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http://biotech.nature.com • AUGUST 2002 • VOLUME 20 • nature biotechnology
Harnessing microbially generated power
on the seafloor
Leonard M.Tender
1
*, Clare E.Reimers
2
*, Hilmar A.Stecher III
2
, Dawn E.Holmes
3
, Daniel R.Bond
3
,
Daniel A.Lowy
4
, Kanoelani Pilobello
4
, Stephanie J.Fertig
4
, and Derek R.Lovley
3
Published online:1 July 2002, doi:10.1038/nbt716
In many marine environments, a voltage gradient exists across the water–sediment interface resulting from
sedimentary microbial activity. Here we show that a fuel cell consisting of an anode embedded in marine sed-
iment and a cathode in overlying seawater can use this voltage gradient to generate electrical power in situ.
Fuel cells of this design generated sustained power in a boat basin carved into a salt marsh near Tuckerton,
New Jersey, and in the Yaquina Bay Estuary near Newport, Oregon. Retrieval and analysis of the Tuckerton
fuel cell indicates that power generation results from at least two anode reactions: oxidation of sediment sul-
fide (a by-product of microbial oxidation of sedimentary organic carbon) and oxidation of sedimentary organic
carbon catalyzed by microorganisms colonizing the anode.These results demonstrate in real marine environ-
ments a new form of power generation that uses an immense, renewable energy reservoir (sedimentary
organic carbon) and has near-immediate application.
RESEARCH ARTICLE
An immense energy reserve sits on the seafloor in the form of oxidiz-
able organic carbon resulting primarily from sedimentation of phy-
toplankton detritus. Vast regions of the seafloor have accumulated
sediments meters thick containing 0.1–10% organic carbon by weight
1
.
The typical energy density of such sediments based on 2.0% organic
carbon content
2
and complete oxidation by oxygen is 6.1 × 10
4
J/L
(17 W h/L)
3–5
, a remarkable value if sediment volume (6.3 × 10
14
liters
for the Gulf of Mexico
1
, for example) is considered. Microorganisms,
limited by the oxidant supply of overlying seawater, use a small portion
of this energy reserve, and in doing so create a voltage drop as large as
0.8 V within the top few millimeters to centimeters of sediment sur-
faces
6,7
. This voltage gradient results from depletion of oxygen by sedi-
ment surface–dwelling microorganisms and metazoans causing
microorganisms farther down to use a succession of less potent oxi-
dants (such as sulfate) and generate, as by-products, more potent
reductants (such as sulfide) with increasing sediment depth
3,4
.
Here we report the in situ use of benthic voltage gradients to generate
sustained electrical power. Two fuel cells were deployed in two coastal
marine environments: a boat basin carved into a salt marsh near
Tuckerton, New Jersey and the Yaquina Bay Estuary near Newport,
Oregon. Each fuel cell consisted of a graphite anode embedded in sedi-
ment and a graphite cathode suspended in overlying seawater. The
anode and cathode of each fuel cell were connected by an external cir-
cuit containing a resistive load capable of dissipating power at either
constant voltage or constant current by feedback control of resistance.
The natural separation of oxygen-rich seawater (cathodic reactant) and
organic carbon-rich sediment (anodic reactant) eliminates the need for
a two-compartment cell and a semipermeable membrane for power
generation. Placement of the anode into sediment initially disrupts the
voltage gradient, which re-establishes itself on the order of days.
Analysis of sediment collected at the Tuckerton site indicated reduced
carbon contents of 4–6% (dry weight) arising from marine phyto-
plankton detritus and the marsh grass Spartina alterniflora. Sediment
at the Newport site had reduced carbon contents of 2–6% by weight
(highest values near the sediment surface) derived primarily from
marine phytoplankton and macroalgae, and from the sea grass
Zostera marina. A second fuel cell through which no current flowed
(open circuit) was deployed at each site to serve as a control for post-
retrieval sediment and microbial analysis.
Results and discussion
Power generation. Voltage–current and power–current properties of
both fuel cells (Fig. 1) indicate the optimal current or voltage ranges at
which each fuel cell can be operated to maximize power generation.
