S. Rink á M. KuÈ hl á J. Bijma á H. J. Spero

Microsensor studies of photosynthesis and respirationin the symbiotic foraminifer Orbulina universa

Received: 26 January 1998 /Accepted: 11 April 1998

Abstract Oxygen and pH microelectrodes were used toinvestigate the microenvironment of the planktonicforaminifer Orbulina universa and its dino¯agellateendosymbionts. A di�usive boundary layer surroundsthe foraminiferal shell and limits the O2 and protontransport from the shell to the ambient seawater andvice versa. Due to symbiont photosynthesis, high O2

concentrations of up to 206% air saturation and a pH ofup to 8.8, i.e. 0.5 pH units above ambient seawater, weremeasured at the shell surface of the foraminifer at sat-urating irradiances. The respiration of the host±sym-biont system in darkness decreased the O2 concentrationat the shell surface to <70% of the oxygen content inthe surrounding air-saturated water. The pH at the shellsurface dropped to 7.9 in darkness. We measured a meangross photosynthetic rate of 8.5 � 4.0 nmol O2 h

)1

foraminifer)1. The net photosynthesis averaged5.3 � 2.7 nmol O2 h

)1. In the light, the calculated res-piration rates reached 3.9 � 1.9 nmol O2 h

)1, whereasthe dark respiration rates were signi®cantly lower(1.7 � 0.7 nmol O2 h

)1). Experimental light±darkcycles demonstrated a very dynamic response of thesymbionts to changing light conditions. Gross photo-synthesis versus scalar irradiance curves (P vs Eo curves)showed light saturation irradiances (Ek) of 75 and137 lmol photons m)2 s)1 in two O. universa specimens,respectively. No inhibition of photosynthesis wasobserved at irradiance levels up to 700 lmol photons

m)2 s)1. The light compensation point of the symbioticassociation was 50 lmol photons m)2 s)1. Radial pro®lemeasurements of scalar irradiance (Eo) inside the fora-minifera showed a slight increase at the shell surface upto 105% of the incident irradiance (Ed).

Introduction

Planktonic symbiont-bearing foraminifera often occur inoligotrophic ocean waters. Probably due to their closerelationship with autotrophic dino¯agellates, they cansurvive in nutrient-limited environments. Symbiont-bearing foraminifera populate the euphotic zone, wherethe symbionts are exposed to light levels su�cient forphotosynthesis. It has been suggested that the zooxan-thellae live well-protected in the cytoplasm of the hostwhere they bene®t from the respired CO2 as well as fromnitrogen and phosphorus from prey digested by the for-aminifer (Be 1977; Jùrgensen et al. 1985; Gastrich andBartha 1988). The density of endosymbionts can reacha mean of 3300 cells per foraminifer, and speci®c photo-synthetic rates of 1.72 pmol C symbiont)1 h)1

were measured at saturating irradiances (Spero andParker 1985). The importance of the endosymbionts forthe host was demonstrated in experiments, where thesymbionts were treated with the photosynthetic inhibitorDCMU [3-(3,4-dichlorophenyl)-1,1-dimethylurea]. Be etal. (1982) thus found signi®cantly shorter survival times,reduced shell growth rates, and a smaller ®nal shell sizeafter inhibition of zooxanthellae photosynthesis.

Spinose planktonic foraminifera have a perforatecalcareous shell with thin spines. The spines can reach alength of several millimeters and enlarge the e�ectivesurface area of the foraminifera, thereby increasing thechance of capturing prey with its sticky rhizopodialnetwork (Be 1977). Due to the enormous productivity offoraminiferal shells large parts of the ocean ¯oor arecovered with them and constitute the so-called ``glob-igerina ooze''. Because the geochemical composition, i.e.the stable carbon and oxygen isotope composition, of

Marine Biology (1998) 131: 583±595 Ó Springer-Verlag 1998

Communicated by O. Kinne, Oldendorf/Luhe

S. Rink (&) á M. KuÈ hlMax-Planck-Institute for Marine Microbiology,Microsensor Research Group, Celsiusstr. 1,D-28359 Bremen, Germany

J. BijmaAlfred-Wegener-Institute for Polar and Marine Research,Columbusstr., D-27568 Bremerhaven, Germany

H.J. SperoDepartment of Geology, University of California, Davis,California 95616, USA

planktonic foraminiferal shells can be used for paleo-environmental reconstructions of the last 120 � 106

years of the world's oceans, these organisms have becomea major tool in geology to reconstruct the productivity ofpast oceans. However, photosynthesis of endosymbiontscan a�ect the isotopic composition of the foraminiferalshells due to the higher a�nity of the CO2 ®xing enzymefor 12CO2 (see e.g. Spero and de Niro 1987).

