ORIGINAL RESEARCH ARTICLE
published: 24 February 2015
doi: 10.3389/feart.2015.00006
Assessing the utility of trace and rare earth elements as
biosignatures in microbial iron oxyhydroxides
Christine Heim1*, Klaus Simon2, Danny Ionescu3,4, Andreas Reimer1, Dirk De Beer3,
Nadia-Valérie Quéric1, Joachim Reitner1 and Volker Thiel1
1 Department of Geobiology, Geoscience Centre, University of Göttingen, Göttingen, Germany
2 Department of Geochemistry, Geoscience Centre, University of Göttingen, Göttingen, Germany
3 Max-Planck-Institute for Marine Microbiology, Microsensor Research Group, Bremen, Germany
4 Leibnitz Institute for Freshwater Ecology and Inland Fisheries, IGB, Experimental Limnology, Stechlin, Germany
Edited by:
Dina M. Bower, Carnegie Institution
of Washington, USA
Reviewed by:
Doug LaRowe, University of
Southern California, USA
Jakob Zopfi, University of Basel,
Switzerland
*Correspondence:
Christine Heim, Department of
Geobiology, Geoscience Centre,
University of Göttingen
Goldschmidtstr. 3,
D-37077 Goettingen, Germany
e-mail: cheim@gwdg.de
Microbial iron oxyhydroxides are common deposits in natural waters, recent sediments,
and mine drainage systems. Along with these minerals, trace and rare earth elements
(TREE) are being accumulated within the mineralizing microbial mats. TREE patterns
are widely used to characterize minerals and rocks, and to elucidate their evolution and
origin. However, whether and which characteristic TREE signatures distinguish between
a biological and an abiological origin of iron minerals is still not well-understood. Here
we report on long-term flow reactor studies performed in the Tunnel of Äspö (Äspö Hard
Rock Laboratory, Sweden). The development of microbial mats dominated by iron-oxidizing
bacteria (FeOB), namely Mariprofundus sp. and Gallionella sp were investigated. The
feeder fluids of the flow reactors were tapped at 183 and 290m below sea-level from two
brackish, but chemically different aquifers within the surrounding, ∼1.8 Ga old, granodioritic
rocks. The experiments investigated the accumulation and fractionation of TREE under
controlled conditions of the subsurface continental biosphere, and enabled us to assess
potential biosignatures evolving within the microbial iron oxyhydroxides. After 2 and 9
months, concentrations of Be, Y, Zn, Zr, Hf, W, Th, Pb, and U in the microbial mats were
103- to 105-fold higher than in the feeder fluids whereas the rare earth elements and Y
(REE+Y) contents were 104- and 106-fold enriched. Except for a hydrothermally induced
Eu anomaly, the normalized REE+Y patterns of the microbial iron oxyhydroxides were very
similar to published REE+Y distributions of Archaean Banded Iron Formations (BIFs). The
microbial iron oxyhydroxides from the flow reactors were compared to iron oxyhydroxides
that were artificially precipitated from the same feeder fluid. Remarkably, these abiotic
and inorganic iron oxyhydroxides show the same REE+Y distribution patterns. Our results
indicate that the REE+Y mirror closely the water chemistry, but they do not allow
to distinguish microbially mediated from inorganic iron precipitates. Likewise, all TREE
studied showed an overall similar fractionation behavior in biogenic, abiotic, and inorganic
iron oxyhydroxides. Exceptions are Ni and Tl, which were only accumulated in the microbial
iron oxyhydroxides and may point to a potential utility of these elements as microbial
biosignatures.
Keywords: biosignatures, microbial mats, microbial iron oxides, trace elements, rare earth elements,
microbe–metal interaction, banded iron formation
INTRODUCTION
The structure, properties and formation of iron oxyhydroxides
have attracted the attention of many researchers during the last
decades, due to their ubiquitous occurrence in natural settings
and anthropogenic biotopes, as well as their properties as an
efficient sorbent for (heavy) metals, with a resulting potential
for technical applications (Stumm and Morgan, 1996; Ferris
et al., 2000; Cornell and Schwertmann, 2003; Katsoyiannis and
Zouboulis, 2006; Michel et al., 2007; Cao et al., 2012; Chi Fru
et al., 2012; Savchenko et al., 2013).
In natural environments, iron oxyhydroxide precipitates
typically originate from the chemically or biologically controlled
oxidation of Fe2+ to Fe3+, where the oxidation rate strongly
depends on the redox conditions and the pH of the aqueous
solution (e.g., Fortin et al., 1997; Morgan and Lahav, 2007).
Microbial precipitation of iron oxyhydroxides can be specified
as ‘biologically induced mineralization’ (Frankel and Bazylinski,
2003). Three modes of biologically induced mineralization occur:
(i) Extracellular mineral nucleation and growth are processes
from which the microorganisms gain energy by direct redox
conversion of specific ions like iron. (ii) Indirect mineral pre-
cipitation takes place due to gradual changes in the chemical
equilibrium of the surrounding solution which may also be
supported by the release ofmetabolic products from themicrobial
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EARTH SCIENCE
Heim et al. TREE as biosignatures
community (Thompson and Ferris, 1990; Fortin and Beveridge,
1997; Fortin et al., 1997; Southam, 2000). (iii) Passive mineraliza-
tion is induced by non-living organic matter such as cell debris or
extracellular polymeric substances (EPS). Thereby, exposed neg-
atively charged surfaces act as adsorption and nucleation sites
for metal cations (Urrutia and Beveridge, 1993; Anderson and
Pedersen, 2003; Ercole et al., 2007; Chan et al., 2009).
While these biologically induced mineralization pathways
can be well-specified in theory, they are often difficult to
recognize and distinguish in natural samples (Ionescu et al.,
2015b). Nevertheless, such biogenic processes may produce
minerals different from their inorganically formed varieties in
shape, size, crystallinity, isotopic, and trace element composi-
tion (Konhauser, 1997; Ferris et al., 1999, 2000; Weiner and
Dove, 2003; Bazylinski et al., 2007; Haferburg and Kothe, 2007;
Takahashi et al., 2007). In studies of contemporary mineral
deposits such biosignatures may be specified and utilized for the
identification of related biological processes in geological sam-
ples throughout the Earth history. Massive deposition of banded
iron formations (BIFs), for instance, occurred at 2.7–2.4 Ga, after
molecular oxygen started to be available due to the increasing
photosynthetic activity of cyanobacteria (Anbar et al., 2007). The
mechanisms of BIF formation are widely discussed and hypothe-
ses involving abiotic and biotic processes have been proposed
(e.g., Morris and Horwitz, 1983; Bau and Möller, 1993; Krapez
et al., 2003; Kappler et al., 2005; Lewy, 2012). In the “biologi-
cal” scenarios, a central role has been assigned to iron oxidizing
bacteria (FeOB) living under both anoxic and oxic conditions,
(Konhauser et al., 2002). Therefore, studying modern microbial
iron oxyhydroxides and the bacteria involved in iron oxidation are
crucial for a better understanding of BIF deposition (Konhauser
et al., 2002; Kappler et al., 2005).
