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Applied and Environmental Microbiology, February 2004, p. 1231-1233, Vol. 70, No. 2
0099-2240/04/$08.00+0 DOI: 10.1128/AEM.70.2.1231-1233.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Effects of Temperature and Pressure on Sulfate Reduction and Anaerobic Oxidation of Methane in Hydrothermal Sediments of Guaymas Basin
Jens Kallmeyer1,2 and Antje Boetius3,4*
Max Planck Institute for Marine Microbiology, 28359 Bremen,1
Geoforschungszentrum Potsdam, 14473 Potsdam,2
Alfred Wegener Institute for Polar and Marine Research, 27515 Bremerhaven,3
International University Bremen, 28759 Bremen, Germany4
Received 30 June 2003/
Accepted 31 October 2003

ABSTRACT
Rates of sulfate reduction (SR) and anaerobic oxidation of methane
(AOM) in hydrothermal deep-sea sediments from Guaymas Basin
were measured at temperatures of 5 to 200°C and pressures
of 1
x 10
5, 2.2
x 10
7, and 4.5
x 10
7 Pa. A maximum SR of several
micromoles per cubic centimeter per day was found at between
60 and 95°C and 2.2
x 10
7 and 4.5
x 10
7 Pa. Maximal AOM
was observed at 35 to 90°C but generally accounted for less
than 5% of SR.