Current can be sustained in both marine environments, indicating
that net oxidation of marine sediment constituents occurs at the
anodes and net reduction of seawater constituents occurs at the cath-
odes. This result is consistent with the use of noncorroding graphite
electrodes as anodes in microbial fuel cells
8
and as cathodes in seawater
batteries
9
. Furthermore, the results are consistent with the expected
voltage–current and power–current density properties of a fuel cell
10
:
power seems limited by electrode kinetics at low current density, by
mass transfer of charge-compensating ions between electrodes at
intermediate current density, and by mass transfer of one of the elec-
trode reactants at high current density.
The Newport deployment (maintained from January 2001 to
January 2002) was used in part to assess the long-term ability of a fuel
cell to produce current at a constant voltage. From July 17 to November
30, 2001, power density averaged ∼28 mW/m
2
at 0.27 V (Fig. 2A), and
was steady except for three incidents of power reduction attributed to
1
Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Code 6900, 4555 Overlook Avenue, SW, Washington, DC 20375.
2
Hatfield Marine
Science Center, Oregon State University, 2030 SE Marine Science Drive, Newport, OR 97365.
3
Department of Microbiology, 106N Morrill IV N, University of
Massachusetts, Amherst, MA 01003.
4
Nova Research, Inc., 1900 Elkin Street, Alexandria, VA 22308.
*Corresponding authors (lmt@cbmse.nrl.navy.mil and clare.reimers@hmsc.orst.edu).
821
© 2002 Nature Publishing Group http://biotech.nature.com
episodic coverage of the cathode with sediment and macroalgal detri-
tus as a result of the fuel cell’s location in a sheltered cove seasonally
inundated by benthic macroalgae. Small to moderate sinusoidal power
anomalies observed on a tidal timescale (Fig. 2B) are likely due to com-
binations of environmental variables associated with estuaries such as
temperature, salinity, water velocity, short-term sedimentation events,
and sediment bioirrigation.
The Tuckerton deployment (January 2001–August 2001) was used
to test power output under varying current and voltage conditions.
After 224 days of operation, we retrieved the active (power generating)
and control fuel cells from the Tuckerton site and collected core sam-
ples of sediment from above and below each anode and scrapings of the
graphite anodes. During the first and last weeks of operation, the active
fuel cell generated maximum power of 26.6 mW/m
2
and 25.4 mW/m
2
,
respectively, indicating remarkable stability in spite of expected elec-
trode fouling. Typical power generation in response to programmed
periods of fixed current was constant, whereas voltage rose rapidly
upon reduction in current, characteristic of the rechargeable reaction
environment around both electrodes (Fig. 3).
Chemical analysis. Comparison of pore-water chemistry near the
active and control anodes of the Tuckerton fuel cells yields insights
into the mechanisms of power generation. A linear sulfide gradient
above and below the active anode and complete sulfide depletion at
the anode surface (Fig. 4A) indicate mass transfer–limited oxidation of
sediment sulfide at the anode. Application of Fick’s first law of diffu-
sion
11
using the average sulfide gradient (2.8 ×10
–7
mol/cm
4
) and a dif-
fusion coefficient for sulfide through sediment of 1.3 × 10
–5
cm
2
/s at
25°C (the average temperature at the Tuckerton deployment site dur-
ing August 2001)
12
yields a sulfide flux to the surface of the active
anode of 3.6 × 10
–4
mol/cm
2
s. Assuming that two electrons are trans-
ferred per sulfur atom in the oxidation of sulfide to elemental sulfur
13
,
a current density of 14 mA/m2 (based on footprint area of the anode)
is expected from the calculated sulfide flux. (Oxidation of sulfide to
elemental sulfur is consistent with scanning electron microscopy
(SEM) observations of a precipitate on the active anodes and with lab-
oratory studies we have conducted in which electricity was generated
from anodes immersed in sterile sulfide-rich media.) Comparison to
the measured average current density (35 mA/m
2
) during the last 23
days of operation (the minimum time required to maintain the
observed sulfide depletion zone, t) indicates that other mechanisms
contribute to power generation (t = d
2
/4D where d is the thickness of
the sulfide diffusion layer (10 cm), and D is the diffusion coefficient
for sulfide through sediment)
11
.
