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The role of benthic foraminifera in deep-sea food webs at the
sediment-water interface: Results from in situ feeding experiments
in Sagami Bay
Hiroshi Kitazato1, Hidetaka Nomaki2, Petra Heinz3 and Takeshi Nakatsuka4
Research Program for Paleoenvironment, Institute for Frontier Research on Earth Evolution (IFREE)
Ocean Research Institute, University of Tokyo
3
Institute of Geology and Paleontology, University of Tuebingen
4
Institute of Low Temperature Sciences, Hokkaido University
1
2
iments in laboratory and in the field, respectively. They found
that fresh organic material derived from the surface ocean was
consumed rapidly by benthic foraminifera. The foraminifera
were able to utilize sinking organic material within a day. It is
obvious that benthic organisms play an important role in the
early stages of consumption of sinking organic carbon. However, species-specific metabolic characteristics of benthic
foraminifera have not been determined until now.
In this study, we quantitatively examine how organic carbon
is consumed by benthic foraminifera at the sediment-water
interface based on in situ feeding experiments with 13C-labeled
organic material. The sediment was sampled both vertically
(i.e., different sediment layers) and temporally in order to
determine how much of the added organic carbon was ingested
by the foraminifera. Stable isotopes provide a useful tracer to
track labeled material on short to long time scales, and to
examine ingestion rates quantitatively.
Introduction
The deep-sea floor is a dark, cold, oligotrophic environment
characterized by high hydrostatic pressures. Despite these
extreme conditions, a wide variety of benthic organisms dwell
on the deep-sea floor. Since Billett et al. (1983) reported that
organic matter produced at the ocean surface directly reached
the deep-sea floor as fresh phytodetritus, a considerable
research effort has been devoted to elucidating how deep-sea
ecosystems are sustained by sinking organic matter. Benthic
organisms consume fresh phytodetritus as a primary food
source and mineralize the organic carbon to carbon dioxide by
respiration (Gage, 1990; Gooday and Lambshead, 1989;
Altenbach, 1992). Excess organic material that is not consumed
by benthic organisms is buried in the sediments. Thus, benthic
activity around the deep-sea sediment-water interface exerts an
important control on both the sedimentation and consumption
of organic carbon. However, quantitative data on how much
organic material is ingested by benthic organisms, and what
percentages of organic carbon is mineralized to carbon dioxide
during respiration and other forms of metabolic activity, are not
available. Thus, it is necessary to clarify the nature of benthic
activities at the sediment-water interface in order to evaluate
the carbon budget in the ocean, particularly on the sea floor.
Benthic foraminifera are among the most common organisms living on the deep-sea floor. They sometimes comprise
more than 50% of the benthic biomass (Snider et al., 1984;
Gooday et al., 1992). Benthic foraminifera may play an important role in the deep-sea carbon budget. However, the metabolic activities of these benthic foraminifera are not understood in
relation to the carbon budget. How much freshly supplied phytodetritus do benthic foraminifera ingest? What place do benthic foraminifera occupy in deep-sea food webs?
Most previous studies have qualitatively inferred benthic
foraminiferal activity based on changes in population size or
individual growth rate before and after the phytodetritus deposition by a year-round observation (Gooday, 1988; Kitazato
and Ohga, 1995; Ohga and Kitazato, 1997; Drazen et al.,
1998). However, there are few observations that address the
short-time benthic response to phytodetritus on the time scales
(shorter than a day) over which benthic organisms are thought
to respond to sinking organic carbon.
Experimental approaches provide the best way to understand
the short-term responses of benthic foraminifera to phytodetritus deposition, particularly in deep-sea settings. Linke et al.
(1995) and Levin et al. (1999) carried out in situ feeding exper-
Materials and methods
Locality
The in situ feeding experiment was carried out at a deep-sea
permanent station (St. OBB2, 1445m water depth) in Sagami
Bay, central Japan (Fig. 1). The physico-chemical environment
at Station OBB2, where Pacific Deep Water occupies areas
below 1000m depth, is stable throughout the year (Kitazato and
Ohga, 1995).