INTRODUCTION
Hydrothermal sediments of the Guaymas Basin contain highly diverse
anaerobic thermophilic microorganisms, including methanogens,
sulfate-reducing bacteria, and presumably also methanotrophs
(
2,
6,
15,
18,
19,
23). Thermogenic reactions in the subsurface
sediments provide a complex mixture of methane, other hydrocarbons,
and volatile fatty acids as substrates for these microorganisms
(
8). Acetate concentrations in Guaymas sediments are extremely
variable, ranging from 10 to >1,000 µM, with maximum
concentrations occurring in hot sediments (
12). Methane concentrations
are 12 to 16 mM (
22), but various other hydrocarbons derived
during thermal alteration of organic material have also been
found in high concentrations (
11,
16,
17). In previous investigations,
high rates of sulfate reduction (SR) were found at sediment
surfaces, where freshly deposited organic material as well as
bottom-water sulfate is available. Subsurface maxima in SR are
found where upward moving fluids advect high concentrations
of volatile fatty acids and a variety of hydrocarbons (
4,
7,
8,
12,
16,
21). It is not known whether methane is a relevant
electron donor for SR in hydrothermal sediments because the
activity of thermophilic methanotrophs in Guaymas sediments
has not yet been investigated. So far, high rates of methane-dependent
SR have been found only in cold environments, at in situ temperatures
of -1.5 to +12°C (
5). Accordingly, physiological experiments
with anaerobic-methanotrophic (ANME) groups from cold seeps
have shown temperature optima for the anaerobic oxidation of
methane (AOM) of between 5 and 15°C (
13,
14). However, typical
biomarker and 16S rRNA gene signatures of AOM consortia suggest
the presence of thermophilic methanotrophs in hydrothermal surface
sediments of Guaymas Basin (
15,
19). The primary aims of the
present study were to reveal the effects of temperature and
pressure on SR and AOM in such sediments and to test whether
SR is fueled by methane.
Hydrothermal sediment was retrieved from a vented site, covered with a Beggiatoa mat, by the submersible ALVIN (dive 3780, cruise AT-07, 5 May 2002; 27°0'32"N, 111°24'26"W; 2,013 m water depth). The maximum temperature recorded at this site was 130°C at a sediment depth of 20 cm (D. Albert, personal communication), a setting similar to that of previous sampling of vent cores (3, 5, 19). The core was very gassy and expanded during retrieval. Immediately after recovery, the complete sediment sample was stored under anoxic conditions at 4°C until further measurements were performed in the home laboratory. For the experiments, 1 part of Guaymas sediment was mixed and diluted with 4 parts of a standard mineral salt solution (14). All preparations were carried out inside a glove box under an atmosphere of N2-CO2. Methane-saturated slurries were prepared by equilibrating the sediment slurry with a 100% methane headspace in a glass flask. Because AOM occurs in two steps (oxidation of methane to carbon dioxide and reduction of sulfate to sulfide), the radiotracers 14CH4 and 35SO4 were added in trace amounts to replicate subsamples for measurement of carbon dioxide and sulfide production. Measurements of SR and AOM were performed according to methods described elsewhere (10, 20). All data were calculated as activity per volume of undiluted sediment. For SR, two replicates were incubated for each temperature or pressure setting. For AOM, two (experiments 2 and 3) or four (experiment 4) replicates were used for each temperature setting. Abiotic controls, obtained by fixing the sample prior to addition of tracer, were also employed. The average control value plus the standard deviation was substracted from each data point value before calculation of activity. The effects of temperature and pressure on SR and AOM were studied by using a high-pressure thermal gradient block (10). Experiment 1 was carried out to measure the combined effects of pressure and temperature on SR (Table 1). In addition, temperature gradient experiments were carried out with different preincubations times (experiment 2, 1 day; experiments 3 and 4, 7 days) and methane concentrations (experiments 2 and 3, 0.1 mM; experiment 4, 1 mM).
The temperature maximum of SR was between 60 and 90°C with
a peak at 80°C, similar to results for intact sediment cores
in previous studies (
3,
21). At 80°C and 2.2
x 10
7 Pa, which
represent the in situ conditions at the sampled site, SR was
around 650 nmol cm
-3 day
-1 (Table
1). A maximum SR of almost
6,700 nmol cm
-3 day
-1 was reached at 4.5
x 10
7 Pa and 95°C.
Under these conditions, the SR rate was 40 times higher than
that in the 10
5- and 10
6-Pa incubations at the same temperatures.
These rates are among the highest SR ever observed in a marine
setting, comparable to the methane-driven SR values measured
in
Beggiatoa mats at cold seeps (
1). SR ceased above 102°C
even at high pressures (Table
1). Experiment 2 showed that a
1-day preincubation was insufficient for the microorganisms
to adjust to the original temperature conditions, as SR and
AOM rates were around 1 nmol cm
-3 day
-1 over the whole temperature
range. In experiment 3, SR reached values similar to in situ
(
21) or ex situ (
3) SR in samples from similar settings. The
SR rate was significantly lower (
P < 0.001) at low to intermediate
temperatures (5 to 53°C; SR range, 0 to 15 nmol cm
-3 day
-1;
average, 3.2 ± 2.5 nmol cm
-3 day
-1;
n = 12) than at higher
temperatures (62 to 85°C; SR range, 75 to 200 nmol cm
-3 day
-1; average, 95 ± 37 nmol cm
-3 day
-1;
n = 8). Most
interestingly, AOM rates were around 1% of SR rates at all temperatures.
However, both processes exhibited a similar trend, as AOM rates
in 62 to 85°C incubations (average, 1.6 ± 0.5 nmol
cm
-3 day
-1;
n = 8) were significantly higher (
P < 0.001)
than those for samples incubated at 5 to 53°C (average,
0.6 ± 0.2 nmol cm
-3 day
-1;
n = 12). However, the identification
of a clear temperature maximum was difficult because of the
very low AOM rates. In experiment 4, samples were preincubated
for a week at a methane concentration of ca. 1 mM. AOM rates
increased to an average of 12 ± 5 nmol cm
-3 day
-1 at
temperatures of 31 to 62°C (
n = 8). At higher temperatures
(>87°C), AOM rates declined again, with only one of four
replicates showing measurable activity. At lower temperatures
(<31°C), AOM rates were significantly lower (
P < 0.005;
average, 1.4 ± 1.7 nmol cm
-3 day
-1;
n = 8), comparable
to those for experiment 3 at similar temperatures (
P = 0.17).
Hence, our results suggest that a maximum for anaerobic oxidation
of methane occurs at 30 to 60°C in the hydrothermal sediments
of Guaymas Basin (Fig.
1). However, AOM contributed only 1 to
5% to SR at all temperatures, which is far from the 1:1 stoichiometry
usually attributed to active AOM zones, where methane is the
main substrate fueling SR (
1,
5,
13,
14). The AOM rate was much
higher in sediments from active cold seeps, reaching several
hundred to 1,000 nmol cm
-3 day
-1 (
13,
14,
20), compared to the
turnover of a few nanomoles at Guaymas vents. However, at a
cold-seep site, where other hydrocarbons were available in addition
to methane, AOM was also reduced to 1 to 10% of SR (
9). This
may indicate that C2:C5 or higher hydrocarbons are a favorable
substrate for sulfate reducers, outcompeting anaerobic methanotrophs
in such environments.
There remains the question of whether the ANME-1 and ANME-2
groups detected in the surface sediments of
Beggiatoa cores
from Guaymas Basin (
15,
19) are also present in hydrothermal
subsurface sediments and are responsible for AOM at temperatures
above 30°C. The previously observed substantial decrease
in archaeal methanotroph biomarker lipids with increasing sediment
depth (i.e., increasing temperature) (
15,
19) indicates that
the methane-rich, hot subsurface sediments are not a preferred
environment for ANME populations. In conclusion, although AOM
proceeds at higher temperatures in subsurface sediments of the
Guaymas basin, it is clearly not the dominant carbon cycling
process.

ACKNOWLEDGMENTS
We thank Dan Albert for providing samples, Imke Müller
for carrying out AOM analyses, and Beth Orcutt, Samantha Joye,
and Andreas Teske for fruitful discussions and comments on the
manuscript.
This study was supported by the program MUMM (Mikrobielle Umsatzraten von Methan in gashydrathaltigen Sedimenten; FN 03G0554A) of the Bundesministerium für Bildung und Forschung (BMBF; Germany). Further support came from the Max Planck Society (Germany).

FOOTNOTES
* Corresponding author. Mailing address: MPI, Celsiusstr. 1, 28359 Bremen, Germany. Phone: 49 421 2028 648. Fax: 49 421 2028 690. E-mail:
aboetius{at}mpi-bremen.de.

This is publication GEOTECH-42 of the program GEOTECHNOLOGIEN of the Bundesministerium für Bildung und Forschung and the Deutsche Forschungsgemeinschaft. 

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Applied and Environmental Microbiology, February 2004, p. 1231-1233, Vol. 70, No. 2
0099-2240/04/$08.00+0 DOI: 10.1128/AEM.70.2.1231-1233.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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