Symbiotic associations of planktonic spinose fora-minifera with microalgae have been reported for at leastseven species. The predominant algal symbionts are coc-coid dino¯agellates (Hemleben et al. 1989). They arefound in the species Orbulina universa, Globigerinoidessacculifer, G. ruber and G. conglobatus (Spindler andHemleben 1980; Hemleben and Spindler 1983; Spero1987). The endosymbiont ofO. universa, an opportunisticspecies from the temperate to tropical provinces (BeÂ1977), is the dino¯agellate Gymnodinium beÂii. The fora-miniferaGlobigerinella aequilateralis,Globigerina cristataand G. falconensis host symbiotic chrysophycophytes(Spindler and Hemleben 1980; Gastrich 1987; Faber et al.1988). All symbionts exhibit a diurnal migration from theshell interior to the distal parts of the foraminiferal spinesin the light. Spero (1987) suggested that the migrationalso involves a daily endo-/exocytotic cycle.

Oxygen and pH microelectrodes have already beenused to study symbiotic associations, such as the for-aminifer Globigerinoides sacculifer and the hermatypiccorals, Favia sp. and Acropora sp. (Jùrgensen et al. 1985;KuÈ hl et al. 1995). Microsensor techniques proved to beuseful tools for measuring the processes of photosyn-thesis and respiration with a high spatial and temporalresolution in these symbiotic associations (Revsbech andJùrgensen 1986). The light±dark shift technique (Revs-bech et al. 1981; Revsbech and Jùrgensen 1983) mea-sures gross photosynthetic rates independent of therespiration process, and light and dark respiration ratesin symbiont±host systems can be assessed independently.Due to their small tip diameter, microsensors can beused without any destruction of the organism, and sev-eral measurements in one specimen, e.g. under changinglight or temperature conditions, are possible.

Photosynthesis in planktonic symbiotic foraminiferahas previously been investigated with two di�erenttechniques. Jùrgensen et al. (1985) used O2 microsensorsto measure the gross and net photosynthetic rates ofGlobigerinoides sacculifer (Jùrgensen et al. 1985). Radiotracer 14C methods have been used to estimate the cell-speci®c carbon uptake of symbionts of two di�erentspecies (Spero and Parker 1985; Gastrich and Bartha1988). It was estimated that a single Orbulina universaspecimen would contribute approximately 0.2% of the®xed carbon in 1 m3 of seawater (Spero and Parker1985). The foraminifer±algal association has been char-acterized as a ``hot spot'' of productivity in oligotrophicseawater.

Symbiont-bearing planktonic foraminifera are cos-mopolitan calcifying organisms, but there are still a lotof open questions about their biology and the physio-

logical and biochemical interactions of the host±sym-biont association. Several hypotheses about their mutualbene®t, e.g. the nutritional relationship, the transport ofmetabolic gases, and the calci®cation process, are dis-cussed in the literature (e.g. Erez 1983; Jùrgensen et al.1985; Hemleben et al. 1989; Lea et al. 1995). Althoughthe photosynthetic rates of the symbionts of Orbulinauniversa have been studied (Spero and Parker 1985), themicroenvironment of this foraminifer and its importancefor host±symbiont interaction are still unknown. Weused O2 and pH microsensors and a ®ber-optic scalarirradiance microprobe to investigate the physico-chem-ical microenvironment of this symbiotic system. Ourstudy demonstrates the in¯uence of changing lightconditions on the foraminiferal±algal symbiosis and aclose coupling of photosynthesis and respiration inO. universa.

Materials and methods

Collection

Adult Orbulina universa with sphere diameters ranging between 290and 550 lm (Fig. 1A) were hand-collected by SCUBA divers fromthe surface waters of the Southern California Bight, near SantaCatalina Island, California between July and August 1995. Indi-vidual specimens were sampled in glass jars at a depth of 5 to 10 m(Fig. 1B). During the collection period the mean water temperaturewas 19.2 °C. Light measurements at the collection site showed anaverage downwelling irradiance of 2100 lmol photons m)2 s)1 atthe water surface in full sunlight (S. Anderson, personal commu-nication). After sampling in the morning hours (09:00 to 11:00 hrs),individual foraminifera were kept in separate glass vessels at 22 °Cand �80 lmol photons m)2 s)1 without feeding. Experiments wereconducted within less than 24 h after collection, in the laboratoryof the Wrigley Institute for Environmental Studies (WIES).

Experimental setup

For microsensor measurements, a specimen was placed on a nylonmesh in a small Plexiglas chamber (10 ml volume) with ®lteredseawater (Figs. 1C, 2A). The microsensors were manually posi-tioned with a micromanipulator (MaÈ rtzhaÈ user, Germany). Theangle of inclination of the microsensor was 30° relative to thevertically incident light. Positioning of the microsensor tip relativeto the foraminiferal shell surface was adjusted under a dissectionmicroscope. Measurements were performed at room temperature(20 to 22 °C) in a dark room under de®ned light conditions. Thelight source was a ®ber optic halogen lamp (Schott KL-1500)equipped with a collimating lens, and incident irradiance (0 to1000 lmol photons m)2 s)1) was adjusted by neutral density ®lters(Oriel). Downwelling quantum irradiance (400 to 700 nm) wasmeasured with a quantum irradiance meter (LiCor, LI 189). Thelight was controlled by a mechanical shutter, installed in the lightpath of the halogen lamp, without in¯uencing the light quality. Thespecimens were allowed to adapt to conditions in the measuringchamber for 0.5 to 1 h, and the experiments were started when thesymbionts were distributed in a concentric halo around the shell(Figs. 1A, 2B).

