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Appl Environ Microbiol, July 1998, p. 2691-2696, Vol. 64, No. 7
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Microbial Community Composition of Wadden Sea
Sediments as Revealed by Fluorescence In Situ Hybridization
Enric
Llobet-Brossa,
Ramon
Rosselló-Mora,* and
Rudolf
Amann
Max Planck Institut für Marine
Mikrobiologie, 28359 Bremen, Germany
Received 25 February 1998/Accepted 14 April 1998
 |
ABSTRACT |
The microbial community composition of Wadden Sea sediments of the
German North Sea coast was investigated by in situ hybridization with
group-specific fluorescently labeled, rRNA-targeted oligonucleotides. A
large fraction (up to 73%) of the DAPI
(4',6-diamidino-2-phenylindole)-stained cells hybridized with the
bacterial probes. Nearly 45% of the total cells could be further
identified as belonging to known phyla. Members of the
Cytophaga-Flavobacterium cluster were most abundant in all
layers, followed by the sulfate-reducing bacteria.
 |
TEXT |
Marine sediments cover 70% of the
total earth; consequently, they play an important role in the global
cycling of carbon and nutrients (36). Early diagenetic
processes are catalyzed mainly by the microorganisms that colonize the
marine sediments (41). However, despite their environmental
importance, the bacterial community structures of marine sediments
remain poorly studied (23). Several attempts to describe
marine sediment microbial communities have already been made. Most of
these have been based on cultivation (see, e.g., references 6,
20, and 30) and were therefore subject to
restrictions and biases leading to a distorted representation of the
true community composition (2). Molecular techniques have
greatly increased our knowledge of marine microbial diversity. For
example, 16S rDNA libraries of marine plankton (see, e.g., references
7, 8, 15, and 33) and sediment
(see, e.g., references 16 and 25)
suggested the presence of hitherto-uncultured organisms. Techniques
such as reassociation analysis of DNA (45), denaturing
gradient gel electrophoresis (44), and restriction fragment
length polymorphism (25) have yielded insight into bacterial
diversity and community composition. However, phylogenetically based
oligonucleotide hybridization techniques permit not only the monitoring
of individual phylogenetic groups but also a quantification of their
abundance in the natural habitats (2). Marine sediment
microbial diversity has been studied by using the quantitative slot
blot hybridization technique (9, 24, 35). However, these
results cannot be directly translated into cell numbers because of the
differences in absolute rRNA content per cell among the different
members of the community (2). In situ hybridization with
rRNA-targeted fluorescent oligonucleotide probes, in contrast, permits
the identification and quantification of individual cells
(2) and has demonstrated great power in the analysis of
bacterial community composition in several environments (14, 32,
40, 42, 51). To date this method has not been tested with marine
sediments.
In the present work we describe, to our knowledge for the first time,
the community composition and vertical distribution of a marine
sediment determined by using the in situ hybridization technique. The
sampling area is located within the Jadebusen Bay, which is a part of
the German Wadden Sea that forms the southern boundary of the North
Sea, extending from The Netherlands to Denmark. The area of study is
under the influence of the fluvial input of the river Weser, although
it is not exposed to the extreme seasonal changes of salinity observed
in the Weser estuary (37). The sediment is silty and
experiences tides which expose it to air for about 5 h and leave
it inundated for about 7 h, with some variability due to the wind
velocity and direction (37).
Two cores were obtained from the near shore intertidal mud and sand
flats of Dangast on 9 November 1997. One core was completely composed
of mud (mud core). The second core originated from an artificial beach
