta1
                                                                                                            present at successively greater sediment
          Diverse microbial communities and numerous energy-yielding activities                             depths (9–12). This succession of redox zones
          occur in deeply buried sediments of the eastern Pacific Ocean. Distributions                      has been ascribed to competition between meta-
          of metabolic activities often deviate from the standard model. Rates of                           bolic pathways; electron-accepting reactions
          activities, cell concentrations, and populations of cultured bacteria vary                        that yield successively less negative standard
          consistently from one subseafloor environment to another. Net rates of                            free energies are hypothesized to predomi-
          major activities principally rely on electron acceptors and electron donors                       nate at successively greater depths because
          from the photosynthetic surface world. At open-ocean sites, nitrate and                           electron acceptors with higher free energy
          oxygen are supplied to the deepest sedimentary communities through the                            yields are depleted at shallower depths [e.g.,
          underlying basaltic aquifer. In turn, these sedimentary communities may                           (9, 11)]. In shallow marine sediments that
          supply dissolved electron donors and nutrients to the underlying crustal                          exhibit this zonation, terminal electron ac-
          biosphere.                                                                                        ceptors ultimately enter the sediment from
                                                                                                            the overlying ocean. As the reduced prod-
                                                                                                            ucts from below enter successively shallower
   Microbial life is widespread in the marine            er are largely unknown. Its relationship to        zones of SO42–, Fe(III), Mn(IV), NO3–, and
   sediments that cover more than two-thirds of          the photosynthetic surface world is not fully      O2 reduction, vertical cascades of electron-
   Earth_s surface. Intact cells (1) and intact          understood. Its relationship to the deeper world   accepting processes are sustained (11). For
   membrane lipids (2, 3) provide evidence of            of the underlying basaltic crust has not been      example, O2 may be used to oxidize Mn(II)
   prokaryotic populations in sediments as deep          tested.                                            to Mn(IV), which can oxidize Fe(II) to Fe(III),
   as 800 m below the seafloor (mbsf). Prokaryotic           To explore life in deeply buried marine        which might oxidize reduced sulfur, which
   activity, in the form of sulfate (SO42–) re-          sediments, we undertook Ocean Drilling Pro-        ultimately could oxidize hydrogen or organic
   duction and/or methanogenesis, occurs in              gram (ODP) Leg 201. The expedition sites           carbon.
   sediments throughout the world ocean (4).             are located in the equatorial Pacific Ocean            In many respects, dissolved chemical pro-
   The prokaryotes of subseafloor sediments              and on the continental margin of Peru (Fig. 1)     files of Leg 201 sites exhibit this standard
   have been estimated to constitute as much as          (6). These sites are typical of subsurface en-     zonation. However, they also depart from it
   one-third of Earth_s total living biomass (5).        vironments that exist throughout most of           in four important ways. First, at site 1229,
       Despite the ubiquity of life in subseafloor       Earth_s ocean. Their water depths range from       the introduction of dissolved SO42– from be-
   sediments, little is known about it. The di-          150 m on the Peru Shelf to 5300 m in the           low reverses the standard redox zonation by
   versity of its metabolic activities, the com-         Peru Trench. The sampled sediments ranged          sustaining a zone with abundant SO42–
   position of its communities, and the nature of        in subseafloor depth from 0 to 420 m, in tem-      beneath a sulfate-depleted methane-rich zone
   its variation from one environment to anoth-          perature from 1- to 25-C, and in age from 0        (Fig. 2). At this site and at nearby site 1228,
                                                         to 35 million years ago (Ma) (6, 7). Pro-          SO42– is introduced at depth by upward dif-
    Ocean Drilling Program Leg 201 Shipboard Scien-      karyotic cells occur throughout the sampled        fusion from ancient brine (6). This deep brine
   tific Party. 2Institut fur Chemie und Biologie des
   Meeres, Universitat Oldenburg, D-26111 Oldenburg,     sediment column at every site (Fig. 1) (8).        is present along much of the Peru Shelf (13).
   Germany. 3University of Rhode Island Graduate             Diversity of metabolic activities. Dis-        At sites 1225 and 1226, SO42– similarly dif-
   School of Oceanography, Narragansett, RI 02882,       solved electron acceptors such as SO42– and        fuses upward into the deepest sediments from
   USA. 4Department of Environmental Science, Roger      nitrate (NO3–) exhibit subsurface depletion,       water circulating through the underlying ba-
   Williams University, Bristol, RI 02809, USA.
                                                         whereas dissolved metabolic products such          saltic aquifer. Sulfate is supplied to the deep-
   *To whom correspondence should be addressed at        as dissolved inorganic carbon (DIC 0 CO2 þ         est sediments in this manner throughout much
   NASA Astrobiology Institute, University of Rhode
   Island Graduate School of Oceanography, South Ferry
                                                         HCO3– þ CO32–), ammonia (SNH3 0 NH3 þ              of the eastern equatorial Pacific (14).
   Road, Narragansett, RI 02882, USA. E-mail: dhondt@    NH4þ), sulfide (SH2S 0 H2S þ HS–), methane             Second, at some sites, the expected zona-
   gso.uri.edu                                           (CH4), manganese [inferred to be Mn(II)], and      tion is locally reversed by the appearance of