Here we report on a flow reactor experiment investigating
the development of iron-oxidizing microbial mats in the Äspö
Hard Rock Laboratory (HRL). The Äspö HRL, operated by the
Swedish Nuclear Fuel and Waste Management Company (SKB),
is a tunnel system drilled beneath the island of Äspö in south-
eastern Sweden, ca. 400 km south of Stockholm (Figures 1A,B)
and serves as a testing site for the long-term storage of nuclear
waste. It offers a unique window into a subterranean terrestrial
FIGURE 1 | (A) Location of the Äspö Hard Rock Laboratory (HRL); (B) 3D
sketch of the HRL tunnel system beneath the island of Äspö; arrows indicate
the location of the flow reactor experiments at (C) site 1327B (183m bsl) and
(D) site 2156B (290m bsl). (E) Sketch of a flow reactor immediately after
installation, (F) with microbial mats after 2 months, and (G) with microbial
mats after 9 months. (H) Flow reactor used in this study prior to installation;
(I) the same flow reactor after opening, showing microbial mat development
after 2 months.
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Heim et al. TREE as biosignatures
biosphere (e.g., Pedersen and Ekendahl, 1990; Pedersen, 1997,
2012; Ionescu et al., 2015a). The host rock of the Äspö site is part
of the Precambrian Transscandinavian Igneous Belt and consists
of ∼1.8 Ga old granitic to quartz-monzodioritic rocks (Wahlgren
et al., 2006).
We particularly aimed to explore microbial TREE accumula-
tion and fractionation patterns in the microbial mats thriving in
the Äspö HRL and their potential use as biosignatures for micro-
bially induced iron precipitation. The growth of microbial con-
sortia in the ÄspöHRL strongly depends on the depth, the current
velocity, salinity, oxygen content and the chemical composition
of the feeder fluids supplied by different aquifers. Microbial mats
thriving on tunnel walls and in ponds are often dominated by the
FeOBGallionella sp. andMariprofundus sp. Gallionella is an auto-
and mixotrophic, microaerophilic FeOB using Fe(II) as an elec-
tron donor and CO2 or carbohydrates as carbon source (Hallbeck
and Pedersen, 1991; Hallbeck et al., 1993). It lives at circum neu-
tral pH and at temperatures between 5 and 25◦C. Directing the
iron oxidation to twisted extracellular “stalks” made of extracellu-
lar polymeric substances (EPS), at distance from the cells, protects
Gallionella ferruginea against oxygen radicals formed during the
iron metabolism and consequently against encrustation of the
cell surface (Hallbeck and Pedersen, 1991, 1995; Hallberg and
Ferris, 2004). The production of the stalks seems to be limited
to pH values above 6 (Hallbeck et al., 1993). G. ferruginea was
first described by Ehrenberg (1836) who characterized its stalk
as a unique morphological trait. However, the morphologies of
Mariprofundus ferrooxidans, first described in 2007, and G. fer-
ruginea are largely similar, as both have bean-shaped cells and
winded, iron encrusted filamentous stalks (Emerson et al., 2007).
M. ferrooxidans is a marine, microaerophilic lithoautotrophic
FeOB, which uses Fe(II) or Fe(0) as electron donor and CO2 as
carbon source (Emerson et al., 2007). Studies with G. ferruginea
showed that at circum neutral pH the size of the stalks (average
33μm, maximum 60μm length) may exceed the size of the cell
(1–2μm) by far (Hallbeck and Pedersen, 1991). Likewise, a stalk
length of tens ofμm was reported for M. ferrooxidans (Emerson
et al., 2007). Both G. ferruginea and M. ferrooxidans are able to
cast off, and replace, their old, encrusted stalks. Thus, the amount
of iron oxyhydroxide effectively precipitated by these microor-
ganisms may become extremely high as compared to the actual
cell biomass. Therefore, these stalk-forming FeOB are considered
as particularly relevant for iron ore and -rock forming processes
throughout earth history (Konhauser et al., 2002).
Previous studies have shown that the stalk length of G. fer-
ruginea is positively correlated not only with the amount of iron
oxyhydroxides precipitated, but also with the co-precipitation of
lanthanides, Th and U (Anderson and Pedersen, 2003). Here we
demonstrate that microbial mat formation byM. ferrooxidans and
G. ferruginea are also accompanied by a massive enrichment of
TREE, together with the establishment of distinctive element frac-
tionation patterns. To elucidate whether these traits may help to
distinguish microbially induced from inorganically precipitated
iron oxyhydroxides, the patterns found in the microbial mats
were compared to those obtained by abiotic/inorganic precipita-
tion from the same feeder fluids, and also to TREE distributions
reported for ancient BIF deposits.
MATERIALS AND METHODS
FLOW REACTORS
A flow reactor experiment was designed to simulate the environ-
mental conditions in fluid conduits within the granodioritic host
rock and investigate microbial iron biomineralization and TREE
accumulation under monitored conditions. Dark, air-tight flow
reactors (total volume 45 l) non-pressurized and with a flow of
0.7 l/min were constructed (Figures 1B–F) and connected to the
aquifers in the Äspö HRL in 2006. Only chemically inert materi-
als such as polytetrafluoroethylene (PTFE, Teflon®), PTFE—fiber
glass, fluorinated ethylene propylene (FEP) and special PTFE—
foam were used as construction materials to avoid chemical con-
tamination from glass and plastic ware. The flow reactor systems
and connection tubing’s were thoroughly sterilized with ethanol
(70%, overnight) before underground installation to prevent
biological contamination from the surrounding environment.
The geochemistry of feeder fluids was studied at the differ-
ent tunnel levels prior to setup of the flow reactors to select
suitable installation sites. According to the results, and con-
sidering published data (Ferris et al., 1999; Laaksoharju et al.,
1999; SICADA database1), the flow reactors were connected to
aquifers at 183m (site 1327B) and 290m below sea level (site
2156B) (Figures 1B–D). The former shows a major influence of
recent Baltic Sea water, whereas the latter contains a mixture of
recent and ancient Baltic Sea water (retention time of 4–6 years),
and glacial melt water (Laaksoharju et al., 1999). The specially
designed outlet enabled an operation of the flow reactors without
any headspace.
