Journal of Experimental Marine Biology and Ecology 396 (2011) 108–114

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Journal of Experimental Marine Biology and Ecology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j e m b e

Oxygen respiration rates of benthic foraminifera as measured with oxygen microsensors E. Geslin a,⁎, N. Risgaard-Petersen b, F. Lombard c, E. Metzger a, D. Langlet a, F. Jorissen a a b c

Laboratory of Recent and Fossil Bio-Indicators (BIAF), University of Angers, 2 Bd Lavoisier, F-49 045 Angers, and LEBIM, Marine Station of Yeu Island, France Center for Geomicrobiology, Institute of Biological Sciences, Aarhus University Ny Munkegade 114-116 Bldg 1540, Denmark DTU Aqua, Technical University of Denmark, Kavalergården 6, 2920 Charlottenlund, Denmark

a r t i c l e

i n f o

Article history: Received 13 April 2010 Received in revised form 30 September 2010 Accepted 11 October 2010 Keywords: Aerobic carbon mineralisation Benthic foraminifera Oxygen respiration rates

a b s t r a c t Oxygen respiration rates of benthic foraminifera are still badly known, mainly because they are difficult to measure. Oxygen respiration rates of seventeen species of benthic foraminifera were measured using microelectrodes and calculated on the basis of the oxygen fluxes measured in the vicinity of the foraminiferal specimens. The results show a wide range of oxygen respiration rates for the different species (from 0.09 to 5.27 nl cell−1 h−1) and a clear correlation with foraminiferal biovolume showed by the power law relationship: R = 3.98 10−3 BioVol0.88 where the oxygen respiration rate (R) is expressed in nl O2 h−1 and in μm3 biovolume (BioVol) (n = 44, R2 = 0.72, F = 114, p b 0.0001). The results expressed per biovolume unit (1.82 to 15.7 nl O2 10−8 μm−3 h−1) allow us to compare our data with the previous published data showing similar ranges. A comparison with available data for other microbenthos groups (nematodes, copepods, ostracods, ciliates and flagellates) suggests that benthic foraminifera have a lower oxygen respiration rates per unit biovolume. The total contribution of benthic foraminifera to the aerobic mineralisation of organic matter is estimated for the studied areas. The results suggest that benthic foraminifera play only a minor role (0.5 to 2.5%) in continental shelf environments, which strongly contrasts with their strong contribution to anaerobic organic matter mineralisation, by denitrification, in the same areas. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Benthic foraminifera inhabit a wide range of marine environments including brackish, open marine and deep sea environments (Murray, 2006). They are a dominant component of the benthic meiofauna in terms of density and biomass (e.g. Pascal et al., 2008; Gooday, 1986). It has been assumed that they are important for sea floor mineralisation processes, because of their high ingestion rates of organic carbon (Moodley et al., 2002). However, data on foraminiferal oxygen respiration and C mineralisation rates are still very scarce in comparison to other benthic groups such as nematodes, copepods, ostracods and polychaetes (e.g. Marshall, 1973; Lasserre and Renaud-Mornant, 1973; Lasserre, 1976; Warwick and Price, 1979; Price and Warwick, 1980; Herman and Heip, 1982, 1983; Moens et al., 1996, 1999; Moodley et al., 2008). Only few studies on the metabolism of foraminifera exist (e.g. Bradshaw, 1961; Lee and Muller, 1973; Schwab and Hofer, 1979; Hannah et al., 1994; Nomaki et al., 2007; Moodley et al., 2008; Høgslund et al., 2008; Piña-Ochoa et al., 2010) and these studies consider only a limited number of species (see ⁎ Corresponding author. Tel.: +33 2 41 73 54 07; fax: +3 2 41 73 53 52. E-mail addresses: [email protected] (E. Geslin), [email protected] (N. Risgaard-Petersen), fl[email protected] (F. Lombard). 0022-0981/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jembe.2010.10.011

Table 1). It is therefore still very difficult to evaluate the quantitative importance of foraminiferal respiration in marine benthic environments. In fact, this is only possible when very rough generalizations are made, such as an overall, species-independent, relationship between body size and oxygen respiration. Numerous studies on oxygen respiration rates of various benthic micro/meiofauna groups have shown a clear log–log relationship (equivalent to a power law relationship) between oxygen respiration rates and the weight, volume or biomass of the organisms (e.g. Coull and Vernberg, 1970; Teare and Price, 1979; Warwick and Price, 1979; Herman and Heip, 1982,1983; Fenchel and Finlay, 1983; Shirayama, 1992; Moens et al., 1999). However, among the studies of foraminiferal oxygen respiration only a single one (i.e. Bradshaw, 1961) has reported a significant positive correlation between respiration rate and body size for the species Ammonia tepida. The studies of Lee and Muller (1973), Hannah et al. (1994) and Nomaki et al. (2007), who also dispose of larger data sets, do not show such a relationship between oxygen respiration rate and body size, biovolume or biomass. It is not clear whether this lack of correlation is real, and thus represents a specific trait of foraminifera, or reflects limitations of the database, and/or the influence of other factors, such as the physiological state of the organisms, which may also influence oxygen respiration rates. In order to construct a larger data set of foraminiferal oxygen respiration needed for biological models we determined respiration