R
ESEARCH ARTICLE
nature biotechnology • VOLUME 20 • AUGUST 2002 • http://biotech.nature.com822
Table 1. Percentage of 16S rDNA clones recovered from anode
surfaces belonging to major phylogenetic groups
Bacterial group Control Active
(no current) (current)
α-Proteobacteria 17 3
γ-Proteobacteria 21 9
ε-Proteobacteria 3 0
δ-Proteobacteria:
Desulfuromonas spp. 9 45
Desulfobulbus/Desulfocapsa spp. 1 Σ = 23 24 Σ = 76
Other 13 7
Cytophagales 6 5
Clostridium/Bacillus spp. 5 3
Other 25 4
Figure 1. Polarization properties.(A) Voltage and power density vs.
current density characterization of fuel cell deployed at Newport site,
recorded March 21, 2001.(B) Voltage and power density vs.current
density characterization of fuel cell deployed at Tuckerton site, recorded
January 22, 2001.Data was obtained by stepwise reduction of cell voltage
followed by measurement of current through the external circuit after
sufficient time elapsed for current to stabilize (>10 min).Current density
(mA/m
2
) was calculated by normalization of current to the electrode
footprint area (0.183 m
2
);power density (mW/m
2
) was calculated as the
product of voltage and current density.
Figure 2. Long-term power density. (A) Daily record of average power
density of the active fuel cell deployed at the Newport site.Cell voltage
was maintained at 0.27 V.Episodic power reductions are attributed to
sedimentation around the cathode.During one such event, cell voltage fell
below measurement range of the load resulting in the large gap in late
September. On October 2, divers cleared sediment from around and
under the cathode resulting in recovery and stabilization of power.Data
gaps occurred on six other days due to problems with the recording
computer. (B) Eight-day example of tidal time-scale variations in power
density recorded every 10 min.Top record, power density.Bottom record,
tide height.
A
B
A
B
© 2002 Nature Publishing Group http://biotech.nature.com
RESEARCH ARTICLE
Microbial analysis. Analysis of 16S rDNA sequences from
microorganisms found in the anode scrapings provides
insights into such mechanisms (Table 1). Power generation
results in specific enrichment of microorganisms in the delta
subclass of the Proteobacteria colonizing the anode of the
active fuel cell (76% of the 16S rDNA sequences recovered
from the anode of the active fuel cell were in the delta subgroup
of Protebacteria compared with 23% recovered from the
anode of the control; Table 1). Furthermore, 59% of the delta
Protebacteria enriched were most closely related to Fe(III) and
S
0
-reducing microorganisms in the family Geobacteraceae
(>95% similarity to Desulfuromonas acetoxidans). This result
is similar to results in laboratory studies demonstrating that
Geobacteraceae can oxidize acetate, the primary intermediate
in the anaerobic degradation of sedimentary organic carbon,
and directly transfer electrons to graphite anodes without
soluble electron-transfer mediators
14
. Laboratory fuel cells
using graphite anodes and excess acetate, to which
Desulfuromonas acetoxidans was added, could sustain current
densities of 20 mA/m
2
(0.13 V) across a 500 Ω load without
electron-transfer mediators in solution. Current density
increased to 26 mA/m
2
(0.17 V) when anthraquinone
2,6-disulfonate (AQDS, a compound known to serve as an
electron shuttle in the respiration of Geobacteraceae)
15
was added.
Enrichment of dissolved iron near the active anode (Fig. 4B) is con-
sistent with both sulfide depletion (sulfide reacts with soluble Fe(II) to
form iron sulfide precipitates) and with the presence of
Geobacteraceae. These bacteria are capable of reducing precipitates of
Fe(III) to soluble Fe(II). Thus, depletion of sulfide near the active
anode would allow Fe(II), generated by Geobacteraceae and other iron-
reducing genera, to accumulate in pore water near the active anode rel-
ative to the control.
The change in sulfate concentration relative to the change in ammo-
nia (a by-product of microbial oxidation of organic matter) in pore
water adjacent to the anode of the active fuel cell is similar to that of
pore water adjacent to the anode of the control (Fig. 4C). This suggests
that sulfate concentration adjacent to both anodes is primarily deter-
mined by depletion of a seawater source by microbial sulfate reduction.