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                                                                                                                                                RESEARCH ARTICLE
large peaks in dissolved Mn and Fe concen-                                     Third, subsurface CH4 maxima occur                  genesis occurs long before electron acceptors
trations far below the seafloor (e.g., Figs. 1                             within the sediments of all Leg 201 sites, in-          that yield higher standard free energies have
to 3). Such midcolumn peaks demonstrate                                    cluding the open-ocean sites where dissolved            been depleted.
that, in discrete intervals, concentrations of                             SO42– concentrations are high (6) (Fig. 1).                 The fourth important departure occurs
buried iron- and manganese-bearing minerals                                Similar maxima have recently been identi-               only at the open-ocean sites, where the suc-
can be high enough and their rates of dis-                                 fied at many other open-ocean sites (4, 15).            cession of redox zones that extends from the
solution and reduction slow enough to con-                                 They indicate that methanogenesis occurs                seafloor to greater depths is mirrored by a
tinue long after metabolic activities with lower                           deep beneath the seafloor in most, perhaps              similar succession that extends upward from
standard free energies have become predom-                                 all, marine sediments, regardless of SO42–              the basement-sediment interface (Fig. 3). At
inant in shallower sediments.                                              availability. This result indicates that methano-       sites 1225 and 1231, relatively high concen-

Fig. 1. Map of Leg 201 sites and concentration profiles of several dissolved chemical species at five of the sites (17). At sites 1225, 1226, and 1231, the
deepest sample was taken just above the basaltic basement.

                0                                                                                                                                          Fig. 2. Concentration
                                                                                                                                                           profiles of cells and
                                                                                                                                                           some dissolved chemi-
                                                                                                                                                           cals at site 1229 (17).
               50                                                                                                                                          White bands mark
                                                                                                                                                           sulfate-methane tran-
                                                                                                                                                           sition zones. Arrows
Depth (mbsf)

                                                                                                                                                           mark midcolumn peaks
               100                                                                                                                                         in dissolved Mn con-


               200                                  200

                     4   5   6   7   8   9   10   11 0      10        20       30     40 0        1000       2000     3000 0   2       4    6    8    10
                Cell concentration (Log10 cells cm-3)            SO                                 CH ( M)
                                                                                                                               Dissolved Mn ( M)