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Table 1 Respiration rates of benthic foraminifera published in the literature. Data are expressed in nl O2 cell−1 h−1 (as reported in the original papers) and normalized to 13 °C in order to compare with our data. Authors

Studied species

T °C

Diameter (μm)

Method

Respiration rate nl O2 cell− 1 h− 1

Respiration rate (normalized at 13 °C) nl O2 cell− 1 h− 1

Bradshaw (1961)

26 10 to 45 26 26 15–35 15–35 15–35 20 10 10 10 10 10 10 10 5 5 5 5 5 5 5 5 5 5 12

135 to 321 ~ 245 370 162 520 307 413 49 232 185 151 158 131 108 nd 415 639 514 548 451 587 1177 786 537 n.d. 370

Micro-respirometer

Høgslund et al. (2008)

Ammonia tepida (n = 12) Ammonia tepida (n = 10) Rosalina columbiensis Spirilina vivipara Allogromia laticollaris Rosalina leei Spiroloculina hyalina Allogromia laticollaris Ammonia Reophax sp.1 Reophax sp.2 Quinqueloculina Elphidium Buliminella Allogromia Cibicidoides wuellerstorfi Uvigerina akitaensis Bolivina spissa Bolivina pacifica Stainforthia apertura Textularia kattegatensis Globobulimina affinis Chilostomella ovoidea Bulimina subornata Ammonia beccarii Nonionella stella

0.4 to 3.1 0.1 to 2.1 3.7 0.4 2.5 to 114 2.3 to 21.8 6.36 to 59.3 8.3 7.7 10.8 13.1 14.3 9.7 12.0 13.3 9.5 5.5 3.0 0.9 2.7 5.1 6.1 1.2 1.9 9.9 0.8

0.05 to 0.78 0.01 to 0.26 0.9 0.09 11.8 to 30.2 3.3 to 5.98 8.95 to 5.96 3.84 10.5 14.9 18.0 19.7 13.3 16.5 18.3 24.0 13.8 7.5 2.2 6.8 12.86 15.5 3.15 4.85 25.1 0.84

Moodley et al. (2008)

Ammonia beccarii

20

334

4.9

2.26

Piña-Ochoa et al. (2010)

Valvulineria cf. laevigata Bolivina seminuda Stainforthia sp.

12.3 12.3 12.3

n.d. n.d. n.d.

0.76 0.37 0.83

0.80 0.39 0.88

Lee and Muller (1973)

Schwab and Hofer (1979) Hannah et al. (1994)

Nomaki et al. (2007)

rates for 16 common species of foraminifera and one gromid species (an amoeboid protist with an organic test, thought to be a sister-group of foraminifera (Longet et al., 2004)). We then determined the relationship between estimated biovolume and oxygen respiration rate, as many authors did for various kinds of animals. Finally we evaluated the contribution of benthic foraminifers to benthic C mineralisation in various marine environments. 2. Material and methods 2.1. Collection of foraminifera and sample preparation Living specimens were collected at 4 different localities: 1) an intertidal rocky area at Yeu Island on the French Atlantic coast, in April 2007, 2) intertidal mud flats in Aiguillon Bay (Atlantic coast), in April 2009, 3) two sites in the Rhône prodelta (37 and 60 water depth), in August 2006 and April 2007, and 4) a 450 m deep station in the axis of Cap Breton canyon in the Bay of Biscay, in June 2006. Living foraminifera from Yeu Island and Aiguillon Bay were sampled during low tide and were stored at in situ temperature (about 13 °C). Samples from open marine environments (Rhône prodelta, Bay of Biscay) were collected with a Barnett multi-corer allowing sampling of an undisturbed sediment surface (Barnett et al., 1984). The superficial sediment with the overlaying seawater was incubated on board at in situ temperature (12–14 °C) and was transported in cool boxes to the laboratory. In the laboratory, sediment samples containing living organisms were incubated at in situ temperature until the start of the experiment (in May 2007 except for those of Aiguillon Bay, which were measured in April 2009). No additional food was added. Just before starting the measurements, the sediment was sieved (using a 150 μm mesh) with