During the experiment the reactors were kept strictly undis-
turbed, except for sampling of reactor water and microbial mats
after 2 and 9 months, respectively. The reactors were regularly
controlled for maintenance/tightness, and in- and outflowing
waters were routinely sampled and analyzed for physicochemical
fluctuations.
Artificial precipitation experiments
As a reference, artificial precipitation experiments were per-
formed using 30 l of aquifer water sampled from site 1327B
that was split into three aliquots (i–iii), as follows. (i) To study
the abiotic precipitation of iron oxyhydroxides, i.e., without any
influence of living organisms, a 10 l aliquot was treated imme-
diately after sampling with concentrated and distilled HNO3
(Merck, triple distilled in the GZG geochemistry lab, purity∗∗∗)
to kill the microbial content (final concentration 2% HNO3).
However, tests with other, parallel samples showed that the
HNO3 treatment does not destroy the microbial EPS, which
may also contribute to the precipitation of iron oxyhydroxides.
(ii) Therefore, to assess the inorganic precipitation of iron oxy-
hydroxides, without any influence of both, living cells as well
as dead biomass, concentrated and distilled HNO3 and H2O2
(Sigma-Aldrich, 35%) was used for the second aliquot to kill the
microorganisms and oxidize all organic matter. In aliquots (i) and
(ii), iron oxide precipitation was initiated by addition of NH4OH.
(iii) As a biotic (living) control, the third 10 l aliquot was closed
1SICADA. Database of the Swedish organisation for nuclear fuel and waste
management (SKB), owner of the Äspö HRL. See also www.skb.se.
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Heim et al. TREE as biosignatures
and kept at the temperature of the sampling site (15◦C) for 15
days, thus enabling microbial iron oxidation to proceed.
The iron oxides precipitated from the three aliquots were sep-
arated from the aquifer water by centrifugation. The precipitates
were washed three times with deionized water to remove the salt
content. Afterwards the samples were lyophilized and weighed. A
fraction was analyzed for the organic carbon (Corg) content of the
precipitates. For TREE analysis the lyophilized precipitates were
processed like the microbial mat samples, as described below.
MICROBIOLOGICAL DIVERSITY
DNA extraction
Biofilm matter was collected in sterile 50ml tubes. After cen-
trifugation in the field lab shortly after collection, the samples
were frozen at −20◦C until further processing. After thawing in
the home laboratory, the samples were resuspended (to 50ml)
in an iron dissolution solution (0.35M acetic acid; 0.2M sodium
citrate; 25mM sodium dithionite; Thamdrup et al., 1993). The
cleaned cells (and remaining iron precipitates) were filtered
onto polycarbonate filters and washed with 1X PBS. DNA was
extracted using the hot-phenol method as described elsewhere
(Ionescu et al., 2012).
454 Pyrosequencing
DNA samples were sent for Roche 454 pyrosequencing at the
MrDNA lab (Shallowater, TX, USA). Sequencing of the 16S
rRNA gene was done using the 28F and 519R general primers
(Lane, 1991). 374,666 sequences were obtained and were pro-
cessed as described in Ionescu et al. (2012). Shortly, the sequences
were checked for quality (length, homopolymers, and ambigu-
ous bases) and aligned against the SILVA reference database.
Sequences with poor alignment score were removed while the
rest was clustered (per sample) into operational taxonomic units
(OTUs) at a similarity value of 98%. Reference sequences of each
OTU ware assigned a taxonomy using BLAST against the SILVA
ref database (Version 111; Quast et al., 2013). Full OTU map-
ping of the sample was done by clustering the reference sequences
of each OTU at 99% similarity. The sequences were deposited
at the European Nucleotide Archive (ENA) under study acces-
sion number PRJEB4914 (http://www.ebi.ac.uk/ena/data/view/
PRJEB4914).
CHEMICAL ANALYSIS
Oxygen in the feeder fluids was measured using the Winkler
method (Hansen, 1999). Conductivity and pH were deter-
mined together with anion measurements by titration and
ion chromatography. Spectrophotometrical Fetotal/Fe(II) mea-
surements were performed on acidified samples immediately
after sampling on-site by the certified SKB chemistry lab.
TREE were analyzed using Inductive Coupled Plasma Mass
Spectrometry (ICP-MS; Perkin Elmer SCIEX Elan DRCII) and
Optical Emission Spectroscopy (ICP-OES; PerkinElmer Optima
3300 DV). For sample conservation and TREE measurements,
concentrated HNO3 (Merck, triple distilled in GZG geochemistry
lab, purity∗∗∗) was added to 50ml water samples (final concentra-
tion 2% HNO3). After sampling, the microbial mats were frozen,
transported in dry ice, and stored at −20◦C until analysis. To
quantify REE in the mineral precipitates, 4ml of H2O2 (35%)
and 2ml of concentrated, distilledHNO3 were added to 500mg of
lyophilized sample. The resulting solutions of themineral precipi-
tates were diluted in 50ml of deionized water (final concentration
4% HNO3). These solutions, a reference sample (matrix matched
calibration solution) containing all chemicals used, and the water
samples were spiked with internal Ge, Rh, In and Re standards
and analyzed by ICP-MS and ICP-OES. The data obtained were
mean values of triple measurements. The internal reproducibility
was better than 2%. External accuracy was checked by measur-
ing and comparing to international standard reference materials
(GJA-2, Japanese Andesite). Furthermore, the results were drift
corrected by monitoring one calibration point through the course
of the run, and also corrected for the major oxygen interferences
(BaO, REEO, etc.). For a better comparison between the different
sites and ages, the TREE data were normalized on the respec-
tive feeder fluids (Figures 5, 6, 8). To enable a comparison with
data from other publications the TREE data are normalized on
PAAS (Figure 7; Post-Archaean average Australian sedimentary
rock; McLennan, 1989).
Carbon (Ctot), total nitrogen (Ntot), and sulfur (Stot) were
analyzed using a CNS Elemental Analyzer (HEKAtech Euro
EA). Inorganic carbon (Cinorg) and organic carbon (Corg) were
analyzed with a Leco RC 412 multiphase carbon analyzer.
Duplicate measurements were performed routinely, and appro-
priate internal standards were used for the Cinorg (Leco 502-
030), Corg (Leco 501-034), and CNS (2,5-Bis(5-tert-butyl-2-
benzoxazolyl)thiophene, SA990752; atropine sulfate SA990753;
IVA Analysentechnik) analyses.