which contained a superficial layer (1 to 2 cm) of a thick-grained sand
(beach core). Samples were transported at 4°C and processed
immediately upon return to the laboratory, within 1 h of sampling.
Sediment cores were sliced in 0.5-cm sections and fixed directly in
ethanol (96%) or in 4% formaldehyde-phosphate-buffered saline (PBS)
(composed of 0.13 M NaCl, 7 mM Na2HPO4, and 3 mM NaH2PO4 [pH 7.2 in water]) for 2 to 4 h on ice. The formaldehyde-fixed samples were then washed in PBS and
stored in ethanol-PBS (1:1) at
20°C. Samples were diluted and
treated by mild sonication with an MS73 probe at a setting of 20 for
30 s (Sonopuls HD70; Bandelin, Berlin, Germany). Samples were then
mixed with 0.05% agarose, and 10 µl was dropped onto glass slides
and dried at room temperature. Glass slides were immersed in 50, 80, and 96% ethanol for 3 min each. The inclusion in agarose did not
result in artifacts such as autofluorescence and did not impede the
access of the probes to the sample, in contrast to what was observed in
similar studies using antibodies (3).
Oligonucleotide probes were synthesized with Cy3 fluorochrome at the 5'
end (Interactiva Biotechnologie GmbH, Ulm, Germany). Hybridizations and
microscopy counts of hybridized and DAPI
(4',6-diamidino-2-phenylindole)-stained cells were performed as
previously described (40). The probes and formamide
concentrations used are given in Table 1.
The slides were examined with an Axiophot II microscope (Zeiss, Jena,
Germany). For each probe and sample, between 700 and 1,000 DAPI-stained cells and the respective hybridized cells in 10 to 20 independent fields were counted. Standard deviations of counts ranged between 2 and
8%. They are relatively high for those probes that gave low cell
counts, and this is mainly due to the heterogeneity of the sample.
Counting results were always corrected by subtracting signals observed
with the probe NON338 (40).
Regarding the specificity of DAPI staining and fluorescent probes, a
few cells (<1%) showed very weak staining with DAPI after hybridization with EUB338-Cy3. However, by taking micrographs of those
cells, it became clear that they were stained with DAPI, and we
regarded this phenomenon to be a result of effective absorption of DAPI
emission by Cy3. Furthermore, we observed an effect similar to that
reported by Zarda et al. (51). After 3 months of storage of
fixed sediments in PBS-ethanol at
20°C, an increased detection of
members of the
subclass of Proteobacteria with probe
ALF1b was observed. Values increased from about 1% to a maximum of
4.1% of the DAPI counts. For the identification of members of the
subclass of Proteobacteria, we used the probe SRB385, which
is targeted to most of the known sulfate-reducing bacteria of this subclass (1). We are aware that this probe does not target all members of the
subclass and that it is complementary to some
organisms which are not affiliated with the
subclass, such as
numerous gram-positive bacteria (32, 50). Fluorescence in
situ hybridization (FISH) was done on formaldehyde-fixed cells, which
renders most gram-positive bacteria unreactive with fluorescent oligonucleotide probes, and consequently it is likely that most of the
organisms detected by the SRB385 probe belong to the
subclass.
Total cell counts and domain-specific probing.
Total cell
counts determined by DAPI staining in the surface sediments were in
accordance with what has been reported previously for similar
environments (21, 38, 48). As shown in Table 1 and Fig.
1, DAPI-stained-cell counts were
relatively constant in the top 3 cm of the mud core, i.e., 4.3 × 109 to 4.5 × 109 cells/cm3,
and decreased with depth to 1.8 × 109
cells/cm3. In contrast, we detected many fewer
microorganisms (6 × 108 to 8 × 108
cells/cm3) in the top 1 cm of the beach core, which
corresponded to the sand layer. This observation is in accordance with
a lower microbial load in sand flats than in mud flats (19,
23). The total cell counts increased below the sand-mud boundary
at the 1.5-cm depth to 2.5 × 109
cells/cm3, although they remained lower than in the mud
core.