Gilson differential respirometer

Warburg cartesian diver microrespirometry

microrespiration cell + Clark-type microcathode oxygen electrode

microchamber and microelectrodes (Unisense) microrespiration chamber and O2 sensor spot microchamber and microelectrodes (Unisense)

micro-filtrated sea water at in situ temperature (13 °C) and salinity (35 for Yeu Island, Aiguillon Bay and Biscay samples; 38 for Rhône samples). Benthic foraminifera showing cytoplasm inside the shell were picked with a brush under a Leica (MZ 12.5) stereomicroscope. However, even foraminifera containing cytoplasm may have been dead for some hours or days. In order to be sure that we only measured oxygen respiration rates of living and active foraminifera, prior to the experiment all picked foraminifera were placed on a thin layer of sediment (b38 μm) in a small Petri dish (20 ml) at in situ temperature. After some hours, specimens that moved, and formed a burrowing trace in the thin sediment layer, were selected for the experiment. These active specimens were picked and rinsed several times with artificial sea water kept at in situ temperature and salinity in order to prevent bacterial contamination to a maximum. In order to ensure that measurement procedures were not lethal to the studied specimens we placed them all on a thin layer of sediment and observed their activities. 2.2. Oxygen respiration rates Foraminiferal oxygen respiration rates were measured using the same method as Høgslund et al. (2008) and Piña-Ochoa et al. (2010) which is relatively easy for scientists familiar with microsensor profiling. Measurements were carried out in glass micro-tubes we constructed from Pasteur pipette tips (inner diameter approximately 0.9 mm). The micro-tubes were fixed to a small vial, filled with artificial seawater at in situ salinity. The vial was placed in an aquarium with water kept at in situ temperature (13 °C). One to eight living foraminiferal specimens (depending on their size) were placed in the glass micro-tube with a brush in which air bubbles were carefully removed. The number of foraminifera used for each measurement

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Table 2 Respiration rates of benthic foraminifera studied in the present paper with detailed data on their size (average size of maximal elongation and estimated biovolume), average respiration rate measured and standard error of the replicate measurements as well as the number of specimens used for each measurement. Species

Sampling

Shell shape

Average size of maximal elongation (μm)

Rhône Yeu Rhône

Cylinder Half of a sphere Half of a sphere

1180 417 188

Aiguillon Bay of Biscay Rhône Bay of Biscay Rhône Rhône Aiguillon Yeu Rhône Rhône Rhône Rhône Rhône Rhône

Half of a sphere Cone with a flatted base Spheroide prolate Spheroide prolate Spheroide prolate Spheroide Half of a sphere Spheroide prolate Spheroide prolate Spheroide prolate Spheroide prolate Spheroide prolate Cylinder Spheroide prolate

541 617 369 369 251 380 285 940 459 247 406 625 396 493

Location Gromia sp. Ammonia beccarii Ammonia beccarii forma inflata Ammonia tepida Bolivina subaenariensis Bulimina aculeata Bulimina marginata Cassidulina carinata Eggerella Scabra Haynesina germanica Massilina secans Nonion scaphum Nonionella turgida Nouria polymorphinoides Pyrgo elongata Rectuvigerina phlegeri Valvulineria bradyi

is noted in Table 2. After approximately 10 min, micro-profiles of O2 were measured with a Clark type O2 microsensor (Revsbech, 1989) with a 50 μm tip mounted on a motorized micromanipulator. The tip of the sensor was placed about 200 μm above the foraminifer(a) in the centre of the micro-tube. The exact position was determined by inspection with two dissection microscopes located at both sides of the aquarium. Thereafter, the O2 concentration was measured in 200 μm steps until approximately 1000 μm above the micro-tube containing the foraminifer(a) (Fig. 1). Each oxygen profile was repeated 5 to 10 times (with 5 minute intervals) in order to confirm the stability of the oxygen gradient (Fig. 1) and after complete stability was obtained, the last profile was selected to calculate the oxygen O2 flux.