Dissimulatory sulfate reduction is commonly described by the follow-
ing net reaction:
(CH
2
O)
x
(NH
3
)
y
(H
3
PO
4
) +
1
/
2
xSO
4
2–
→ xHCO
3
–
+ yNH
3
+ H
3
PO
4
+
1
/
2
xH
2
S
If this is the only process affecting sulfate and ammonia concentra-
tions, then:
∆SO
4
2–
/∆NH
4
+
= –
1
/
2
(x/y) = constant
Nonetheless, modest sulfate enrichment occurs near the active
anode (Fig. 4C). This result may reflect microbial oxidation of S
0
to
sulfate with the electrode serving as the electron acceptor, in a man-
ner analogous to previously described microbial oxidation of S
0
to
sulfate with Mn(IV)
16
. This is consistent with an observed enrich-
ment of 16S sequences most similar to species in Desulfobulbus or
Desulfocapsa genera (24% of sequences recovered from anodes), as
these organisms are known to be capable of sulfur oxidation and dis-
proportionation
17
.
Conclusions. The results described here have implications for the
development of power supplies that harvest energy from marine
environments. Long-term sustained power generation appears lim-
ited by reactant flux to the anode (sulfide and organic carbon). This
is an environmental factor that promises high power output in cer-
tain environments (such as at sulfide-rich vents or near or atop
methane hydrate deposits
18,19
). Furthermore, long-term steady-state
power generation is not limited by fouling, which often dramatical-
ly changes the surface properties of objects placed in marine envi-
ronments. Microbial colonization is an important component of
fouling. Further investigation of the influence of oxidative current
on the anode-colonizing microorganisms may lead to better strate-
gies toward antifouling surfaces. Many marine oceanographic
instruments such as integrated conductivity, temperature, and
depth sensors, hydrophones, and underwater inductive modems
have power requirements on the order of 0.1–1 W with deployment
times limited by battery life. A fuel cell that uses abundant fuel and
oxidant as they are naturally found has now been shown to operate
without fouling for many months in two marine environments. In
the one case analyzed, observed microbial enrichment and sulfide
depletion suggest that both microbial metabolism with coupled
direct electron transfer to the anode and oxidation of sediment sul-
fide contributed to the observed current. In this specific environ-
ment, ∼40% of the observed current density (14 mA/m
2
of
35 mA/m
2
) could be attributed to oxidation of sediment sulfide. The
balance of observed current density (21 mA/m
2
) is consistent with
that sustained in laboratory studies (20–100 mA/m
2
) by catalytic
oxidation of sedimentary acetate by microorganisms colonizing the
anode. Most certainly, the relative and absolute contributions of each
http://biotech.nature.com • AUGUST 2002 • VOLUME 20 • nature biotechnology 823
Figure 3. Voltage and current-density record of the active fuel cell
deployed at the Tuckerton site recorded August 7, 2001.Data points
represent 10-min averages.
Figure 4. Anode chemical impacts. Comparative pore-water chemistries surrounding
the anodes of active (♦) and control () cells from the Tuckerton site.Vertical error
bars represent the thickness of sediment sections centrifuged to extract pore water.
AB C
© 2002 Nature Publishing Group http://biotech.nature.com
mechanism with respect to overall power generation will depend
strongly on the specific environment. Other anode reactions, possi-
bly involving microbial consortia and various oxidation states of sul-
fur, cannot be ruled out and need further investigation. By scaling up
the approach described here, long-term (indefinite) power may be
supplied by such fuel cells to marine instruments in the near future.
Experimental protocol
Fuel cells. Electrodes consisted of graphite discs (LG graphite grade 10,
Graphite Engineering & Sales, Greenville, MI) of 48.3 cm diameter and 1.3 cm
thickness drilled with a pattern of 790 evenly spaced holes of 0.64 cm
diameter. Electrodes were used as received or lightly sanded. Electrical
connection to the anode and cathode of each active fuel cell was made in
the following manner. A water-insulated, pluggable, single-conductor,
oceanographic, electrical bulkhead connector (Impulse Enterprise, San
Diego, CA) was joined to each electrode by silver epoxy (Epoxy
Technology, Billerica, MA) encapsulated with water-insulating epoxy
(Dexter Corporation, Seabrook, NH). Matching single-conductor oceano-
graphic cables attached to these connectors electrically connected the
anode and cathode of each active fuel cell to pluggable bulkhead connec-
tors wired to a computer-controlled resistive load (870, Scribner
Associates, Southern Pine, NC) contained within a custom-built oceano-
graphic instrument enclosure. Connections to land-based power (for
operation of the load) and computer were provided by additional cables to
the enclosure.