                                                          www.sciencemag.org            SCIENCE      VOL 306        24 DECEMBER 2004                                               2217
   trations of NO3– and traces of O2 were dis-                    sulfate reduction) is similarly consistent with       net fluxes of SH2S out of those sediments,
   covered at the base of the sediment column                     this hypothesis.                                      and estimated Fe reduction rates within the
   (16, 17). This NO3– and O2 presumably enters                       The occurrence of methanogenesis in               sediments are much higher at the ocean-
   the sediments from oxic water circulating                      sulfate-replete porewaters and the occur-             margin sites than at the open-ocean sites. In
   through the underlying basaltic basement.                      rences of Mn and Fe reduction in deep methano-        contrast, estimated Mn reduction rates and net
       A local peak in dissolved Mn occurs just                   genic zones and deep sulfate-reducing zones           NO3– reduction rates in the subsurface sed-
   above this deep nitrate-reducing zone at both                  require different explanation(s) than the             iments (from 91.5 mbsf to the base of the
   sites 1225 and 1231 (Fig. 3). A similar peak                   reversed redox zones. There are at least              drilled sediments) are higher at the open-ocean
   occurs in the basal sediments of site 1226.                    three possible explanations of these occur-           sites than at the ocean-margin sites. These net
   At all three sites, these Mn concentration                     rences: (i) The organisms that undertake these        NO3– reduction rates entirely result from the
   maxima are stratigraphically overlain by maxi-                 different processes may rely on noncompet-            introduction of NO3– from the underlying ba-
   ma in dissolved Fe concentrations. These Mn                    itive substrates (different electron donors)          saltic aquifers.
   and Fe maxima mark successive intervals of                     in deep marine sediments; (ii) organisms                  These rates of subseafloor activities vary
   Mn and Fe reduction.                                           that rely on electron-accepting pathways with         predictably from open-ocean sites to ocean-
       The deep occurrences of successive O2,                     higher standard free energies may have higher         margin sites (Table 1). At each site, the pre-
   NO3–, Mn, and Fe redox zones at open-ocean                     energy requirements than organisms that rely          dominant energy-yielding pathways may be a
   sites have three immediate implications. First,                on pathways with lower standard free energy           function of total electron-accepting activity.
   the transport of O2 and NO3– through the                       yields; or (iii) the in situ free energies of these   For example, if the electron donor is C(0) for
   underlying basaltic aquifer sustains aerobic                   reactions may differ greatly from their stan-         electron-accepting reactions, carbon oxidation
   and nitrate-reducing prokaryotic communities                   dard free energies (for example, all of these         by net SO42– reduction appears to greatly
   in the deepest (11 to 35 Ma) sediments of                      reactions may yield similar free energies in          outpace carbon oxidation by metal (Mn and
   these sites, although anaerobic communities                    deep subseafloor sediments where they co-             Fe) reduction at the high-activity ocean-margin
   are active in the overlying sediment. Second,                  occur). Whichever explanation ultimately              sites and the most active open-ocean site
   this deep introduction of NO3– and O2 may                      applies, subseafloor occurrences of many              (Table 1) (20). In contrast, at the open-ocean
   cause Mn and Fe oxidation fronts and                           electron-accepting activities cannot be pre-          site where net activities are lowest (site 1231),
   thereby sustain continued Mn and Fe cycling                    dicted by simple extrapolation of the shallow         SO42– reduction is not detectable and net res-
   at the base of the sediment column. Third,                     marine redox zonation to sediments at                 piration in sediments deeper than 1.5 mbsf
   respiration along the flow path through the                    greater depths.                                       may principally rely on reduction of Mn(IV)
   underlying basalts is insufficient to strip O2                     Rates of electron-accepting activities. At        and Fe(III).
   and NO3– from the circulating water. Respi-                    steady state, fluxes of SO42– and NO3– into a             Biogeochemical linkages to the surface
   ration in these basaltic aquifers may be                       sediment column are respectively equal to             world and to life in underlying aquifers. The
   limited by electron donor availability (18).                   net reduction rates of SO42– and NO3– within          activities in Table 1 ultimately rely on elec-
       These discoveries indicate that the energy-                that column. Also at steady state, the mini-          tron acceptors from the photosynthetically
   yielding activities of deep subseafloor sedi-                  mum rate of metal (Mn or Fe) reduction is             oxidized surface world. O2, NO3–, and SO42–
   mentary ecosystems are far more diverse                        equal to the flux of the dissolved metal from         ultimately enter these sediments by diffus-
   than can be predicted from the standard redox                  zones of net reduction (marked by local con-          ing down past the seafloor and, at the open-
   zonation of shallow marine sediments. Al-                      centration peaks) to zones of net precipita-          ocean sites, by transport upward from seawater
   though unexpected, the reversed redox succes-                  tion or oxidation (marked by concentration            flowing through the underlying basalts. The
   sions of the deepest open-ocean sediments are                  minima) (19).                                         oxidized Mn and Fe were originally intro-
   consistent with the hypothesis that electron-                      Biogeochemical flux models based on con-          duced to the sediments by deposition of Mn
   accepting pathways with successively lower                     centration data and sediment physical prop-           and Fe at the seafloor.
   standard free energies predominate at suc-                     erties were used to quantify rates of these               The activities in Table 1 probably also
   cessively greater distances from a source of                   electron-accepting activities at most Leg 201         principally rely on electron donors from the
   oxic water. The reversed subseafloor succes-                   sites (Table 1) (17, 19). Net rates of SO42– re-      photosynthetically oxidized surface world. The
   sion of site 1229 (from methanogenesis to                      duction in subseafloor sediments (91.5 mbsf),         ultimate electron donors for subsurface eco-

   Fig. 3. Dissolved concentration                       0
   profiles of NO3– (red diamonds),
   Mn (light blue squares), and Fe
   (dark blue circles) at open-ocean
   sites 1225 (A), 1226 (B), and 1231                   100
   (C). Arrows mark midcolumn peaks
   in dissolved concentrations of Mn
   and Fe at site 1226. At each site,
                                         Depth (mbsf)

   the deepest sample was taken just                    200
   above the basaltic basement.