Incubation of 13C-labeled algae
The unicellular alga, Dunaliella tertiolecta, was used as a
food material. This species was incubated at 20°C with sterilized
seawater that contained f/2 medium and 99.9% 13C-enriched
NaHCO3 (Shoko Tsusyo). The final concentration of 13C in algae
was 5.25%. Algal cells were centrifuged and then frozen at
–20°C until in situ culture experiments were performed.
Experimental procedure
The feeding experiment was carried out from Nov. 23 to
Nov. 29, 2001 during R/V Natsushima cruise, NT01-11. Either
late autumn or early winter is the best season for in situ feeding
experiments in Sagami Bay because the amount of “natural”
sinking organic carbon is relatively low (Kitazato et al., 2000).
On Nov. 23rd, five culture devices were placed on the undis227
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δ13C values (around –20‰) from the sediment surface to a
depth of 3cm. δ13C values within the sediment changed to –5‰
in the top 0.5cm layer of the 2-hour core, although values
below 2cm were the same with those of the background core.
In the 2-day core, the δ13C value increased to 80‰ in the top
0.5cm. Labeled algae were mixed to 1.5-2.0cm sediment depth
2 days after the start of the experiment. The influence of added
algae extended to a depth of 5cm in the 4-day core where the 01cm and 3-5cm layers both yielded high δ13C values. The deeper peak is thought to be due to the burrowing activity of
megabenthos. The value of δ 13 C in the top 0.5cm layer
decreased in the 4-day core compared to the 2-days core. The
maximum value of δ13C was found at 0.5-1.0cm depth in the 6day core, while the δ13C value was close to that of the background core at a depth of 4-5cm. The δ13C peak became deeper
as the experiment progressed. This clearly indicates that fresh
phytodetritus is rapidly ploughed 5cm into the sediment within
a couple of days by benthic organisms.
turbed sea floor using a manipulator arm of the manned submersible “Shinkai 2000”. The locality for the experiment is 2
meters away from the permanent station, OBB2. The culture
devices consisted of acrylic push cores, 5cm in diameter, with
two syringes on top. Each syringe held 6ml of labeled algae.
After setting the culture devices on the seafloor, the trigger of
each syringe was released to inject the algae (12ml in total)
onto the sediment surface within the core tube. In each case,
the amount of introduced alga was 1.03gC/m2, which corresponded to 2-5 times the daily amount of sinking organic carbon during the spring season at the center of Sagami Bay
(Kitazato et al. 2000). Two hours after deployment on the
seafloor, one culture device was recovered with a manipulator.
Two devices were recovered after 2 days (Nov. 25th), and the
remaining two were recovered 6 days after the start of the
experiment (Nov. 29th). On Nov. 25th, one device was placed
on the seafloor in the same manner, and then recovered after 4
days (Nov. 29th). In this way, we obtained time-series samples
at 2 hours, 2 days, 4 days and 6 days after food had been added
to the core surface. Three push core samples (4.2cm in diameter) were taken separately to determine natural 13C concentrations, both in the foraminiferal cell and in the sediment.
Temporal changes of foraminiferal distribution
Fig. 3 shows temporal changes in the vertical distributions of
three selected foraminiferal species during the experiments.
Three species, Bulimina aculeata, Bolivina pacifica and
Globobulimina affinis, inhabit shallow infaunal, intermediate
infaunal and deep infaunal microhabitats respectively (Nomaki
et al., in pep.). This figure also shows the number of individuals
that contain green-colored cytoplasm, indicating the ingestion of
fresh algal cells. In the case of B. aculeata and G. affinis, the
vertical distributions changed during experimental runs. Distribution patterns for both species shifted from deep to shallow in
response to the availability of food on the sediment surface. In
the 6-day core, Globobulimina affinis occurred at a shallower
depth than in background core. The peak in abundance was
located at 1.5 to 2.0cm depth in the case of the 6-day core.
More than 80% of B. aculeata individuals had green-colored
cytoplasm after 2 days of feeding. They ingested added algae
not only in the 0cm of the core but also at 2cm depth. Bolivina
pacifica also ingested algae after 2 days. Six days after feeding,
78% of B. pacifica individuals in deeper layers had also ingested algae. Globobulimina affinis did not ingest algae in the 2day core. After six days, however, G. affinis had ingested alga
in every layer, even at a depth of 2-3cm. More than 70% of living individuals of this species took up the algae. Chilostmella
ovoidea, a deep infaunal species, did not show clear ingestion
of alga even after 6 days (Fig. 4).