Biovolume (μm3)

Respiration rates (nl O2 cell− 1 h− 1)

Mean

SE

Mean

SE

9.1E + 07 9.7E + 07 9.7E + 06

1.5E + 07 4.3E + 06 1.4E + 05

2.78 5.27 0.63

3.1E + 07 2.8E + 06 4.6E + 06 8.0E + 06 3.1E + 06 6.2E + 06 2.6E + 06 8.6E + 07 1.4E + 07 3.5E + 06 7.4E + 06 4.0E + 07 4.6E + 06 1.8E + 07

3.2E + 06 1.3E + 05 2.4E + 04 6.4E + 05

2.01 0.25 0.45 0.42 0.12 0.76 0.41 4.25 0.99 0.16 0.14 1.97 0.09 0.75

7.3E + 05 5.7E + 05 1.1E + 07 1.7E + 06 1.0E + 06 3.7E + 06 4.2E + 05 2.2E + 06

Number of replicates

Specimen number by replicates

0.45 0.52 0.15

2 3 2

1 to 2 1 1

0.07 0.05 0.07 0.07

4 3 3 3 1 3 3 2 3 1 2 3 3 3

2 8 3 6 to 8 3 to 4 1 1 to 3 3 to 1 to 5 to 2 to

0.27 0.08 0.39 0.38 0.04 0.66 0.02 0.28

7 5

3 5 2 8 4

A blank without foraminifera was also measured (Fig. 1). For each analyzed species, 1 to 3 replicates were performed with different specimens (Table 2). The O2 flux ( J) was calculated using the first law , where D is the free diffusion coefficient for O2 at of Fick: J = −D dC dx the experimental temperature and salinity (Li and Gregory, 1974) and dC the O2 concentration gradient. The O2 concentration gradient was dx determined from the O2 profiles. The rate of total O2 consumption was calculated as the product of the O2 flux and the surface area of the micro-tube. Then this rate was divided by the number of foraminifera in the micro-tube to obtain the cell specific rate.

0.12 blank 10:14:34 Allogromia sp.

0.10

Distance from the test (cm)

10:20:30 Allogromia sp. 10:25:11 Allogromia sp.

0.08 10:29:13 Allogromia sp. 10:32:55 Allogromia sp.

0.06

Masselina secans (1 ind.) Nonion scaphum (3 ind.)

0.04

Bulimina acuelata (3 ind.) Pyrgo elongata (1 ind.)

0.02

0.00 50

100

150

200

250

300

350

400

450

O2 (µM) Fig. 1. Time series of oxygen profiles measured on Gromia sp are represented with white markers. Blank oxygen profile without foraminifera is represented with cross symbols. Examples of oxygen profiles in micro-tubes with different foraminiferal species are represented by filled markers.

E. Geslin et al. / Journal of Experimental Marine Biology and Ecology 396 (2011) 108–114

In order to compare our results with previous published oxygen consumption measurements obtained for other temperatures (Table 1, Fig. 1), the published respiration rates (R(Tobs)) for different Tobs temperatures were normalized to a 13 °C temperature (T13) using the Arrhenius relationship (Lombard et al., 2009):  RðT13 Þ = RðTobs Þ exp

TA T − A Tobs T13



All temperatures are expressed in degrees Kelvin; the Arrhenius temperature (TA; 9206°K), has been identified on the basis of Bradshaw's (1961) measurements in the 10–34 °C range. 2.3. Biovolume determination For every species, the total foraminiferal volume was estimated by using the best resembling geometric shape (Table 2). We then estimated the cytoplasmic volume (or biovolume) by assuming that the internal test volume corresponds to 75% of the total foraminiferal test volume (Hannah et al., 1994) and that the internal test volume of the shell is entirely filled with cytoplasm. For example, shell volume of Ammonia was approximated with the equation for a half sphere. The shell size parameters (length, width, and height) of each specimen were measured using a micrometer mounted on a Leica stereomicroscope (MZ 12.5). 2.4. Total benthic foraminiferal oxygen respiration and contribution to the local diffusive oxygen uptake (DOU) Total foraminiferal oxygen respiration was estimated for different locations where the measured foraminiferal species are dominant (cumulative percentage N 65%, see Table 3), and where we have quantitative data on their vertical distribution in the superficial sediment column (e.g. the Bay of Biscay and Rhône prodelta). Total foraminiferal respiration was calculated as the product of the number of individuals of the various taxa in the oxic sediment layer and their respective oxygen respiration rates, extrapolated to the total fauna by multiplying with (100/cumulative percentage of the considered species). In order to estimate the foraminiferal contribution to benthic ecosystem respiration, the total foraminiferal oxygen respiration was compared with the diffusive oxygen uptake (DOU) estimated from vertical oxygen profiles obtained at the same locations. Diffusive oxygen uptake of the sediments was calculated from oxygen profiles using PROFILE software (Berg et al., 1998). The two boundary conditions used for the calculations correspond to the overlying water oxygen concentration and the zero flux at the bottom of the oxic zone. The bulk sediment molecular diffusion coefficient (Ds) was estimated according to DS = φ2D0 (Ullman and Aller, 1982) where ϕ is the sediment porosity and D0 is the diffusion coefficient in water at in situ temperature (Li and Gregory, 1974). Sediment oxygen profiles were determined using Clark-type electrodes (Revsbech, 1989) with tip diameters of 100 μm and measured in situ with a benthic lander in the Rhône prodelta (Goineau et al., accepted for publication) and on board of the ship at in situ temperature in the Bay of Biscay (Anschutz et al., 2002).