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FIG. 1.
Vertical profiles of the mud core (left) and beach core
(right). In both cases the absolute numbers of bacteria detected are
given. The drafted diagram of the cores is given on the left of each
DAPI-EUB profile, indicating the transition zone to the iron-sulfide
precipitation layer (black).
|
|
FISH resulted in the detection of a large fraction of the microbial
community living in the top 5 cm of the Wadden Sea sediment.
Up to 73%
of the DAPI-stained cells hybridized with our set of
probes (Table
1).
Our detection yields are comparable to those
obtained for activated
sludge (
40) and freshwater (
31) but
are higher
than those obtained for soil (
51) or seawater
(
32).
In addition, hybridized cells were visualized with
strong fluorescent
signals (Fig.
2),
which directly demonstrates a high cellular
rRNA content
(
2). The Wadden Sea sediments of the German North
Sea coast
are highly influenced by the discharges of eutrophic
freshwater from
the Ems, Weser, and Elbe rivers. They are among
the most active areas
for decomposition of organic material in
the German Bight
(
17). These environmental conditions would
explain the high
abundance and activity of microorganisms observed.

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FIG. 2.
Epifluorescence micrographs of bacteria in sediment
samples from the Jadebusen Bay of the German Wadden Sea. (a)
Hybridization with probe EUB338, specific for Bacteria. (b)
Same microscopic field as in panel a with UV excitation (DAPI
staining). (c and d) Identical microscopic fields with probe SRB385 (c)
and DAPI staining (d). (e) Hybridization with probe DNMA657, specific
for Desulfonema. (f) Specific hybridization for
Arcobacter with probe ARC94. Bar, 10 µm (applies to all
panels).
|
|
The microbial communities analyzed were dominated by
Bacteria (probe EUB338), whereas
Archaea
(probe ARCH915) and
Eucarya (probe EUK516) were
detected only in numbers that remained below
the detection limit, set
at 1% of the DAPI-stained cells. Although
similar low counts of
members of the domain
Archaea were observed
in bulk soil
(
51), results of other molecular studies on marine
and
nonmarine environments indicated higher abundance (
8,
11,
29). With the still relatively large amount of cells that are
not
detected by FISH, we currently cannot exclude the possibility
that
Archaea make up a large part of those DAPI-stained cells
which did not hybridize with ARCH915.
The fraction of detectable bacteria in the mud core strongly decreased
over the vertical profile (Table
1; Fig.
1), from
73% in the uppermost
layer to 28.6% at the 5-cm depth. Similar
results were obtained for
the beach core. However, we found lower
detection rates in the sand
layer of the core (40 to 45%) than
in the mud layers. These yield
differences may be related to the
organic-matter content of the
sediment (
19,
23). Below the
sand-mud interface, detection
yields were similar to those for
the mud core (Table
1; Fig.
1). Thus,
not only the absolute cell
number but also the fraction of cells
detectable by FISH decreased
thorough the profiles. Our results are in
accordance with the
correlation between total cell number and total
bacterial production
and activity, with the uppermost layers of mud
flats showing the
highest values (
5,
48).
Abundances of major bacterial groups.
With a set of eight
probes for major phyla within the domain Bacteria, we could
affiliate between 17 and 44% of the total DAPI cell counts with known
bacterial groups (Table 1; Fig. 1). This means that the majority of the
detectable bacteria could be affiliated to a known group, and only
between 8.5 to 35.8% of the EUB338 counts remained unaffiliated.
The most abundant phylogenetic group in Wadden Sea sediments was the
Cytophaga-Flavobacterium cluster. This is remarkable,
since
high numbers of
Cytophaga-Flavobacterium had so far not
been
found in marine sediments by either molecular methods (
16,
36) or culture-based analysis (
6,
10). Most of the
cells
identified within this cluster showed a homogeneous morphology
of
thin long rods. Their relative abundance ranged from 5 to 6.2%
of the
DAPI counts in the deepest layers to 18.1% in the uppermost
layer of
the mud core. This result means that between 15 and 25%
of the total
detectable bacteria could be affiliated to this group.
Significant
numbers of
Cytophaga-Flavobacterium members in marine
environments have so far been found only in the water column associated
with macroscopic marine aggregates (
7) or with alga blooms
in sea ice (
4). The members of the
Cytophaga-Flavobacterium cluster are mainly aerobic,
gram-negative bacteria which are specialized
for the degradation of
complex macromolecules (
18,
34). Since
the bacterial use of
electron acceptors in sediments is stratified
according to decreasing
redox potentials (
41) and since the
oxygen depletion in the
Wadden sediments occurs within the first
5 mm (
37), we can
only speculate on the energy metabolism of
the
Cytophaga-Flavobacterium cells found below the oxic zone.
However, considering the brightness of the hybridization
(
2),
the cells detected by probe CF319a seem to be intact
and metabolically
highly active.
The sulfate-reducing members of the

