Geology Reference
In-Depth Information
found in natural gel-like mucilaginous structures
(Chafetz & Buzcynsky, 1992), such as micro-
bial mats or biofi lms. Biofi lms comprise bacteria
immersed in a matrix of extracellular polymeric
substances (EPS) (Decho, 2000; Riding, 2000).
The bacteria studied were inoculated in triplic-
ate onto the surface of semi-solid media. Control
experiments, consisting of non-inoculated culture
media or media inoculated with autoclaved
bacterial cells, were run in parallel.
in the laboratory: samples were fi xed with 2.5%
glutaraldehyde in 0.2 M Na-cacodylate buffer for
90 minutes at 4°C. Subsequently, the samples were
washed twice with distilled water, 30% ethanol,
dehydrated in acetone, and fi nally critical-point
dried in liquid CO
2
. Samples were analysed by a
fi eld emission SEM (Leo 1530, 143 eV resolution,
LEO Electron Microscopy LTD, Germany).
Stable carbon and oxygen isotopes
The carbon and oxygen stable isotopic composi-
tions of powdered bulk carbonate sediment sam-
ples were determined. Samples were dissolved
using an on-line common acid bath attached to
a VG PRISM mass spectrometer. The reaction
time was set at 10 minutes. The isotope data are
reported in the standard
Recovering of minerals from plates
The Petri dishes were sealed with parafi lm to
avoid water evaporation and incubated aerobic-
ally at 30°C for 30 days. In order to detect the
presence of precipitates during incubation, Petri
dishes were observed once a day with light
microscopy (20
notation relative to
the international standard V-PDB (Craig, 1953,
1957). The analytical precision of the mass spec-
trometer is
). After the incubation period was
completed, precipitates were recovered by scrap-
ing the colony from the agar surface. Subsequently,
they were washed several times with distilled
water to eliminate the nutritive solution, remain-
ing agar and cellular debris, and dried at 37°C.
Microscopic observation demonstrated that this
treatment does not alter the morphology of the
crystals. Measurements of pH were performed
at the end of the growth and carbonate forma-
tion experiments. The pH-indicator paper (Merck
Spezial-Indikatorpapier) was applied directly to
the semi-solid surface.
18
O.
The isotopic composition of the water was deter-
mined by equilibration with CO
2
through the use
of an automated ISOPREP 18 equilibration device
coupled to a FISONS-OPTIMA mass spectro-
meter. Analytical reproducibility of
13
C, and
0.1‰ for
0.2‰ for
18
O water is
0.06‰. The oxygen isotopic results are reported in
the conventional per mille notation with respect
to Vienna standard mean ocean water (V-SMOW).
All
18
O values have been corrected for dolomite -
phosphoric acid fractionation at 90°C using the
fractionation factor of 1.0093 (Rosenbaum &
Sheppard, 1986). Conversions between
18
O V-PDB
X-ray diffraction analysis
18
O V-SMOW were calculated by using the
equation in Coplen
et al
. (1983). The sediment and
pore-water isotope data are listed in Table 1.
and
The purifi ed minerals formed in the laboratory
culture experiments by the two different bacteria
strains and the carbonate sediment sampled at 2.5
and 21 cm depths in the Brejo do Espinho lagoon
were powdered and analysed for their mineral-
ogy on a Scintag XDS 2000 X-ray diffractometer
(XRD). The samples were scanned by continuous
scan at 1° min
1
from 5 to 70° with Cu-K
Total organic carbon and sediment pore-water pH
Sediment samples were analysed using UIC, Inc.
coulometer system. The amount of total organic
carbon (TOC) in the samples was obtained as the
difference between the measured inorganic carbon
and the total carbon. The percentage TOC data are
given in Table 1. Standard pH-microelectrodes
(Methrom) were used in the fi eld to measure the
pH of the sediment pore water.
radia-
tion. From the d104 of the diffraction spectra, the
Mg/Ca ratio of the carbonate minerals was calcu-
lated after Lumsden (1979).
Scanning electron microscope analysis
The minerals formed in the laboratory experiments
and the Brejo do Espinho sediment were prepared
for scanning electron microscopic (SEM) obser-
vation. In order to preserve the biological struc-
ture as best as possible, the following treatment
was applied to the uppermost sediment sample
RESULTS
Description of the sediment and mineralogy
The 25-cm sediment core, taken from the cen-
tre of Brejo do Espinho lagoon, contains fi ne