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3. Results and discussion 3.1. Ecological characteristics of the investigated species In order to determine accurately the relation between oxygen respiration rates and biovolume (which is related to test size) we have chosen a selection of foraminiferal species with a large variation in test size. The smallest measured specimens were found for Haynesina germanica with a biovolume of 2.6 × 106 μm3 (285 μm maximal elongation). The largest biovolume, of 9.7 × 107μm3, was found for specimens of Ammonia beccarii (417 μm maximal elongation) (Table 2). Masselina secans and Ammonia beccarii are both large species (8.6 to 9.7 × 107 μm3) dominant in the meiofauna of the intertidal rocky shore of Yeu Island. They are living in/on the sediment, at the basis of red algae (Corallina officinalis and Gigartina acicularis) (Debenay et al., 1998). Ammonia tepida and Haynesina germanica are both living in the transitional zone between marine and continental realms (e.g. Debenay et al., 2000). These species strongly dominate the fauna in the mud flats of Aiguillon Bay (Pascal et al., 2008). They are resistant to extreme variations of temperature, salinity and oxygen concentration during tidal and/or seasonal cycles. Foraminifera from the Rhône delta are generally smaller sized (3.1 × 106 to 4.0 × 107 μm3 , Table 2) and live in muddy sediment enriched with organic matter (Mojtahid et al., 2009). At the sampling site, the sediment is oxic until 5 mm depth. Some species, such as Ammonia beccarii, Cassidulina carinata and Eggerella scabra, show a marked preference for the superficial sediment layer, whereas others, such as Nonion scaphum and Nonionella turgida live in strongly hypoxic or anoxic niches deeper in the sediment. Still other species (B. aculeata, N. polymorphinoides, R. phlegeri, V. bradyi) do not show a preferential microhabitat (Mojtahid et al., 2010). Recently, PiñaOchoa et al. (2010) have shown that N. scaphum, N. turgida, R. phlegeri, V. bradyi are facultative anaerobes capable to shift to nitrate respiration under anoxic conditions. Foraminifera from the Bay of Biscay have been sampled at about 450 m depth in the axis of Cap Breton Canyon. At this station, the sediment consists of silty mud with abundant plant debris and micas (Hess and Jorissen, 2009). The oxygen penetration depth is 5 mm. The exceptionally rich fauna is strongly dominated by Bolivina subaenariensis. Although this species shows a maximum density in the uppermost sediment layer (~3400 ind. 72 cm−2 in the first half cm layer), numerous specimens are found deeper in the sediment in anoxic conditions. According to Piña-Ochoa et al. (2010), also B. subaenariensis is a facultative anaerobe able to respire nitrate. 3.2. Oxygen respiration rates Because for most organisms, physiological processes are strongly influenced by temperature (Bishop, 1950), we normalized all published data to 13 °C, at which our measurements were performed (Table 1). The influence of temperature on metabolic rates has been confirmed by experimental measurements of oxygen respiration rates in both planktonic (Lombard et al., 2009) and benthic foraminifera (Bradshaw, 1961).

Table 3 Estimation of the foraminiferal contribution to benthic oxic respiration in the studied areas. OPD: oxygen penetration depth. DOU: diffusive oxygen uptake. Location

Rhône prodelta Rhône prodelta Bay of Biscay

Station

18 30 G

Depth

OPD

DOU

Nber of forams in the O2 zone

Taxa for which we dispose RR

Respiration rates by foraminiferal fauna

Forams contribution to DOU

m

mm

Μmol O2 m− 2 d− 1

ind. 72 cm− 2

%

Μmol O2 m− 2 d− 1

%

37 60 450

4.6 6.8 5.0

7390 3930 13,900

460 600 5000

78 66 73

42.3 47.0 87.7

0.6 1.2 0.6

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E. Geslin et al. / Journal of Experimental Marine Biology and Ecology 396 (2011) 108–114