subclass of
Proteobacteria detected with the probe SRB385 (SRBs) made up
the second-largest
group, with a maximum of 6.5% of DAPI counts. We
observed positive
signals through the whole vertical profile, with a
maximum at
the 2-cm depth. The relative abundance of SRB counts through
the
sediment profile, together with their relatively high amounts
in
the upper layers of the sediment where sulfate reduction should
not be
the predominant process (
37), was in accordance with
results
obtained for comparable environments (
9,
20,
39).
The
morphology of SRBs was quite variable (Fig.
2). Among them
were large
filamentous bacteria whose affiliation with the genus
Desulfonema (
49) was confirmed with the probe
DNMA657 (Fig.
2). A maximum of 2.7 × 10
7 cells
cm
3 was found at a depth of 2 to 2.5 cm in the mud core
(Table
1).
Although
Desulfonema organisms made up only 9%
of all SRBs, these
bacteria contribute significantly to the total
bacterial biomass
in the Wadden sediments due to their large size (Fig.
2).
In contrast to the high abundance of members of the
Cytophaga-Flavobacterium cluster, we found relative low
numbers of
Proteobacteria (

,

, and

subclasses),
Planctomycetes, and gram-positive bacteria
with a high G+C
content (each accounted for 1 to 5.7%). The relatively
low numbers of
members of

-subclass
Proteobacteria in sediments
was
unexpected, since they have been described as a predominant
group in
marine plankton (
13,
15,
26).
One of our most surprising results was the presence of members of the
genus
Arcobacter at >10
7 cells
cm
3 (1.3% of DAPI counts) (Table
1) in the upper layers
of the sediments.
All cells detected with the probe ARC94 showed the
small, bow-shaped
rod morphology (Fig.
2) characteristic of the members
of this
genus in the

subclass of
Proteobacteria
(
46). There was a
clear stratification, with higher counts
in the upper 3 cm (Fig.
1). Almost no
Arcobacter organisms
were detected at below 3.5
cm. Although members of this genus have
recently been detected
in different natural ecosystems (
40,
43,
46,
47), they
have not previously been reported to be significant
in marine
sediments. Arcobacters represent the most aerotolerant of the
former campylobacters (
46). An ability for denitrification
has
been reported (
43). It is therefore not surprising that
Arcobacter spp. could be found only in the upper layers.
Even though the
relative abundance is low, the total cell counts for a
single
genus (exceeding 10
7 cm
3) are high,
and the number is nearly identical to that for
Arcobacter organisms observed in activated sludge (
40).
Overall, these results indicate that the community structure in the
sediment differs significantly from that in the overlying
water column.
However, there should be a direct interaction between
the water phase
and the sediment, which has implications for the
community development.
In this respect, we think that one of the
most interesting findings in
our work is the unexpected high abundance
of members of the
Cytophaga-Flavobacterium cluster. This group
has also been
found to be a major constituent of the macroaggregate-attached
bacterial communities in marine environments (
7). Indirect
observations (
28) indicate that the sedimentation of
microbial
aggregates plays an essential role in the formation of the
microbial
communities in marine sediments, not only by the input of
organic
material but also by the input of established microbial
communities.
Our results support this hypothesis, but there is a
question as
to whether a high abundance of
Cytophaga and
Flavobacterium is
common in marine sediments.
Future investigations on marine sediments will combine a larger set of
specific probes with the analysis of biogeochemical
processes to more
fully understand the structure and function
of marine sediments.
 |
ACKNOWLEDGMENTS |
This study was supported by funds of the Max Planck Society.
Bo Barker Jørgensen and Jakob Pernthaler are acknowledged for
critically reading the manuscript and for helpful comments on earlier
versions.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Molecular
Ecology Group, Max Planck Institut für marine Mikrobiologie,
Celsiusstrasse 1, 28359 Bremen, Germany. Phone: 49-421-2028-940. Fax:
49-421-2028-580. E-mail: rrossell{at}mpi-bremen.de.
 |
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Copyright © 1998, American Society for Microbiology. All rights reserved.
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