SCANNING ELECTRON MICROSCOPY AND ENERGY DISPERSIVE X-RAY
ANALYSIS (SEM-EDX)
For SEM-EDX analysis, samples were fixed in 2% glutardialde-
hyde immediately after sampling and stored at 4◦C. Prior to
analysis, the samples were dehydrated in rising ethanol concen-
tration, by rinsing with successive solutions of 15, 30, 50, 70%
ethanol (30min each), followed by 90 and 99 % (60min each),
and 99% ethanol (12 h). After the dehydration series, the samples
were mounted on SEM sample holders and sputtered with Au-Pd
(7.3 nm for 120 s). Samples were analyzed using a field emission
SEM (LEO 1530 Gemini) combined with an INCA X-act EDX
(Oxford Instruments).
RESULTS
FEEDER FLUID CHEMISTRY
Basic water chemistry data for the feeder fluids are given in
Table 1. Replicate analyses (data not shown) revealed that their
chemical properties remained virtually stable over 15months, i.e.,
over the total duration of the experiments (STD, 2σ , below 10%).
The feeder fluids consisted of waters with salinities of 5.8% at site
1327B (183m bsl), and 7% at site 2156B (290m bsl). Oxygen
measurements revealed stable O2-concentrations between 0.28
and 0.38mg/l in the feeder fluids and thus, suboxic conditions.
The fluid at the shallower site, being influenced by recent Baltic
Sea water, contained higher amounts of sulfate and showed a
higher alkalinity than the fluid from the deeper site. At both
sites, Fe in the feeder fluids was exclusively present as ferrous iron
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Heim et al. TREE as biosignatures
Table 1 | Basic chemical parameters of the feeder fluids and Baltic Seawater (for comparison).
pH Cond. HCO−3 Cl
− Br− F− SO2−4 Sulfide Fetotal Fe
2+ O2
mS/m [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l] [mg/l]
1327B feeder fluid 7.33 978 209 3059 14.3 1.54 425 0.07 1.69 1.68 0.28
2156B feeder fluid 7.41 1188 139 3926 19.4 1.51 313 0.03 0.60 0.59 0.38
Baltic Sea water 6.90 278 26 692 2.4 0.42 501 2.98 0.57 0.34 3.47
Mean values (<10% STD) obtained from replicate sampling over the duration of the experiment. Cond., conductivity.
FIGURE 2 | Composition of themicrobial communities performing iron-oxidation or being involved in the deposition of iron oxyhydroxides in themats.
At site 1327B, main FeOB were Mariprofundus sp. and Gallionellaceae, whereas at site 2156B Mariprofundus sp. dominated and Gallionellaceae were absent.
(Fe2+, Table 1). The Fe2+ concentration was considerably higher
at site 1327B (1.68mg/l) than at site 2156B (0.59mg/l).
MICROBIAL MAT DEVELOPMENT
Within 2 months, several cm-thick microbial mats developed
in both flow reactors (Figures 1F,I). Macroscopically, the mats
consisted of ochrous, fluffy material. Sequencing data revealed
that the shallower flow reactor contained Mariprofundus sp.
and Gallionella-like organisms as the dominating FeOB, whereas
the deeper flow reactor contained Mariprofundus sp. but not
G. ferruginea (Figure 2). Other iron oxidizers detected in both
reactors were Paracoccus sp., Acidithioacillus sp., Leptothrix sp.,
Thiobacillus sp., and Sideroxydans sp.
After 9 months, both flow reactors were almost completely
filled with microbial mats which showed a massive abundance
of stalks characteristic of Mariprofundus sp. and G. ferruginea
(Figure 3A). These stalks showed a delicate filamentous structure
with only few mineral precipitates after 2 months (Figure 3B).
After 9 months, the flow reactors at both sites still exhibited
macroscopically similar microbial mats, but in both cases the
FeOB stalks were extensively encrusted with mineral precipitates
(Figure 3C). XRD measurements showed two broad humps with
maxima between 25 and 70 ◦2θ, indicating two-line ferrihydrite
as dominating mineral form (data not shown). Although micro-
bially formed ferrihydrites are relatively stable mineral phases
(Kennedy et al., 2004; Toner et al., 2012), their water content
may vary (Schwertmann et al., 1999), and we therefore use
the comprehensive term “iron oxyhydroxides” hereafter. Further,
SEM/EDX investigations revealed the presence of authigenic gyp-
sum crystals (Figure 3D).
The organic carbon concentrations of the microbial mats
remained fairly constant during the experiment and did not
exceed 3.7 %dry weight at site 2156B and 7%dry weight at site 1327B
(Table 2). At both sites, the content of nitrogen remained stable
at 0.3 %dry weight. Carbon phases analysis indicated that Cinorg
(Table 2) was bound as siderite in the young as well as in the aged
microbial mats at both sites.
TREE ACCUMULATION AND FRACTIONATION WITHIN THE MICROBIAL
MATS
The concentrations of all cations analyzed in the feeder fluids
and in the microbial mats are given in Table 3. In both flow
reactors, the main element patterns of the microbial mats were
always dominated by iron (Figure 4). Other main elements, such
as Al, Si, Na, Ca, and Mg showed considerably different rel-
ative amounts at both sites after the initial 2 months period
(Figures 4A,C). After 9 months, however, Si, Na, Ca, and Mg
approached similar abundances (Figures 4B,D). The bivariate
plots provided in Figure 5 illustrate the relative element accumu-
lation in the microbial mats normalized on the feeder fluid over 2
and 9 months. Compared to the feeder fluids, the microbial mats
showed accumulations of most TREE. In both flow reactors, the
REE and the trace elements Be, Y, Zr, Nb, Hf, Pb, Th, and W were
103–104-fold enriched after 2 months. After 9 months, these ele-
ments showed up to 106-fold enrichments (Figure 5). In contrast,
Ca, Na, Mg, and K did not accumulate during the first 2 months
in the mats though these elements were available at high concen-
trations in the feeder fluids. The accumulation of these elements
in both flow reactors was only observed after 9 months and was
most likely related to gypsum precipitation (Figures 3D, 4D, 5).
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Heim et al. TREE as biosignatures
FIGURE 3 | (A) SEM micrograph of the 2 months old microbial mat from the
flow reactor at site 1327B, showing the twisted EPS-stalks of Gallionella sp.
and/or Mariprofundus sp. (B) Detail of young EPS-stalks sampled after 2
months, still showing a pristine filamentous structure; (C) Aged EPS-stalks
sampled after 9 months, showing heavy iron oxyhydroxide impregnation; (D)
authigenic gypsum crystals in a 9 months old microbial mat.
Table 2 | Concentrations (%dry weight) of organic carbon (Corg), inorganic carbon (Cinorg), total nitrogen (Ntot), and total sulfur (Stot) in the
mineralized microbial mats after 2 and 9 months, respectively.