                                                              0    40       80      120      160 0       40       80    120      160 0       40      80      120     160
                                                                   Concentration (µM)                    Concentration (µM)                  Concentration (µM)

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                                                                                                                                  RESEARCH ARTICLE
systems have been hypothesized to include                    trations are highest at sites where concen-            site 1231 (25), this isolate demonstrates that
buried organic matter from the surface world                 trations of metabolic products (SNH3, CH4,             previously undiscovered prokaryotes exist in
(9, 10), reduced minerals [such as Fe(II)-                   DIC) and net rates of SO42– reduction and Fe           deep subseafloor sediments of the open ocean.
bearing silicates] (21, 22), and thermogenic                 reduction are highest, and cell concentra-                 Although the cultured bacteria constitute
CH4 from deep within Earth (23). Comparison                  tions are lowest at sites where these rates and        only a small fraction of the total cell count in
of our subseafloor carbon oxidation estimates                metabolic product concentrations are lowest            each sample (up to 0.1%), these results hint
to published estimates of organic burial rates               (Fig. 1 and Table 1). The open-ocean sites             of consistent patterns in the community com-
(24) suggests that buried organic carbon from                contained some of the lowest average cell              position of subsurface sediments. Some lin-
the overlying photosynthetic world is abun-                  concentrations ever observed in deep-sea sed-          eages appear to be cosmopolitan members
dant enough to fuel most or all of the esti-                 iments, whereas sediments recovered from               of subseafloor sedimentary communities. The
mated electron-accepting activities (Table 1).               the Peru Shelf contained the highest concen-           most commonly cultured taxa are Firmicutes
    The role of reduced minerals cannot be                   trations ever observed beneath the seafloor            that are most closely related to the spore-
directly assessed with our data. However,                    (Fig. 2).                                              forming bacterium Bacillus firmus and a-
thermodynamic considerations preclude oxi-                       Cell concentration profiles are also close-        Proteobacteria that are most closely related
dation of Fe(II) or Mn by Fe(III) or SO42–.                  ly related to dissolved reactant distributions         to Rhizobium radiobacter. These taxa were
Consequently, reduced Fe and Mn are unlikely                 within individual sites. For example, at site          often recovered from open-ocean sediments
to be important subseafloor electron donors                  1229, high cell concentrations occur in sub-           with abundant dissolved SO42– and little CH4
at any sites where SO42– or Fe(III) is the                   seafloor sulfate-methane transition zones              (Table 2). They were also often recovered
principal electron acceptor. In the sediments                (Fig. 2). Diffusion of the two reactants               from ocean-margin sediments with abundant
of site 1231, where Mn(IV) appears to be the                 (SO42– and CH4) to these zones provides an             CH4 and no dissolved SO42–. Close relatives
principal electron acceptor, Fe(II) may be an                interface of high biochemical energy supply.           of R. radiobacter have also been recently iso-
important electron donor.                                    At one such zone (92 mbsf), this interface             lated from Mediterranean subseafloor sedi-
    Our data clearly indicate that activity in               supports cell densities that are an order of mag-      ments (26). Recent surveys of archaeal 16S
the sediments of our open-ocean sites is not                 nitude higher than at the seafloor (Fig. 2).           genes in subseafloor sediments suggest that
fueled by thermogenic CH4 from deep within                       Bacteria were successfully cultured and            some archaeal lineages [the Deep-Sea Archaeal
Earth. Maximum concentrations in the mid-                    isolated from multiple depths at every site            Group and the Marine Benthic Group A] are
dle of each open-ocean sediment column and                   (Table 2) (17). These cultures indicate that           similarly cosmopolitan members of subsea-
minimum concentrations near the sediment-                    living bacteria are present throughout the en-         floor sedimentary communities (27).
basement interface (Fig. 1) indicate that bio-               tire range of subseafloor depths sampled by                Other lineages appear to be more selec-
genic CH4 and SNH3 are produced deep in                      Leg 201 (1 to 420 mbsf) (Table 2). As as-              tive in their subseafloor environmental affini-
these sediments, and, at sites where chemical                sessed from the 16S ribosomal RNA (rRNA)               ties. For example, cultured g-Proteobacteria
transport is dominantly diffusive, actually mi-              genes of 168 isolates, these bacteria belong           were consistently found at ocean-margin sites
grate downward toward the underlying basalts.                to at least six distinct lineages (Table 2) (17).      (Table 2), where concentrations of organic mat-
In this manner, the deep sedimentary com-                    Most of these isolates are closely related to          ter, cell concentrations, and net metabolic rates
munities may provide electron donors and                     known marine organisms. Others are more                are high. However, they were rarely found at
biologically accessible nitrogen to commu-                   distant from known organisms. Most striking-           open-ocean sites, where organic concentra-
nities in the underlying basaltic aquifers.                  ly, the 16S gene of one isolate from open-             tions, cell counts, and net metabolic rates are
    Environmental variation in cell abun-                    ocean site 1225 differs from the 16S gene of           low. In contrast, Actinobacteria were most
dance and cultured isolates of subseafloor                   its nearest known relative (a member of the            consistently found in sulfate-reducing sedi-
sedimentary communities. Cell concentra-                     Bacteroidetes) by 14% (Table 2). In combi-             ments of the open-ocean sites (sites 1225,
tions vary with metabolic reaction rates and                 nation with the recent discovery of deeply             1226, and 1231) and ocean-margin site 1227.
metabolic product concentrations from site                   rooted but previously unknown archaeal 16S                 In short, subseafloor sedimentary com-
to site (Fig. 1). For example, cell concen-                  gene sequences in subseafloor sediments of             munities contain some taxa that inhabit a