Sample treatment
Following recovery, each culture device was kept at 4°C in
an incubator on board until the core sediments could be
processed. Each core was sliced into 0.5cm thick layers from 0
to 2cm depth and every 1cm between 2 and 5cm. Both 2-hour
cores and one of the 2-day cores were sliced into 1cm thick
layers between 2 and 3cm depth. From every sliced sediment
fraction, 0.5cm3 was analyzed in order to determine the 13C
concentration in the sediment. Sediment samples were frozen
at –20°C. The remaining parts of the slices were used for benthic organism analysis. These samples were sieved on a 63µm
mesh screen with artificial seawater, and then frozen at –20°C
prior to picking living individuals.
All living foraminifera, in which the test cavity was filled
with cytoplasm, were removed, together with some metazoans
(i.e., nematode, copepod and polychaetes), using a binocular
microscope. Living foraminifers were sorted at the species
level. If foraminiferal cytoplasm showed a green color due to
ingestion of added alga, we noted these specimens as green
individuals. Every individual was cleaned with artificial seawater to remove sediment particles attached to the test. All separated foraminifera was transferred to a petri dish and kept in a
freezer until isotope analysis was conducted.
Sediment samples were dried at 50°C in an oven to measure
dry weight. Total organic carbon concentrations were measured
with an elemental analyser (NA-1500, Fisons Instrument).
13
C/12C ratios were determined with an isotope ratio monitoring
mass spectrometer (Delta plus, Thermo Quest) at Institute of
Low Temperature Science, Hokkaido University, and shown as
δ-notation against PDB standard.
δ13C values in foraminiferal cytoplasm
Changes in δ13C values of foraminiferal cytoplasm clearly
reflect the ingestion of labeled algae during the experimental
period (Fig. 3). Bulimina aculeata took up the labeled algae
most actively. The δ13C value for B. aculeata cytoplasm in the
2-day core was about 1100‰ (0 to 1cm layer). This suggests
that organic carbon originating from labeled algae occupies
29% of the cytoplasm. Bolivina pacifica and G. affinis showed
slightly increased δ13C values in the 2-day core. δ13C values
increased more in the 4-day core in comparison to the 2-day
core for both B. pacifica and G. affinis. Values increased distinctly in the 6-day core for these two species. Concentrations
Results
δ13C in sediment: The distribution pattern of δ13C values in
the surface sediment changed after the algae were added (Fig.
2). The background core exhibited a constant vertical profile of
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of isotopes were found in individuals from 2-3cm depth in the
case of B. pacifica and G. affinis. Ingestion rates were similar,
even though B. pacifica took up more labeled material than G.
affinis in the 1-2cm layer. Chilostmella ovoidea was the only
species that did not show a significant increase of δ13C in the 2day core (Fig. 4). However, clear ingestion was seen in the 02cm layer of the 6-day core, even though δ13C values were 5 to
10 times smaller than other species.
Species-dependent response to sinking organic carbon
Responses of deep-sea benthic foraminifera to sinking organic materials varied between species, according to their microhabitat preferences. Two shallow-infaunal species, U. akitaensis
and B. aculeata, quickly ingested alga in comparison to intermediate and deep infaunal species. Kitazato and Ohga (1995)
undertook feeding experiments in the laboratory. These experiments indicate that deep infaunal species appeared to prefer
more degraded food. Rudnick (1989) conducted feeding experiments that addressed the metazoan meiobenthos and reported
that food assimilation times differed between epifaunal and
infaunal taxa. The difference in response times may reflect food
preferences; for example, for either fresh or altered phytodetritus, detritus or bacteria. Such food preferences may reflect the
vertical distributions of species within sediments. Different
types of responses toward fresh food are probably important
aspects of the trophic structure at the sediment-water interface.