Respiration rates were calculated on the basis of oxygen gradients measured in the vicinity of the foraminifera as described in the experimental section. The successive gradients realized during the same experiment showed parallel lines (Fig. 1) indicating a good repeatability for the gradients but a progressive impoverishment of the oxygen content in the micro-environment close to the foraminifera. Our observations indicate that oxygen saturation changes did not affect respiration rates. This is an important observation since some foraminifera live in micro-environments with highly variable oxygen saturation values. The average oxygen respiration rates obtained at 13 °C for the 17 studied species varied strongly, from 0.09 nl O2 cell−1 h−1 for small species such as Rectuvigerina phlegeri to 5.27 nl O2 cell−1 h−1 for the much larger Ammonia beccarii (Table 2). In order to compare our data with those of the literature, we expressed the respiration rate in biovolume units (1.58 to 15.8 nl O2 10−8 μm−3 h−1). The data set of Bradshaw (1961) and Nomaki et al. (2007) are of the same order of magnitude (0.5 to 26.6 nl O2 10−8 μm−3 h−1 and 1.61 to 83.2 nl O2 10−8 μm−3 h−1 respectively) whereas that of Hannah et al. (1994) shows much higher values (153 to 2400 nl O2 10−8 μm−3 h−1). For some of the measured genera (e.g. Valvulineria), our measurements are close to previously published data, whereas for other taxa, such as the various Ammonia species, the data are very different (Tables 1 and 2). In fact, several authors measured oxygen respiration rates for the genus Ammonia. The data show large differences (see Table 1) between the maximum values published by Hannah et al., 1994 (10.5 nl O2 cell−1 h−1 corresponding to 153 nl O2 10−8 μm−3 h−1) and Nomaki et al., 2007 (25.1 nl O2 cell−1 h−1; no data on volume or size are available to make the conversion to biovolume units) and the much lower values found by Bradshaw (1961) (0.1 nl O 2 cell−1 h−1 corresponding to 4.3 nl O2 10−8 μm−3 h−1). The values of Moodley et al. (2008) are comparable to our data obtained for Ammonia tepida from Aiguillon Bay (Tables 1 and 2). However, the oxygen respiration rates measured for Ammonia beccarii forma inflata specimens (average value 0.63 nl O2 cell−1 h−1) from the Rhône prodelta are much lower. These large differences are probably due to the fact that the various studied Ammonia species have very different test sizes (see next paragraph for the relation with biovolume). In order to evaluate whether the large variability described in the previous paragraph is indeed due to large differences in test size, we investigated the relationship between oxygen respiration rate and biovolume (Fig. 2). We found a strong correlation between estimated biovolume and oxygen respiration rates (our data n = 44, R2 = 0.72, F = 114, p b 0.0001), described by the equation R = 3.98 10−3 BioVol0.88 (with respiration rate (R) expressed in nl O2 h−1 and biovolume (BioVol) in μm−3) (Fig. 2). In Fig. 2, we plotted some literature data on benthic foraminiferal respiration rates for which biovolume or size data are available. We estimated biovolumes for all species using size data reported in the cited papers, and applied the same method as for our material. Most published data plot close to our observations even if the types of data are different (for example data of Bradshaw (1961) are measurements of each single individuals whereas those of Nomaki et al. (2007) are average of each species). In the literature, a clear relationship between test size and oxygen respiration rates was previously observed by Bradshaw (1961), who studied Ammonia tepida specimens of various sizes. Hannah et al. (1994) have shown that as cell size (μm3) increased the respiration rate (μl O2) per unit of protozoan biomass decreased. Surprisingly, Nomaki et al. (2007) did not find a clear relationship between oxygen respiration rates and biomass (related to biovolume), probably because of the narrow size range of the foraminiferal assemblages they studied. The dependency of oxygen respiration of biovolume, as described by the aforementioned power law relationship (Fig. 2) is close to the

Fig. 2. Plot of respiration rates versus biovolume data from our study and comparable data from the literature. The regression lines and equations describe the relationships between biovolume (Biovol) and respiration rates (R) of foraminifera for the data presented in this paper (stippled line; R = 3.98 10−3 BioVol0.88 (n = 44, R² = 0.72, F = 114, p b 0.0001)).

mean power value (0.75) found in similar models applied to a wide range of organisms (e.g., Hemmingsen, 1960; Warwick and Price, 1979). The comparison of the foraminiferal power law equation with similar power law equations determined for other benthic groups (nematodes, copepods, ostracods, ciliates and flagellates) is presented in Fig. 3. It is remarkable that the slope is very similar for all investigated groups. However, the equations for other benthic groups all show an offset to more positive values. This offset is higher for ciliates/flagellates than for copepods and nematodes. This strongly suggests that benthic foraminifera have a lower oxygen respiration rate than other groups, even when standardized for biovolume. This lower respiration rate may reflect a lower metabolic rate, which could also explain the much lower physical activity of foraminifera compared to more mobile benthic groups. In view of their lower respiration rates, it can also be expected that the foraminiferal contribution to aerobic organic matter mineralisation is rather limited.