Table 1. Estimated reduction rates and carbon oxidation equivalents at ODP Leg 201 sites. BDL, below detection limit; ND, not determined.

                                                                                                  Potential    Potential
                                                                      SH2S flux    Potential                                                Potential     Organic
                                                                                                 C oxidation C oxidation
                Water Net NO3 – Estimated       Estimated Net SO42–      out of   C oxidation                                              C oxidation    carbon
                                                                                              – by estimated by estimated
Leg 201         depth reduction Mn reduction Fe reduction reduction sediment by net NO3                                                    by net SO42–    burial
                                –2          –2          –2 (mol cm –2                              Mn(IV)        Fe(III)
location      (m below (mol cm     (mol cm     (mol cm                  column     reduction                                                reduction    rate (24)
                              –1)         –1)         –1)         –1)          –2                 reduction    reduction
              sea level) year        year        year        year     (mol cm     (mol cm  –2                                              (mol cm –2   (mol cm –2
                                                                                                 (mol cm –2 (mol cm –2
                                                                        year –1)    year –1)                                                 year –1)     year –1)
                                                                                                   year–1)      year –1)

                                                                            Peru margin sites
Shelf site        427         BDL        2.2 Â 10 –11    1.0 Â 10–7* 0.9 Â 10 –6 –0.7 Â 10 –6      BDL        1.1 Â 10 –11   2.5 Â 10 –8    1.8 Â 10 –6   3.1 Â 10 –6
Slope site       5086         ND         1.4 Â 10 –10    2.5 Â 10 –7* 2.5 Â 10–6 –2.0 Â 10 –6       ND        0.7 Â 10–10    6.3 Â 10 –8    5.0 Â 10 –6   3.1 Â 10 –6
                                                                             Open Pacific sites
Equatorial       3760      1.3 Â 10 –9   2.9 Â 10–8        1 Â 10 –8* 1.9 Â 10 –8   BDL         1.6 Â 10 –9   1.5 Â 10 –8    2.5 Â 10 –9    3.8 Â 10 –8   4.5 Â 10 –7
  site 1225
Equatorial       3297         ND         5.9 Â 10 –9       7 Â 10 –8* 1.4 Â 10–7 –1.3 Â 10 –9       ND        3.0 Â 10 –9    1.8 Â 10 –8    2.8 Â 10 –7   4.6 Â 10–7
  site 1226
Peru Basin       4813      8.0 Â 10 –9   6.1 Â 10 –8     3.9 Â 10 –8     BDL          BDL       1.0 Â 10 –8   3.0 Â 10 –8    1.0 Â 10 –8       BDL        9.4 Â 10 –7
  site 1231
*Inferred from net S burial. Assumes all buried S goes to FeS2.

                                               www.sciencemag.org          SCIENCE      VOL 306      24 DECEMBER 2004                                                   2219
   Table 2. Cultured bacterial isolates from Leg 201 sediments. Species listed are type species from GenBank database.

