sediments have usually followed the track of vertical gradients that are produced by redox
potentials and pO
2
(Novitsky and Kepkay, 1981). The resulting vertical patterns of
distinct microhabitats, however, may often be superimposed or even disturbed by burrowing activities of the macrofauna, viz. bioturbation. Beyond that, it has been claimed that
bacterial growth rates and metabolic activities are considerably enhanced as a result of
infaunal reworking of the sediment (Yingst and Rhoads 1979). Yet, an apparent lack of
quantitative data makes it impossible to get an idea of the magnitude and significance of
this impact.
In largely anoxic sediments, formation of «internal» oxidized surface areas by the activity
of burrowing macrofauna should provide secondary niches for many aerobic or microaerophilic bacteria in an otherwise not inhabitable environment. This may apply particularly to chemoautotrophic bacteria that depend on a constant supply of oxidizable
inorganic compounds as supposed by Yingst and Rhoads (1979).
A first approach to understand the impact of burrowing on bacteria with key roles in the
carbon cycle focussed primarily at the de novo biosynthesis of organic carbon (CO
2
dark
fixation) and secondly at certain aspects of the decomposition of particulate organic
matter. Largely anoxic sediment from a shallow lagoon that was intensely bioturbated by
the polychaete worm Nereis diversicolor, was chosen for a comparative bacteriological
study of different areas in 10 cm cores.
MATERIALS AND METHODS
Sediments from Kiel Bay (Boknis Eck) and Limfjorden were sampled by divers in 60 mm
wide Plexiglass tubes. Cores of sandy sediment from Stein lagoon (grain sizes : 50.7 %>
500 um, 37.1 %> 250 urn) were taken in 7 x 16.5 x 30 cm metal boxes. Redox potentials
(measured with an Ingold Pt/ Ag-AgCl electrode) as well as coloration differences served
to localize selected sampling areas within the box core samples. In addition, pO
2
gradients
in the burrow walls were measured using an oxygen microelectrode (Revsbech et ai,
1980). Most of the investigations described were carried out with lagoon sediment
obtained during a sampling series on 7 days in August 1984.
Assays were carried out with subsamples separated from selected areas in the cores.
Subsamples were obtained by breaking the cores into pieces to expose several longitudinal halfs of the burrows. Metal spatulas with U-shaped profiles fitting the open burrow
halfs were used to peel off burrow wall layers of roughly one mm thickness from sediment
layers showing the same coloration. In certain cases, successive subsamples of approximately 0.5 mm thickness were collected, the uppermost subsample consisting merely of
mucous, particle-incrusted pellicles. Subsamples were transferred immediately to sawedoff 1 cm
3 syringes and sealed with Parafilm r to avoid gross changes of E
h
. Cell-free
extracts for enzyme assays were prepared at the sampling site. All assays were initiated
within 1-2 h after sampling.
Enzyme extraction for assays of RuBPC ase were carried out with 1 cm3 of sediment per
5 ml of ice-cold extracting buffer containing 12 ‰ of artificial seawater at pH 7.8, 2 %
triton X 100, and 2 g.l -1 polyvinylpyrrolidone as stabilizing agent. Assays for RuBPC ase
followed essentially the technique of Glover and Morris (1979). In vivo fixation rates (v)
of inorganic carbon (CO
2
) were determined by injecting 100 µl (1µCi) per cm3 sediment of
a stabilized isotonic solution of 1.7 mM Na H14 CO
3
into either entire 5 cm3 - subcores or
0.1 -0.5 cm
3 segments of these, (Reichardt, in prep.), rates were calculated using the
following formula:
416
potentials and pO
2
(Novitsky and Kepkay, 1981). The resulting vertical patterns of
distinct microhabitats, however, may often be superimposed or even disturbed by burrowing activities of the macrofauna, viz. bioturbation. Beyond that, it has been claimed that
bacterial growth rates and metabolic activities are considerably enhanced as a result of
infaunal reworking of the sediment (Yingst and Rhoads 1979). Yet, an apparent lack of
quantitative data makes it impossible to get an idea of the magnitude and significance of
this impact.
In largely anoxic sediments, formation of «internal» oxidized surface areas by the activity
of burrowing macrofauna should provide secondary niches for many aerobic or microaerophilic bacteria in an otherwise not inhabitable environment. This may apply particularly to chemoautotrophic bacteria that depend on a constant supply of oxidizable
inorganic compounds as supposed by Yingst and Rhoads (1979).
A first approach to understand the impact of burrowing on bacteria with key roles in the
carbon cycle focussed primarily at the de novo biosynthesis of organic carbon (CO
2
dark
fixation) and secondly at certain aspects of the decomposition of particulate organic
matter. Largely anoxic sediment from a shallow lagoon that was intensely bioturbated by
the polychaete worm Nereis diversicolor, was chosen for a comparative bacteriological
study of different areas in 10 cm cores.
MATERIALS AND METHODS
Sediments from Kiel Bay (Boknis Eck) and Limfjorden were sampled by divers in 60 mm
wide Plexiglass tubes. Cores of sandy sediment from Stein lagoon (grain sizes : 50.7 %>
500 um, 37.1 %> 250 urn) were taken in 7 x 16.5 x 30 cm metal boxes. Redox potentials
(measured with an Ingold Pt/ Ag-AgCl electrode) as well as coloration differences served
to localize selected sampling areas within the box core samples. In addition, pO
2
gradients
in the burrow walls were measured using an oxygen microelectrode (Revsbech et ai,
1980). Most of the investigations described were carried out with lagoon sediment
obtained during a sampling series on 7 days in August 1984.
Assays were carried out with subsamples separated from selected areas in the cores.
Subsamples were obtained by breaking the cores into pieces to expose several longitudinal halfs of the burrows. Metal spatulas with U-shaped profiles fitting the open burrow
halfs were used to peel off burrow wall layers of roughly one mm thickness from sediment
layers showing the same coloration. In certain cases, successive subsamples of approximately 0.5 mm thickness were collected, the uppermost subsample consisting merely of
mucous, particle-incrusted pellicles. Subsamples were transferred immediately to sawedoff 1 cm
3 syringes and sealed with Parafilm r to avoid gross changes of E
h
. Cell-free
extracts for enzyme assays were prepared at the sampling site. All assays were initiated
within 1-2 h after sampling.
Enzyme extraction for assays of RuBPC ase were carried out with 1 cm3 of sediment per
5 ml of ice-cold extracting buffer containing 12 ‰ of artificial seawater at pH 7.8, 2 %
triton X 100, and 2 g.l -1 polyvinylpyrrolidone as stabilizing agent. Assays for RuBPC ase
followed essentially the technique of Glover and Morris (1979). In vivo fixation rates (v)
of inorganic carbon (CO
2
) were determined by injecting 100 µl (1µCi) per cm3 sediment of
a stabilized isotonic solution of 1.7 mM Na H14 CO
3
into either entire 5 cm3 - subcores or
0.1 -0.5 cm
3 segments of these, (Reichardt, in prep.), rates were calculated using the
following formula:
416