Fig. 3. Respiration rates (R) in function of biovolume. Our data of respiration rates (black diamonds) are compared to previous published relationships obtained for other meiofaunal groups.

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3.3. Foraminiferal oxygen respiration rates and contribution to DOU Foraminiferal contribution to aerobic benthic respiration was studied for two stations from the Rhône prodelta, which are contrasted in terms of influence of the Rhone River (Goineau et al., accepted for publication). Stations 18 (water depth 37 m) and 30 (60 m deep) were located at 10 km and 20 km southwest of the river mouth, respectively. They showed different DOU (7390 and 3930 μmol O2 m−2 d−1, respectively) and foraminiferal densities (460 and 600 ind. 72 cm−2, respectively) (Table 3). As explained before, we restricted our calculations to the foraminifera inhabiting the oxic surface layer of the sediment (0–0.5 cm for station 18, 0–1 cm for station 30) and the species for which we actually measured the individual oxygen respiration rate (representing 78 and 66% of the total assemblages for stations 18 and 30, respectively). In order to obtain the overall foraminiferal oxygen respiration rate, we multiplied this value by (100/cumulative percentage of measured species). Foraminiferal respiration rates were about the same at both stations (42.3 and 47.0 μmol O2 m−2 d−1) while DOU was twice higher at station 18. The foraminiferal contribution to DOU would be 0.6 and 1.2%, respectively. Station G (450 m depth) from the Cap Breton canyon in the Bay of Biscay was chosen because of the uncommonly high densities of foraminifera (see Hess and Jorissen, 2009, station OB5G), which made us expect a higher foraminiferal contribution to the benthic oxygen uptake. The foraminiferal density at station G (about 5000 ind. 72 cm−2), was one order of magnitude higher than in the Rhône prodelta station and DOU was two times higher than at station 18 (Table 3). The taxa for which we dispose of oxygen respiration rate measurements accounted for 73% of the foraminiferal fauna of the oxic 0–0.5 cm level (Table 3). In spite of the high foraminiferal density at station G, the estimated foraminiferal contribution to total DOU was only 0.6%. We think that the contrast between the very high standing stocks and the low foraminiferal contribution to DOU is due to the small size of the individuals (for instance, compared to the faunas found in the Rhône prodelta stations). The results obtained in these two contrasting areas suggest that the foraminiferal contribution to the DOU is very small. However, our analysis only pertains to foraminifera from the N150 μm size fraction. A calculation of the theoretical contribution of foraminifera from the 63–150 μm fraction, using the relationship between oxygen respiration rate and biovolume (Fig. 2), shows that even if this small size fraction is 5 to 10 time richer in abundance than the N150 μm fraction (Mojtahid et al., 2009), its contribution to the total oxygen uptake is still minimal. If foraminiferal activity in the laboratory is similar to that in situ, our estimates should give a fair representation of the foraminiferal contribution to carbon mineralisation in the investigated environments. We think that the foraminiferal contribution to benthic carbon mineralisation is also low in other shelf environments. For example, in Whittard canyon (North Atlantic) foraminiferal oxygen respiration (N150 μm fraction) accounts for 2.5% of the DOU at 300 m water depth and for 0.5% at 500 m water depth, respectively (Duros, unpublished data). In the literature, Moodley et al. (2002) and Nomaki et al. (2007) have estimated the contribution of foraminiferal carbon mineralisation using two different methods. In the first study, the authors estimated the foraminiferal contribution to fresh organic carbon mineralisation by measuring the short-term ingestion of fresh food input (13 C-enriched diatoms) in the NE Atlantic. Their data suggest that foraminifera may play a central role in the rapid initial processing of fresh organic carbon in deep sea sediment (Moodley et al., 2002). Nomaki et al. (2007) have estimated the total foraminiferal carbon mineralisation rate using a method taking into account the oxygen respiration rates of foraminifera (using the following equation Rmineral = Rresp × q × 12 (gC)× N where Rmineral, Rresp, q and N are the mineralisation (mg Cm−2 d−1),