% 1327B 1327B 2156B 2156B Biotic (15 Abiotic Inorganic (HNO3 and
2 months 9 months 2 months 9 months days activity) (HNO3 added) H2O2 added)
Corg± 0.05% 7.0 5.7 3.6 3.7 1.1 0.46 0.04
Cinorg± 0.05% 1.3 0.4 0.1 0.2 0.01 0.02 0.01
Ntot 0.3 0.31 0.11 0.17 n.a. n.a. n.a.
Stot 0.37 0.51 0.24 0.36 n.a. n.a. n.a.
Corg and Cinorg values of the iron oxyhydroxides from the artificial precipitation experiment. n.a., not analyzed.
At both sites, REE+Y patterns of the microbial mats plot-
ted against the feeder fluids showed a nearly consistent 104-fold
(2 months) and 106-fold (9 months) accumulation of these ele-
ments (Figure 6). However, minor anomalies were observed for
Eu, Y, Yb and Lu, and the rate of accumulation was somewhat
higher at site 2156B compared to site 1327B. The Fe normalized
REE values from 1327B showed an increasing REE accumula-
tion with the iron oxyhydroxides during the first 2 months,
and a significant drop during the 9 months period. At 2156B
the Fe normalized REE uptake with the iron oxyhydroxides was
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Heim et al. TREE as biosignatures
Table 3 | TREE concentrations of the feeder fluids and in the iron oxidizing microbial mats after 2 and 9 months, respectively.
Site 1327B Site 2156B
Feeder fluid Microbial mat Microbial mat Feeder fluid Microbial mat Microbial mat
mean [mg/l] after 2 months after 9 months mean [mg/l] after 2 months after 9 months
[mg/kg] [mg/kg] [mg/kg] [mg/kg]
Li 0.050±0.0068 0.0082±0.0011 1.25±0.169 0.264±0.0356 0.130±0.018 14.2± 1.91
Be 0.00009±0.00001 0.081±0.008 10.4±0.971 0.00004±0.000003 0.174±0.0163 13.1± 1.23
Sc 0.002±0.0002 1.031±0.133 3.98±0.514 0.0019±0.00025 1.38±0.1776 7.74± 1.00
V 0.011±0.0006 0.322±0.018 24.6±1.40 0.0159±0.00090 0.183±0.0104 9.31± 0.530
Co 0.00037±0.00003 0.011±0.0008 0.21±0.015 0.0010±0.00007 0.025±0.0018 1.09± 0.080
Ni 0.0056±0.0002 0.075±0.0025 1.55±0.052 0.0158±0.00053 0.097±0.0033 0.801± 0.0270
Cu 0.00133±0.00007 0.049±0.0027 11.0±0.606 0.0016±0.00009 0.117±0.0065 0.905± 0.0500
Zn 0.00158±0.00012 0.223±0.017 75.0±5.60 0.00056±0.000041 0.216±0.0161 9.95± 0.742
Y 0.00017±0.00001 0.636±0.025 65.9±2.56 0.00046±0.000018 5.65±0.2197 313.0± 12.2
Zr 0.00010±0.00001 0.141±0.018 14.4±1.82 0.000012±0.000002 0.157±0.0198 8.79± 1.11
Nb 0.000008±0.0000001 0.0033±0.00006 0.357±0.006 0.000011±0.0000002 0.015±0.0003 0.963±0.0166
Mo 0.00094±0.00015 0.022±0.003 2.11±0.33 0.0061±0.00096 0.221±0.0348 11.0± 1.73
Cd 0.000019±0.000003 0.00011±0.00002 0.042±0.0060 0.00012±0.000018 0.00076±0.00011 0.0128± 0.00182
Sb 0.0000078±0.0000010 0.00048±0.00006 0.046±0.0056 0.000016±0.000002 0.00099±0.00012 0.0269± 0.00327
Rb 0.025±0.002 0.0089±0.0008 1.10±0.103 0.0202±0.00189 0.016±0.0015 1.45± 0.136
Sr 1.83±0.056 10.0±0.31 807±24.8 5.95±0.183 22.8±0.702 1197± 36.9
Cs 0.0028±0.0003 0.0013±0.0001 0.162±0.015 0.0016±0.00015 0.0017±0.0002 0.12± 0.01
Ba 0.076±0.0008 6.25±0.063 580±5.81 0.0328±0.00033 2.85±0.029 145± 1.45
La 0.000029±0.000002 0.168±0.012 20.5±1.43 0.000074±0.000005 1.30±0.090 80.5± 5.61
Ce 0.000049±0.000004 0.246±0.019 31.9±2.51 0.00010±0.000008 1.96±0.154 126± 9.91
Pr 0.0000074±0.0000004 0.034±0.002 5.42±0.31 0.000013±0.000001 0.250±0.014 20.1± 1.14
Nd 0.000041±0.000005 0.158±0.018 24.9±2.81 0.000061±0.000007 1.10±0.12 88.4± 9.99
Sm 0.000011± 0.000001 0.039±0.0022 6.20±0.346 0.000013±0.000001 0.26±0.015 21.3± 1.19
Eu 0.000005±0.0000004 0.009±0.0006 1.15±0.082 0.0000033±0.0000002 0.037±0.0027 2.99± 0.213
Gd 0.000016±0.000002 0.066±0.0101 9.75±1.496 0.000030±0.000005 0.483±0.074 35.9± 5.50
Tb 0.0000017±0.0000003 0.008±0.0013 1.26±0.213 0.0000031±0.000001 0.070±0.012 5.62± 0.952
Dy 0.000013±0.000002 0.053±0.0064 7.67±0.913 0.000027±0.000003 0.522±0.062 38.2± 4.54
Ho 0.000003±0.000001 0.013±0.0026 1.89±0.367 0.000007±0.000001 0.129±0.025 9.28± 1.80
Er 0.000010±0.000002 0.040±0.0091 5.73±1.30 0.000023±0.000005 0.392±0.089 27.8± 6.31
Tm 0.0000010±0.0000001 0.0050±0.0004 0.70±0.054 0.0000024±0.0000002 0.048±0.004 3.33± 0.258
Yb 0.000014±0.000001 0.032±0.0028 4.28±0.376 0.000019±0.000002 0.275±0.024 18.9± 1.66
Lu 0.0000021±0.00000002 0.0056±0.0001 0.767±0.009 0.0000032±0.00000004 0.042±0.0005 2.87± 0.032
Hf 0.0000014±0.0000002 0.0009±0.0001 0.095±0.011 0.00000018±0.00000002 0.001±0.0001 0.082± 0.010
W 0.000248±0.000043 0.115±0.020 14.5±2.50 0.00030±0.000051 0.306±0.0529 16.7± 2.88
Tl 0.0000056±0.000001 0.000002±0.000001 0.002±0.00039 0.00000052±0.0000001 0.000019±0.000005 0.0018± 0.00042
Pb 0.0000082±0.00000017 0.0016±0.00003 0.787±0.0166 0.000019±0.0000004 0.0058± 0.00012 0.058± 0.001
Bi 0.0000006±0.00000001 0.000035±0.000001 0.005±0.00011 0.00000040±0.00000001 0.000098±0.000002 0.001± 0.000
Th 0.0000004±0.00000001 0.0011±0.00003 0.229±0.00701 0.0000016±0.0000001 0.0043±0.00013 0.612± 0.019
U 0.000351±0.000013 0.0067±0.0002 0.756±0.0273 0.00033±0.000012 0.014±0.0005 0.638± 0.023
Al 0.164±0.0090 8.5±0.47 114±6.28 0.260±0.0143 2139±118 7646± 421
Si 2.682±0.147 61.6±3.39 35478±1951 1.23±0.07 200.1±11.0 40627± 2234
Fe 1.69±0.093 2510±138.0 498425±27413 0.596±0.033 11455±630 375296± 20641