Two deep infaunal species, Globobulimina affinis and C.
ovoidea, show very different responses to added algae. Globobulimina affinis ingested many algal cells after 6 days. In contrast, C. ovoidea rarely ingested fresh alga. This feeding behavior is consistent with observations made during laboratory
feeding experiments (Nomaki et al., in prep.). These results
suggest that these two species have different food preferences,
even though they show similar depth distribution patterns within the sediment. Higher respiration rates of G. affinis in comparison to C. ovoidea may be another aspect of this phenomenon (Nomaki, et al., in prep.). Further observations are needed
to clarify this problem.
Discussion
Rapid uptake of organic carbon by foraminifera
Benthic foraminifera responded to fresh organic materials
within 2 days. In particular, Bulimina aculeata responded
quickly and took up significant amounts of algae, despite its
small size. It has already been suggested that benthic
foraminifera quickly ingest organic matter deposited on the
seafloor (Gooday, 1988; Gooday and Turley, 1990; Linke,
1992; Drazen, 1998). Linke (1992) measured foraminiferal
activity in relation to the episodic supply of food material in
shipboard culture experiments. He reported that the metabolism
of deep-sea foraminifera was activated within a day of food
being added. Levin et al. (1999) conducted in situ culture experiments with 13C labeled algae on the NW Atlantic continental
slope. They reported that agglutinated foraminifera ingested
labeled food materials within 1.5 days after the start of the
experiment. Our experimental results coincide well with these
earlier studies, even though the species involved are different.
Benthic foraminifera may play a large role in the early decomposition of sinking organic carbon at the deep-sea floor since
they quickly ingest fresh phytodetritus into their cells. Quantitative calculations to estimate the rate of degradation of food pulses on the deep-sea floor are underway (Nomaki et al., in prep.).
Acknowledgements. The authors are indebted to both members of
Shinkai 2000 operation team and the crew of R/V Natsushima for their
skillful operation of the submersible. Drs. Christoph Hemleben and
Andrew J. Gooday provided encouragement and support at many stages
during this study. Mr. Yoshiji Imai of JAMSTEC gave useful advice for
mechanical design of feeding apparatus using with submersible Shinkai
2000. Mr. Ryuji Ikeya, a technical specialist of Faculty of Science,
Shizuoka University, skillfully constructed feeding devices. This
research is partly supported by the Grants-in-Aid from Ministry of Education, Science and Culture of Japan (no. 11440154) to H.K.
Rapid mixing of sinking organic carbon into sediment
Benthic foraminifera living 2-3cm deep in the sediment also
ingested added algae within 2 to 6 days. For very slow moving
foraminifera (Kitazato, 1988), this is a considerable distance
from the sediment surface. Our experimental results support the
hypothesis that metazoan megabenthos may transport sinking
organic carbon from the sediment surface deep into the sediment by their activities and/or by particles falling passively into
burrows constructed by metazoan species. Levin et al. (1997,
1999) noted briefly that ingestion of labeled organic carbon was
seen in some metazoan species that dwelled 5-10cm deep. It
indicates that sinking organic carbon deposited on sediment surface can be rapidly transported into deeper layers (at least
10cm) by megabenthos. Our results also showed that labelled
phytodetritus was mixed into the sediment within a couple of
days (Fig. 2), suggesting that deposited organic carbon is rapidly supplied to deep dwelling organisms. Both results indicate
that fresh organic matter is consumed not only by surface
dwelling organisms but also by organisms that dwell deep in the
sediment. The potential consumption of sinking organic carbon
by the infaunal community should be considered when calculating the total carbon budget of the deep-sea floor.
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Figure 2. Down core profiles of δ13C value of sedimentary organic carbon. Except for the 2-hour core, 12ml food materials were supplied to
each core tube. Half this amount of labelled food was supplied in the
case of the 2-hour core.
Figure 1. Map of study area. Closed circle at the central part of Sagami
Bay indicates the location of the in-situ experiment.
Figure 3. Changes in the distribution patterns of living individuals and δ13C values in foraminiferal cells are shown from upper to lower in accordance
with the progress of the experiment. Three species, Bulimina aculeata, Bolivina pacifica and Globobulimina affinis, represent shallow infauna, intermediate infauna and deep infauna species respectively.
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Figure 4. Changes in the distribution patterns of numbers of living individuals and δ13C values in foraminiferal test for the deep infaunal species,
Globobulimina affinis and Chilostomella ovoidea. Feeding behavior is different in these two species.
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