The connector–silver epoxy–graphite union of the active cathode at
the Newport site failed after 4.5 months (January–May 2001). A new
active fuel cell was subsequently deployed at the Newport site (June
2001–January 2002) with electrodes attached directly to the single-
conductor oceanographic cables by wrapping a stripped-end section of
each copper-conducting wire around a #6 stainless steel screw with
32 threads per inch (6-32) threaded into a hole tapped into each electrode.
Each of these cable–graphite unions was then encapsulated into a block of
water-resistant epoxy (West System, Gougeon Brothers, Bay City, MI).
Polyvinyl chloride (PVC) frames were used for both Newport fuel cells to
position each anode ∼15 cm below and parallel to the sediment surface and
each cathode ∼15 cm above and parallel to the sediment surface in overly-
ing seawater. These fuel cells were deployed by divers in water with a mean
midtide depth of ∼4 m.
Electrodes of both Tuckerton fuel cells were enclosed within 167-liter
cylindrical PVC containers (Rubbermaid, Cleveland, OH). Each container
had 80 holes of 5.1 cm diameter lined with fiberglass screen. These holes
ensured sufficient mass transfer of seawater to the cathode while enabling
recovery of sediment above and below the anode with intact geochemical
stratification when each fuel cell was retrieved. Before deployment, each
container was partially filled with ∼50 liters of sediment, the anode was
positioned ∼10 cm below and parallel to the sediment surface, and the cath-
ode was positioned ∼20 cm above and parallel to the sediment surface. The
Tuckerton fuel cells were deployed by lowering the containers until they set-
tled into sediment in water with a midtide mean depth of ∼1.5 m. These fuel
cells settled such that the inner and outer sediment heights were near equal.
An integrated conductivity, temperature, and depth meter and an acoustic
Doppler current meter were deployed at each site to correlate changing
fuel-cell voltage and current with changing environmental properties.
DNA extraction and cloning of 16S rDNA. Electrode surfaces were washed
free of sediment with sterile artificial seawater, and scraped vigorously
with a sterile razor blade into 1.5 ml TE buffer (10 mM Tris-HCl, 1 mM
EDTA, pH 8; Sigma, St. Louis, MO). The miniprep of bacterial genomic
DNA protocol
20
was used for DNA extraction from the recovered graphite
slurry with the following modifications. The pellet was resuspended in
TE/sucrose buffer (50 mM Tris-HCl, 10 mM EDTA, pH 8, 6.7% sucrose),
SDS (0.5%, final concentration) and lysozyme (1 mg/ml final concentra-
tion; Sigma) were added to the suspension, and tubes were incubated at
37°C for 30 min with vortexing every 5 min. After proteinase K treatment
(0.1 mg/ml; Sigma) at 37°C for 1 h, MULTIMIX 2 Tissue Matrix (Bio101
Systems, Carlsbad, CA) was added to the suspension. Tubes were then
placed in a Mini-BeadBeater (BioSpecs Products, Bartleville, OK) for 30 s
at 550 rpm, DNA was extracted once with chloroform/isoamyl alcohol
(24:1; Sigma), and once with phenol/chloroform/isoamyl alcohol (25:24:1;
Sigma). This extracted DNA was further purified with the Wizard DNA
Clean-Up System (Promega, Madison, WI). Extractions typically yielded
20–100 µg DNA per 10 cm
2
of electrode.