                                                                      Numbers of isolates (lowest and highest depth of discovery)
   Closest relative
   (16S rRNA                                       Open Pacific sites                                                                Peru margin sites
   sequence similarity)
                                   1231                1225                     1226                     1227                     1228                    1229                    1230
   Rhizobium                                     7 (1 to 198 mbsf)      2 (1 to 381 mbsf)    14 (12 to 102 mbsf)                                   5 (12 to 70 mbsf)        13 (1 to 124 mbsf)
      radiobacter (98%)
   Rhodobacter                                                                                                                                                               1 (268 mbsf)
      capsulatus (95%)*
   Rhodovulum                   1 (43 mbsf)
      sulfidophilum (96%)
   Bacillus firmus (97%)    14 (2 to 43 mbsf)                           12 (1 to 420 mbsf) 8 (1 to 102 mbsf)                                     34 (1 to 187 mbsf)
   Bacillus simplex (96%)*                                                 1 (1 mbsf)                                                                1 (70 mbsf)
   Alkaliphilus                                                                                                                                      4 (1 mbsf)
      transvaalensis (96%)*
   Paenibacillus                                   1 (198 mbsf)
      glucanolyticus (98%)
       Vibrio                   1 (101 mbsf)                                                                               6 (1 to 114 mbsf) 11 (1 to 187 mbsf)
          mediterranei (99%)
       Vibrio                                                                                                               1 (114 mbsf)                                   4 (1 to 82 mbsf)
          diazotrophicus (99%)*
       Photobacterium                                                                                                           1 (1 mbsf)
          fischeri (94%)
       Psychrobacter                                                                                                                                                       3 (1 to 124 mbsf)
          okhotskensis (98%)
       Marinobacter                                                                                                                                                          1 (268 mbsf)
          aquaeolei (95%)
       Marinobacter                                                                                                                                                          2 (268 mbsf)
          excellens (98%)*
       Micrococcus                               2 (1 to 307 mbsf)         1 (381 mbsf)
         luteus (98%)
       Kocuria palustris (99%)                                                                   4 (21 to 40 mbsf)
       Oerskovia               3 (2 to 101 mbsf)                                                  5 (40 to 55 mbsf)
         paurometabola (92%)
       Desulfomicrobium                             2 (103 mbsf)
         norvegicum (99%)*
       Porphyromonas                               1 (198 mbsf)
          endodontalis (86%)
   *Species names identify the type species in the GenBank database that are the closest relatives of the cultured isolates. The genetic distance between each cultured taxon and its
   closest relative is illustrated by the percent similarity of their 16S rRNA sequences.

   broad array of redox environments and other                 effects on mineral, chemical, and biological                             to be late Eocene on the basis of planktic microfossil
                                                                                                                                        biostratigraphy (6).
   taxa that appear to exhibit consistent prefer-              resources are poorly constrained. The mini-                         8.   Leg 201 microbiological studies relied on samples
   ences for specific subseafloor environments.                mum energy fluxes required to sustain them                               that had very low or undetectable contamination, as
   At least some of the former taxa are cos-                   remain unknown. Their total genetic diver-                               assessed by our contaminant tracing tests (17).
   mopolitan in their distribution. This broad                 sity, rates of population turnover, detailed                        9.   P. N. Froelich et al., Geochim. Cosmochim. Acta 43,
                                                                                                                                        1075 (1979).
   distribution is not entirely surprising, given              metabolic interactions, and community struc-                       10.   K. H. Nealson, Annu. Rev. Earth Planet. Sci. 25, 403
   (i) the occurrence of so many of the same                   tures remain to be determined.                                           (1997).
   metabolic products and reactants at every                                                                                      11.   B. B. Jørgensen, in Marine Geochemistry, H. D. Schulz,
                                                                    References and Notes                                                M. Zabel, Eds. (Springer-Verlag, Berlin, 2000), pp.
   site, (ii) the suitability of spore-forming bacte-                                                                                   173–207.
                                                                 1. R. J. Parkes, B. A. Cragg, P. Wellsbury, Hydrogeol. Rev.
   ria for broad dispersal, and (iii) the pos-                      8, 11 (2000).                                                 12.   H. D. Schulz, in Marine Geochemistry, H. D. Schulz,
   sibility that individual taxa are responsible                 2. K.-G. Zink, H. Wilkes, U. Disko, M. Elvert, B. Horsfield,           M. Zabel, Eds. (Springer-Verlag, Berlin, 2000), pp.
                                                                    Org. Geochem. 34, 755 (2003).                                       85–128.
   for different activities under different sub-
                                                                 3. H. F. Sturt, R. E. Summons, K. J. Smith, M. Elvert, K.-U.     13.   E. Suess et al., Proc. ODP Init. Rep. 112 (1988).
   seafloor conditions (10).                                        Hinrichs, Rapid Commun. Mass Spectrom. 18, 617                14.   P. A. Baker, P. M. Stout, M. Kastner, H. Elderfield, Earth
       Much remains to be learned about life in                     (2004).                                                             Planet. Sci. Lett. 105, 522 (1991).
   subseafloor sediments. We do not yet know                     4. S. D’Hondt, S. Rutherford, A. J. Spivack, Science 295,        15.   T. J. Bralower et al., Proc. ODP Init. Rep. 198 [CD-ROM]
                                                                    2067 (2002).                                                        (2002).
   which organisms (cultured or uncultured) are                  5. W. B. Whitman, D. C. Coleman, W. J. Wiebe, Proc.              16.   Traces of dissolved O2 were detected at the top and
   responsible for which metabolic activities in                    Natl. Acad. Sci. U.S.A. 95, 6578 (1998).                            bottom of the site 1225 and site 1231 sediment
   these sediments. We have probably not yet                     6. S. D’Hondt et al., Proc. ODP Init. Rep. 201 [CD-ROM]                columns (6). These data were too few and too
                                                                    (2003).                                                             imprecise to use for estimating O2 fluxes (17).
   reached the greatest sedimentary depths that
                                                                 7. The youngest sediments are from the seafloor. The             17.   See supporting data on Science Online.
   subseafloor organisms attain. Their effects                      oldest sediment is from the base of the sediment              18.   From alteration textures and chemical traces, pro-
   on global biogeochemical cycles and their                        column at Peru Basin site 1231; its age is estimated                karyotic life has been inferred to occur in the glassy