Mn 0.784±0.049 7.81±0.492 660±41.6 0.608±0.038 11.2±0.70 385± 24.2
Ca 286±17.2 1.4±0.08 39083±2345 764±45.9 1.16±0.07 56101± 3366
Mg 155±8.5 0.305±0.017 4585±252 96.0±5.3 0.422±0.023 3558± 196
Na 1630±89.7 0.128±0.007 26763±1472 1595±87.7 0.312±0.017 54181± 2980
K 25.2±1.39 0.149±0.008 706±38.8 11.64±0.64 0.250±0.014 194± 10.6
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Heim et al. TREE as biosignatures
FIGURE 4 | Distribution of the major cations within the mineralized microbial mats after 2 months (A, C) and 9 months (B, D), respectively. Note the
convergence of the major element patterns over time, although the mats were fed by chemically differing fluids.
almost negligible, even decreased for some of the REE during
the 2 months period. During the 9 months period, however, a
considerable accumulation of REE was observed.
TREE ACCUMULATION AND FRACTIONATION IN ARTIFICIAL
PRECIPITATES
The REE+Y patterns resulting from artificial (i.e., inorganic,
abiotic, and biotic) mineral precipitation using aquifer water
from sites 1327B and 2156B are given in Figure 7. The data
reveal a great similarity between these REE+Y-patterns, and with
those obtained from the microbial mats and the aquifer waters.
Apart from the similarity in REE+Y distributions, however, we
observed that the trace elements Ni, Cd and Tl were below detec-
tion limit in the inorganic precipitate (Figure 8). Ni and Tl were
also missing in the abiotic precipitate. Only Tl was absent from
the 15 days biogenic precipitates whereas both Ni and Cd were
clearly detected (Figure 8). Molybdenum (Mo) concentrations in
the artificial experiments were very low and were in the range of
themeasurement uncertainty; therefore they will not be discussed
further.
DISCUSSION
The microbial mats and the mineral precipitates formed during
the flow reactor experiment showed distinct changes in their mor-
phological structure as well as in their elemental composition
over time (Figures 3B,C, 4A–D, 5A,B). Interestingly, however,
the major chemical traits of both microbial mats converged with
time, although both study sites were spatially separated from
each other and showed a different composition of their microbial
communities. As the feeder fluid compositions did not change
considerably during the experiment, these chemical changes may
be attributed to processes occurring within the microbial mats.
In the following sections, these processes will be interpreted with
respect to their biotic or inorganic nature.
After 2 months most of the EPS-stalks still showed a delicate
structure of twisted filaments and only few iron oxyhydroxide
precipitates outside the stalks, but the TREE and Fe accumulation
within these microbial mats already exhibited a 104-fold accumu-
lation normalized on the feeder fluid. It has been proposed that
during the iron oxidation process in young G. ferruginea stalks,
initial mineralization of hematite (Fe2O3) takes place within the
Frontiers in Earth Science | Biogeoscience February 2015 | Volume 3 | Article 6 | 8
Heim et al. TREE as biosignatures
FIGURE 5 | Bivariate plots of TREE accumulation of the mineralized
microbial mats after 2 and 9 months with respect to the inflowing
water, (A) at site 1327B, and (B) at site 2156B. At both sampling intervals
and at both sites, highest accumulation rates were observed for REE, Be, Y,
Zr, Nb, Hf, Pb, Th, and W, whereas Ca, Na, Mg, and K accumulated only in
low amounts, and only in the 9 months old microbial mats. The accumulation
of the latter elements is attributed to gypsum precipitation occurring in the
aged mats (Figure 3D).
filaments, whereas aging stalks increasingly precipitate iron oxy-
hydroxides outside the filaments (Hallberg and Ferris, 2004).
Indeed, studies have shown that Mariprofundus sp. and G. fer-
ruginea are capable of controlling the iron mineralization process
through localizing the biotic and abiotic mineral precipitation
(Saini and Chan, 2013). The stalk surfaces were observed to be
smooth and pristine during the initial stage of formation, but
with increasing distance from the cell (i.e., age) they became
more and more coated with lepidocrocite (FeOOH) and two-line
ferrihydrite (Chan et al., 2011). In our experiments, observa-
tions of still pristine stalks, with only minor mineral deposits on
their surfaces (Figure 3B), are in good agreement with a con-
trolled iron oxidation process that occurred mainly within the
stalks at an early stage of mineralization (Hallberg and Ferris,
2004; Comolli et al., 2011). The presence of hematite, as pre-
viously described as an early internal precipitate for Gallionella
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Heim et al. TREE as biosignatures
FIGURE 6 | Plot illustrating the REE+Y fractionation in the microbial iron oxyhydroxides. Nearly uniform REE+Y patterns, except for minor negative Eu
and Y anomalies, indicate a largely radius-independent fractionation of REE+Y from the respective feeder fluids.
stalks (Hallberg and Ferris, 2004), was not observed in our
study.