In two separate reactions, 16S rDNA was amplified using primers 27F
21
or 63F
22
with 519R
23
. The total volume of each PCR mixture was 100 µl, and
contained ∼60 µg DNA template, 10 µl Qiagen 10× buffer (15 mM MgCl
2
),
5 µl buffer Q (Qiagen, Valencia, CA), 8 µl 0.25 µM dNTP solution (Sigma),
60 pmol each forward and reverse primers, 5 µl dimethyl sulfoxide (Sigma),
and 3 units Taq polymerase (Qiagen). To ensure sterility, the PCR mixtures
were exposed to UV radiation for 10 min before the addition of template
and Taq polymerase. PCR amplification was carried out in a DNA Engine
thermal cycler (MJ Research, Waltham, MA) with an initial denaturation
step of 94°C for 4 min, followed by 35 cycles of 94°C for 30 s, 50°C for 30 s,
and 72°C for 45 s with a final extension of 72°C for 7 min. The PCR prod-
ucts from all reactions were pooled and cloned into Escherichia coli using
the TOPO TA cloning kit, version K2 (Invitrogen, Carlsbad, CA).
Clones were randomly selected from each clone library, and cloned 16S
rDNA was amplified from the TOPO vector using M13 forward and M13
reverse primers (Invitrogen). These amplified inserts were incubated for
16 h at 37°C with HhaI and MspI (both 4 bp cutters; New England Biolabs,
Beverly, MA). The results from the restriction digests were visualized on a
3% (wt/vol) Metaphor agarose gel (BioWhittaker Molecular Applications,
Rockland, ME), and clones yielding similar restriction-digest banding pat-
terns were assumed to carry similar 16S rDNA inserts. Plasmids were iso-
lated from cultures showing different banding patterns with the QIAprep
Spin Miniprep Kit (Qiagen), and plasmid inserts were sequenced.
Sequences were compared to GenBank and Ribosomal Database Project
databases using the BLAST
24
and SIMILARITY
25
algorithms. At least 60
sequences were analyzed for each clone library, and representative
sequences were submitted to GenBank (accession nos AY123202–
AY123222).
Sediment pore water analysis. Pore waters were extracted by centrifuga-
tion from sections of cores of 8 cm diameter and then filtered (0.45 µm).
All sample handling was done in a glove bag at 25°C ± 3°C (equal to the
average daily in situ temperature at Tuckerton Site in August 2001) under a
N
2
atmosphere. Samples for quantifying dissolved sulfide concentrations
were fixed immediately according to Cline
26
and measured spectrophoto-
metrically (Gilford Stasar II, 1 cm path length). Total iron was measured
after appropriate sample dilution of acidified samples by flow-injection
analysis and spectrophotometric detection following Measures et al.
27
.The
detection of iron is achieved through its catalytic effect on the oxidation of
N,N-dimethyl-p-phenylenediamine dihydrochloride by hydrogen perox-
ide, producing colored semi-quinone derivatives. Sulfate and chloride con-
centrations were measured after dilution of nonacidified samples using a
DX-500 Ion Chromatograph with AG14 columns (Dionex, Sunnydale,
CA). Ammonium was determined using a nutrient autoanalyzer (Alpkem,
Clackamas, OR) after a 1:26 dilution:
∆NH
4
= –(NH
4
)
measured
Pore water sulfate depletion was calculated as
assuming pore water is buried with the conservative ratio of sulfate to
chloride characteristic of seawater.
Acknowledgments
This work was supported by grants from the Office of Naval Research (ONR),
the Naval Research Laboratory (NRL), and the Defense Advanced Research
Projects Administration (DARPA). We are grateful to Rose Petrecca, Joe
Debarro, and staff of the Tuckerton, New Jersey Field Station of Rutgers
University, Institute of Marine and Coastal Sciences for assistance in fuel-cell
deployment and retrieval. We also thank M. Sommer, R. Emmett,
T. Bridgeman, T. Builder, W. Hanshumacher, D. Jacobson, and M. Spencer for
diving assistance at the Newport site and L. Annable for field assistance.
Competing interests statement
The authors declare that they have no competing financial interests.
Received 24 January 2002; accepted 7 May 2002
∆SO
4
=
[
(
SO
4
C1
)
seawater
× [C1]
measured
]
– [SO
4
]
measured
R
ESEARCH ARTICLE
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© 2002 Nature Publishing Group http://biotech.nature.com
RESEARCH ARTICLE
http://biotech.nature.com • AUGUST 2002 • VOLUME 20 • nature biotechnology 825
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