2220                                              24 DECEMBER 2004             VOL 306         SCIENCE www.sciencemag.org
                                                                                                                                               RESEARCH ARTICLE
    rinds of oceanic basalts (21). Fe and S in sub-               with depth (1), more than 99% of the total biomass          27. A. Lauer, A. Teske, Int. J. Astrobiol. 3 (S1), 63 (2004).
    seafloor basalts are strongly oxidized in the first           in sediments deeper than 1.5 mbsf lies within our           28. Samples, shipboard facilities, and expedition support
    10 million to 20 million years of the basalts’ ex-            calculational interval at each site.                            were provided by the ODP. The NASA Astrobiology
    istence (22). Oxidation of these chemical species       20.   Two moles of C(0) (organic carbon) are oxidized by              Institute (NAI) supported postcruise analysis of
    has the potential to support abundant biomass in              reducing one mole of SO4 2– to S2–. Five moles of               biogeochemical data and precruise development of
    basaltic aquifers (22, 23). Our results indicate that         C(0) are oxidized by reducing four moles of NO3 –               shipboard biogeochemical techniques. Postcruise
    by 11 Ma and 35 Ma at sites 1225 and 1231, re-                to two moles of N2. Four moles of Fe(III), or two               culturing studies were supported by grants from the
    spectively, such oxidation is insufficient to strip           moles of Mn(IV), are required to oxidize one mole               Deutsche Forschungsgemeinschaft. We thank three
    dissolved O2 and NO3 – from the circulating water,            of C(0).                                                        anonymous reviewers for very helpful comments.
    perhaps because the mineral surfaces in contact         21.   M. R. Fisk, S. J. Giovannoni, I. H. Thorseth, Science
    with water were largely oxidized when the basalt              281, 978 (1998).                                            Supporting Online Material
    was younger.                                            22.   W. Bach, K. J. Edwards, Geochim. Cosmochim. Acta            www.sciencemag.org/cgi/content/full/306/5705/2216/
19. At each site, the sediment column for which fluxes            67, 3871 (2003).                                            DC1
    were calculated spans the interval from 1.5 mbsf to a   23.   T. Gold, Proc. Natl. Acad. Sci. U.S.A. 89, 6045 (1992).     Materials and Methods
    point midway between the two deepest sample             24.   R. A. Jahnke, Global Biogeochem. Cycles 10, 71 (1996).      References
    depths. At the open-ocean sites, this interval ends     25.   K. B. Sørensen, A. Lauer, A. Teske, Geobiology, in press.
    just above the sediment-basalt contact. Given an        26.        ¨
                                                                  J. Suß, B. Engelen, H. Cypionka, H. Sass, FEMS              7 June 2004; accepted 15 October 2004
    exponential decline in average cell concentrations            Microbiol. Ecol., in press.                                 10.1126/science.1101155