Our finding that the accumulation rates of REE, Be, Y, Zr,
Nb, Hf, Pb, Th, and W in the microbial mat mineral precipitates
were as high as those of Fe (Figures 5A,B) points toward a co-
precipitation of these elements due to scavenging by the iron oxy-
hydroxides. This process is sustained by the strong metal sorption
capacity of iron oxyhydroxides, which is widely used in techni-
cal applications and remediation activities (Dzombak and Morel,
1990; de Carlo et al., 1998; Bau, 1999; Cornell and Schwertmann,
2003; Michel et al., 2007). Laboratory studies on the properties
of inorganic iron oxyhydroxides as metal sorbents have demon-
strated their strong accumulation capacities for REE+Y (de Carlo
et al., 1998; Bau, 1999). These studies also showed that the sorp-
tion of REE+Y onto iron oxyhydroxides increases strongly, up
to 104-fold, with increasing pH. Furthermore, metal ions with a
higher positive charge tend to show enhanced biosorption, inde-
pendently of the absolute amount of metals present (Haferburg
et al., 2007), which plausibly explains the strong enrichments
observed for the three-and four-valent elements Al and Si, REE, Y,
Zr, Hf, Pb, Th, and W (Figure 5). Furthermore, Si adsorption to
iron oxyhydroxides increases with pH and with higher concentra-
tion will lead to surface precipitation of SiO2 and coating of iron
oxyhydroxides (Cornell and Schwertmann, 2003). Si and organic
matter may be important factors that influence the precipitation
of iron oxyhydroxides (Toner et al., 2012). The significant accu-
mulation of Al especially in the 2 months old microbial mat from
2156B might relate to the presence of iron-reducing and fermen-
tative bacteria in the mats (Ionescu et al., 2015a,b) which are
known to selectively mobilize iron oxides and silica under anaer-
obic conditions. In soils, these processes induce an enrichment
of Al in the residual matter which may ultimately lead to the
formation of bauxite (Gadd, 2010).
REE+Y are consistently accumulated in the microbial mats
(Figure 6), with the patterns of the aquifers being preserved in
the iron oxyhydroxides at a 106-fold enrichment. This observa-
tion implies that fractionation of REE+Y from the feeder fluids
and incorporation in, or sorption to, the iron oxyhydroxides is
a radius-independent process, as it has been observed in artifi-
cial precipitation experiments (Bau, 1999). Other studies have
shown that REE accumulation can be closely correlated to the
increasing stalk length of G. ferruginea, whereas the enrichments
of actinides like U and Th show a less pronounced correlation
(Anderson and Pedersen, 2003). The different behavior of the
latter may be explained by their particularly high sorption sus-
ceptibility to organic matter (e.g., Lin et al., 2014). Consequently,
not only the iron oxyhydroxide precipitates, but also the organic
matter of the stalks most likely play a crucial role in TREE pre-
cipitation within “young” microbial mats, because the EPS offers
large reactive surfaces for metal sorption. According to this sce-
nario, the ongoing incrustation of the stalks of Mariprofundus
sp. and Gallionella sp. (Figure 3C) most likely caused a grad-
ual decline of exposed EPS-surfaces, and thus reduced the pas-
sive mineralization capacities of the microbial mats. Instead,
indirect mineral precipitation became increasingly important in
the course of the experiment, and probably accounted for the
significantly higher accumulations of Si and the lower valent
cations Cs, Sr, Se, Li observed in the aged microbial mats
(Figure 4).
The entire water chemistry data of the aquifers were used to
calculate saturation indices (SI). SI values calculated for gypsum
using PhreeqC range between −0.74 and −0.95, indicating that
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Heim et al. TREE as biosignatures
FIGURE 7 | Comparison of PAAS-normalized (Post-Archaean average
Australian sedimentary rock; McLennan, 1989) REE+Y patterns of the
microbial iron oxyhydroxides from all flow reactors and the artificial
precipitation experiments from the feeder fluid at site 1327B, in
comparison to published data for modern seawater and BIFs. Note the
similarity between all REE+Y-patterns regardless of a biological contribution.
direct gypsum precipitation from the aquifers is rather unlikely.
Hence, the authigenic formation of gypsum observed in the
microbial mats (Figure 3D) can only be explained by micro-
environments inducing local supersaturation of Ca2+ and SO2−4 .
The 9 months old microbial mats were quite thick and dense
(Figure 1G) which hampered the fluid circulation in the reac-
tors and likely gave rise to the formation of micro-environments.
The creation of such micro-environments may also be pro-
moted by sulfur oxidizing bacteria (SOB) which oxidize hydro-
gen sulfide (H2S) to sulfur intermediates [i.e., elemental sulfur
(S◦), thiosulfate (S2O2−3 ), sulfite (SO
2−
3 )] and eventually, sul-
fate (SO2−4 ). Indeed, several SOB genera (Thiothrix, Thiobacillus,
Sulfurimonas and Acidithiobacillus) have been identified in the
microbial mats (Ionescu et al., 2015a), and their metabolism may
plausibly account for a local increase in SO2−4 and concomi-
tant gypsum precipitation. Likewise, sorption of SO2−4 onto iron
oxyhydroxides (Dzombak and Morel, 1990; originally described
in soils) may be expected to occur in the microbial mats, and
may further help to create locally distinct supersaturated micro-
environments, thereby inducing gypsum precipitation. The gyp-
sum lattice easily incorporates ions similar to Ca in charge and/or
size, like Mg, K, Na, Cs, and Sr (e.g., Lu et al., 2002 and references
therein) which may have contributed to the relative enrichment
of these elements within the aged microbial mats (Figure 4).
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Heim et al. TREE as biosignatures
FIGURE 8 | Comparison of the TREE (normalized on the feeder fluid at 1327B) accumulated in the microbial and the artificial iron oxyhydroxides. Note
that Ni and Tl are present only in the microbial iron oxyhydroxides including the biotic mineral precipitates (arrows).
Anoxic micro-environments may also provide an explanation
for the observed formation of siderite, as this mineral is only
stable under reducing conditions. Although SI values calculated
for FeCO3 ranged between −0.44 and 0.14 for each flow reac-
tor, siderite formation in the microbial mats was observed at both
sites and during all time points.
When normalized on PAAS (Post-Archaean average Australian
sedimentary rock; McLennan, 1989), the REE+Y plots of the
feeder fluids and the microbial iron oxyhydroxides show a slight
enrichment of the heavy REE (Gd-Lu+Y) over the light REE (La-
Eu) (Figure 6). This pattern is characteristic for REE+Y bound in
carbonate complexes, and is often referred to as “hydrogenous”
(Takahashi et al., 2002). The phenomenon has been explained
by different chemical complex formation of the REE+Y in aque-
ous solutions, rather than a merely charge- and radius-controlled
behavior (CHARAC; Bau, 1996; Takahashi et al., 2002).