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respiration rate (mol O2 d−1 ind.−1), constant respiratory quotient (0.85) and number of benthic foraminifera (m−2, N63 μm) according to Nomaki et al., 2005) in two stations from Sagami Bay (1450 m water depth). The carbon mineralisation rates estimated from the foraminiferal oxygen respiration rates measured by Nomaki et al. (2007) were variable, ranging between 37 and 94 mg C m−2 d−1 corresponding to 9 to 23% of the total organic carbon mineralised (taken into account the fresh and “old” carbon). We suspect that these values may be overestimated because they are based on the entire foraminiferal fauna found in the first 5 cm of sediment (Nomaki, pers. comm.), and not only on the foraminifera inhabiting the oxic superficial sediment layer (2 mm to 20 mm according to Glud et al. (2009a) at the same station). In order to compare our estimation method and that of Nomaki et al. (2007) we applied our estimation method to the autumn faunal assemblages reported in Nomaki et al. (2005) with the oxygen respiration rate values published in Nomaki et al., (2007) for the whole foraminiferal faunas inhabiting the first 3 cm of sediment. We used the DOU values published by Glud et al. (2009) measured at the same station and during the same season. We found a foraminiferal C mineralisation rate corresponding to about 15% of the total oxygen uptake. This value is in agreement with those reported by Nomaki et al. (2007). However, we think it is more accurate to estimate the aerobic C remineralisation taking into account only the faunas inhabiting the oxic zone. If we do this, the foraminiferal contributions to aerobic C remineralisation is only 3%, only slightly higher than the values estimated for our continental shelf and submarine canyon stations. It should be noted that DOU only represents a fraction to the total oxygen consumption of the sediment (Glud, 2008) and the contribution of foraminifera to sediment oxygen consumption might well be even less than shown here. It appears therefore that foraminifera play only a minor role in aerobic C mineralisation process. The contribution of meiofauna to total sediment respiration is also small in coastal sediments (less than a few percent; Middelburg et al., 2005), whereas macrofauna typically contributes 10–30% of total respiration (Herman et al., 1999). For marine shelf and the upper slope environments, Heip et al. (2001) suggested that biotic benthic respiration is mainly due to macrofaunal groups which account for roughly half of the organic matter remineralisation. They indicate that the aerobic bacterial and protist contribution (which includes foraminifera) to the total benthic carbon respiration is about 23% on the shelf and upper slope. However, the contribution of the bacteria and protists (including foraminifera) reaches more than 70% in lower slope and abyssal environments. Combined with our results, this suggests that foraminifera play a minor role in C mineralisation in shelf environments but could be much more important for deep ocean sedimentary carbon recycling. The oxygen respiration rates and consequently the contribution of abyssal foraminifera to C mineralisation cannot be estimated using our equation because our oxygen respiration rate measurements were performed on other species than those found in deep sea environments, and because the metabolism of deep sea species is probably lower as it has been shown for other groups (Mahaut et al., 1995). The relatively low contribution of foraminifera to aerobic mineralisation in shelf environments contrasts with their potentially important role in anaerobic mineralisation processes, such as denitrification. Piña-Ochoa et al. (2010) showed that foraminifera can account for more than 70% of total denitrification in certain shelf areas (e.g. roughly 80% in the Bay of Biscay). A major reason for this is perhaps that nitrate (the electron acceptor used in denitrification) can be stored in the foraminiferal cells, which is not the case for oxygen (Risgaard-Petersen et al., 2006; Piña-Ochoa et al., 2010). Foraminifera which have the ability to accumulate and respire nitrate are therefore not restricted to the narrow oxic zone but can also explore deeper sediment layers, where competition with other organisms is restricted.

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4. Conclusion The present paper presents oxygen respiration rate measurements for 17 species of benthic foraminifera, with strongly varying test sizes. This new data set allowed us to quantify the relationship between oxygen respiration rate and foraminiferal biovolume: R = 3.98 10−3 BioVol0.88 where the oxygen respiration rate (R) is expressed in nl O2 h−1 and biovolume (BioVol) in μm3 (n = 44, R2 = 0.72, F = 114, p b 0.0001). This equation can be used to estimate total foraminiferal respiration and the foraminiferal contribution to aerobic C mineralisation in marine shelf and upper slope environments. Our results obtained in two contrasting environments, the Rhône prodelta (37 to 60 m depth) and Cap Breton canyon (450 m depth) suggest that the foraminiferal contribution to aerobic C mineralisation is low. Our results strongly contrast with the potentially strong foraminiferal contribution to anaerobic C mineralisation, by denitrification (Piña-Ochoa et al., 2010). It suggests that foraminifera relatively contribute more to anaerobic organic matter mineralisation than to aerobic organic matter mineralisation. [RH]

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