                                                                                                                                  Recently, it has been shown in an x-ray
          Electron Coherence in a Melting                                                                                     diffraction experiment that this limitation can
                                                                                                                              be overcome by working on the interface of a
                  Lead Monolayer                                                                                              liquid and a crystalline material, which led to
                                                                                                                              the first experimental observation of the five-
             F. Baumberger,* W. Auwarter,. T. Greber, J. Osterwalder-
                                   ¨                                                                                          fold local symmetry (9), predicted for mon-
                                                                                                                              atomic 3D liquids more than 50 years ago
        We used angle-resolved photoemission spectroscopy to measure the elec-                                                (10). We use a similar idea to directly mea-
        tronic dispersion and single-particle spectral function in a liquid metal. A lead                                     sure the electron dispersion and spectral func-
        monolayer supported on a copper (111) surface was investigated as the tem-                                            tion in a 2D liquid: melted Pb. A crystalline
        perature was raised through the melting transition of the film. Electron spec-                                        Cu(111) substrate serves as a support with
        tra and momentum distribution maps of the liquid film revealed three key                                              minimal influence on the atomic arrange-
        features of the electronic structure of liquids: the persistence of a Fermi sur-                                      ment of the 2D Pb liquid, and at the same
        face, the filling of band gaps, and the localization of the wave functions upon                                       time ensures that the parallel momentum of
        melting. Distinct coherence lengths for different sheets of the Fermi surface                                         the initial Pb states is conserved in the
        were found, indicating a strong dependence of the localization lengths on the                                         photoemission process. In a crystalline envi-
        character of the constituent atomic wave functions.                                                                   ronment, the momentum needed for photo-
                                                                                                                              emission is supplied in discrete quantities by
The transition from the solid to the liquid                 tal). In random systems, such as amorphous                        reciprocal lattice vectors, whereas in a liquid,
state can have substantial effects on a mate-               solids or liquids, the crystal momentum is no                     the photoelectrons gather arbitrary momenta
rial_s electronic properties (1, 2). In the case            longer a good quantum number and the prob-                        in the process. For a liquid monolayer, how-
of semiconducting germanium, for example,                   lem becomes analytically intractable (1, 5, 6).                   ever, the momentum of the initial state can
the forbidden states in the band gap of the                     Despite decades of intense research,                          be retrieved, because the proximity of the
crystal are filled and the melt is metallic                 many fundamental problems of the electron-                        crystalline substrate allows transfer of re-
(3, 4). Understanding the evolution of the                  ic structure of liquids remain unresolved (7).                    ciprocal lattice vectors to the liquid states.
electronic wave functions, which underlie such              In particular, the character of the electronic                        Complete momentum distribution maps of
marked changes of the physical properties,                  wave functions (e.g., to what extent they are                     the liquid film indicate two Fermi surface
represents a prime experimental and theoret-                itinerant or localized) has eluded experimen-                     sheets, and the spectral function (measured
ical challenge. The main conceptual issue is                tal investigation. The primary experimental                       independently for both sheets) reveals novel
the lack of any long-range order in liquid or               problem is the loss of periodicity, which re-                     aspects of the electronic structure of liquids.
amorphous materials. The periodicity of crys-               stricts the information provided by the most                      Contrary to the usual assumptions that ac-
talline solids allows the classification of elec-           important experimental probes. A diffraction                      company the concept of a mobility edge, we
tronic wave functions as Bloch states (i.e.,                experiment, which can retrieve the full three-                    find only a negligible energy dependence of
plane waves, modulated by lattice periodic                  dimensional (3D) atomic structure of a                            the localization length (spatial extension of an
functions, that extend through the entire crys-             crystalline material, yields only a 1D projec-                    exponentially decaying wave function) but a
                                                            tion in the form of a pair-correlation length                     marked momentum dependence. This is in-
Physikinstitut der Universitat Zurich, Winterthurer-
                                ¨                           in a liquid or amorphous material (8). Analo-                     terpreted as a manifestation of the different
strasse 190, CH-8057 Zurich, Switzerland.
                       ¨                                    gously, angle-resolved photoemission spectros-                    symmetries of the constituent atomic orbitals.
*Present address: Department of Applied Physics,            copy (ARPES), which gives direct access to                            The experiments were performed in a mod-
Stanford University, Stanford, CA 94305, USA.               the single-particle spectral function A(k,w)                      ified VG-ESCALAB 220 spectrometer (11)
.Present address: Department of Physics and Astron-         in crystals, only measures the projection of                      using He Ia radiation (21.22 eV). The energy
omy, University of British Columbia, Vancouver,
British Columbia V6T1Z4, Canada.
                                                            the momentum-resolved quantity on the en-                         and angular resolutions were set to 60 meV
-To whom correspondence should be addressed.                ergy coordinate (i.e., the spectral density) in                   and T0.4-, respectively. Pb was evaporated
E-mail: osterwal@physik.unizh.ch                            a liquid.                                                         resistively onto a clean Cu(111) surface held

                                               www.sciencemag.org             SCIENCE         VOL 306         24 DECEMBER 2004                                                            2221

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