When normalized on the feeder fluids, the REE+Y patterns
of the microbial iron oxyhydroxides exactly mirror the source,
thus displaying a consistent radius-independent fractionation
(Figure 6). Interestingly, published REE patterns of Archaean
BIFs (Bau and Dulski, 1996; Frei et al., 2008) range well-within
the REE+Y patterns of the microbial iron oxyhydroxides formed
in our experiment (Figure 7). The only “outlier” is the positive
Eu anomaly of BIFs, which is most likely due to hydrothermal
influence (Michard and Albarède, 1986; Frei et al., 2008) and
is missing in the feeder fluids and in the iron oxyhydroxides
from Äspö. However, this coincidence may not be interpreted
in terms of a “biosignature,” because artificial precipitation of
iron oxyhydroxides (1. inorganic 2. abiotic; 3. biotic, i.e., bio-
logical activity allowed for 15 days) from the Äspö feeder fluids
resulted in largely the same REE+Y pattern. Likewise, the ∼104-
fold REE+Y enrichments observed for the iron oxyhydroxides of
the young (2-months old) microbial mats are quantitatively simi-
lar to the values observed for the artificial precipitates (Figure 7).
Therefore, our data indicate that it is not possible to deduce a
microbial contribution to the deposition of BIFs from REE+Y
patterns. This is in agreement with a recent study on biotic vs.
abiotc iron oxidation indicating that the presence of particulate
organic matter or previously precipitated iron both accelerate
further precipitation (Ionescu et al., 2015b).
Apart from the REE+Y distributions, a comparison of
the complete TREE patterns of the artificial precipitates
and the microbial iron oxyhydroxides yielded interesting
results.
Cd was accumulated in small amounts in the biogenic iron
oxyhydroxides and was highest in the abiotic precipitates. It
was not detected at all in the inorganic iron oxyhydroxides.
Although Cd does not show a preferential sorption to organic
matter (Davis, 1984), in our study Cd is positively correlated to
the organic matter containing iron oxyhydroxides. Tl was only
observed in the microbial iron oxyhydroxides from the flow reac-
tors. A biological function has not been reported so far, but
Tl isotope patterns were suggested as a proxy for an increase
in organic carbon burial in Paleocene marine Fe–Mn Crusts
(Rehkämper et al., 2004; Nielsen et al., 2009).
Ni was only detectable, and accumulated, in microbial iron
oxyhydroxides (all samples from 15 days, 2 and 9months) growth.
Ni is known to fulfill a range of biological functions, especially in
Ni-metalloenzymes. Prominent examples are CO dehydrogenase,
acetyl-CoA synthetase, and methyl-coenzyme M reductase which
all play important roles in microbial carbon metabolism (Ermler
et al., 1998; Dobbek et al., 2001; Voordouw, 2002; Ragsdale, 2007).
The presence of Ni in the biogenic precipitates may thus be inher-
ited from the Ni-content of living microbial cells. Although Ni
is constantly accumulated in the microbial iron oxyhydroxides,
the molar Ni/Fe ratio decreased by an order of magnitude (from
2.8 × 10−5 and 8.0 × 10−6 to 3.0 × 10−6 and 2.0 × 10−6) with
the aging of the mats at both sites, possibly indicating a lower
contribution of microbial cellular matter to the metal content
of the late precipitates. It has been proposed, that BIFs mirror
the source fluid from which they were precipitated and thus, the
elemental composition of the Precambrian Ocean (Konhauser
et al., 2009). In our study, however, the decreasing molar Ni/Fe
ratios of microbial iron oxyhydroxides are not representative for
the constant Ni/Fe supply that was delivered from the feeder
fluids.
Frontiers in Earth Science | Biogeoscience February 2015 | Volume 3 | Article 6 | 12
Heim et al. TREE as biosignatures
CONCLUSIONS
In a suboxic aquifer system hosted by granodioritic rocks and
influenced by Baltic Sea water, the development of iron oxidiz-
ing microbial mats was studied to test the potential usage of
TREE as biosignatures for a microbial involvement in Fe mineral
precipitation. Over the 9 months of the experiment, massive iron-
oxidizing microbial mat systems developed whose predominant
FeOB were the stalk-forming Mariprofundus sp. and Gallionella
sp. The deposition of iron oxyhydroxides in these mat systems
was initially controlled by the metabolic activity ofMariprofundus
sp. and Gallionella sp. and passive mineralization caused by the
high biosorption capacity of newly formed EPS-stalks. Upon
aging, ongoing iron oxyhydroxide impregnation caused a grad-
ual decrease of the biosorption capacities of the EPS-stalks. As a
result, indirect mineral precipitation became increasingly impor-
tant over time.
The iron-oxidizing microbial mats proved to be extremely
efficient in the accumulation of trace and rare earth elements
(TREE), leading to massive (up to 106-fold) enrichments of indi-
vidual metals in the microbial iron oxyhydroxides. The REE+Y
patterns of the resulting microbial iron oxyhydroxides matched
quite closely the feeding aquifer, as well as Archaean Banded
Iron Formations (BIFs). However, inorganic and abiotic iron oxy-
hydroxides precipitated artificially from the Äspö feeder fluids
showed a very similar REE+Y pattern and roughly the same
accumulation rates as the biogenic deposits formed in the flow
reactors. These results argue against the usage of REE+Y patterns
to deduce a biological contribution of FeOB to the deposition
of BIFs. On the other hand, the microbial iron oxyhydroxides
showed significant accumulations of the trace elements Ni and
Tl which were absent in the artificially precipitated iron oxyhy-
droxides. These elements should be studied further as potential
candidate biosignatures to assess a biological contribution of
FeOB to the deposition of BIFs.
ACKNOWLEDGMENTS
We acknowledge with gratitude the detailed and constructive
comments provided by Doug LaRowe and Jakob Zopfi. We are
also grateful to Emmeli Johansson, Magnus Kronberg, Teresita
Morales and the SKB staff for technical, logistic and analyt-
ical support at the Äspö HRL. Volker Karius and Veit-Enno
Hoffmann, Erwin Schiffzyk, and Dorothea Hause-Reitner are
acknowledged for their assistance with the XRD, ICP-OES, and
SEM-EDX measurements, respectively. Andreas Pack helped to
improve the data presentation. Our study received financial sup-
port from the German Research Foundation (DFG). This is pub-
lication No. 75 of the DFG Research Unit FOR 571 “Geobiology
of Organo- and Biofilms.”
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Conflict of Interest Statement: The authors declare that the research was con-
ducted in the absence of any commercial or financial relationships that could be
construed as a potential conflict of interest.
Received: 19 October 2014; paper pending published: 16 November 2014; accepted: 03
February 2015; published online: 24 February 2015.
Citation: Heim C, Simon K, Ionescu D, Reimer A, De Beer D, Quéric N-V, Reitner J
and Thiel V (2015) Assessing the utility of trace and rare earth elements as biosig-
natures in microbial iron oxyhydroxides. Front. Earth Sci. 3:6. doi: 10.3389/feart.
2015.00006
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