Oxygen microelectrodes

Photosynthetic rates and radial concentration pro®les of O2 fromthe ambient seawater to the shell surface were measured with aClark-type O2 microelectrode with a guard cathode connected to a

584

picoammeter and a strip chart recorder (Revsbech 1989). Themicroelectrodes had an outer tip diameter of 5 to 12 lm, a 90%response time of <0.4 to 1.8 s and a stirring sensitivity of 0 to 2%.Linear calibration of the electrode signal was done at room tem-perature in air-saturated seawater and in O2-free seawater (reducedwith sodium dithionite). The O2 concentration of the air-saturatedseawater was determined by Winkler-titration (Grassho� et al. 1983).

pH microelectrodes

pH was measured with glass pH microelectrodes (Revsbech et al.1983) in combination with a calomel reference electrode (Radi-ometer, Denmark), both connected to a high impedance mV meter.The pH electrodes had a pH-sensitive tip of 12 to 25 lm diameterand of 80 to 150 lm length. They were calibrated in NBS bu�ers(Mettler Toledo, pH 4, 7 and 9) at room temperature.

Scalar irradiance measurements

A ®ber optic microprobe (Lassen et al. 1992) connected to a PARmeter (KuÈ hl et al. 1997) was used for measuring radial pro®les ofquantum scalar irradiance (400 to 700 nm) from the surroundingstowards the shell of Orbulina universa. The diameter of the scalarirradiance microprobe tip was <100 lm. Linear calibration of the®ber optic scalar irradiance microprobe was done in darkness andin a collimating light ®eld at a known downwelling irradiance overa black light trap (KuÈ hl et al. 1997). Downwelling irradiance wasmeasured with a quantum irradiance meter (LiCor, LI 189). Alllight measurements in this paper refer to visible light (400 to700 nm), i.e. the available radiation for oxygenic photosynthesis.

Photosynthesis measurements

Oxygen microelectrodes with a fast response time (<0.5 s) wereused for measurements of gross and net photosynthesis. Grossphotosynthesis was estimated with the light±dark shift technique(Revsbech et al. 1981; Revsbech and Jùrgensen 1983) by measuringthe initial decrease of O2 in the ®rst seconds after darkening. TheO2 depletion is equal to the photosynthetic O2 production duringthe previous light period (more details in Revsbech and Jùrgensen1983; Glud et al. 1992; KuÈ hl et al. 1996). Gross photosyntheticrates, P(r), were measured inside the symbiotic swarm at 50-lmintervals starting at the shell surface. Radial pro®les of photosyn-thetic activity were used to calculate the total gross photosyntheticproduction assuming that the symbionts surround the shellin spherical symmetry (Fig. 2B). The total gross photosyntheticrate, Ptotal (nmol O2 h

)1 foraminifer)1), was calculated as the sumof the photosynthetic rates, measured per volume of each concen-tric segment in the symbiotic halo (Jùrgensen et al. 1985):X

i

P�ri� 4

3p �ri � riÿ1�3 ÿ �riÿ1�3h i� �

; �1�

where i � 0; 50; 100 . . . lm.Net photosynthesis and respiration rates were calculated from

the measured steady-state O2 pro®les in light and in darkness,respectively. The area-integrated O2 ¯ux, Qt (nmol O2 h)1 for-aminifer)1), was calculated by the radial gradient, dC/dr, the mo-lecular O2 di�usion coe�cient in seawater, D, and the surface areaof the sphere, 4pr2 (Jùrgensen et al. 1985; Ploug et al. 1997):

Qt � 4pr2DdCdr

: �2�

Respiration measurements

The respiration of the symbiont±host system in the light was cal-culated as the di�erence between total gross photosynthesis and netphotosynthesis (Jùrgensen et al. 1985). In the dark, the O2 ¯ux tothe sphere is determined by the combined respiration rate of theforaminifer and the symbionts, and dark respiration was calculatedfrom the O2 pro®les measured in the dark by using Eq. 2.

P versus Eo curves

Gross photosynthetic rates (nmol O2 cm)3 s)1) were measured with

the light±dark shift method at the shell surface inside the symbiont

Fig. 1A Adult Orbulina universa with dino¯agellate symbiontssurrounding the shell. Juvenile trochospiral shells are visible in thecenter of the transparent spherical chamber (diameter of the sphericalshell was �500 lm) (photo: T. Mashiotta). B Collection of planktonicforaminifera by SCUBA diving. Individual specimens are sampled inglass jars (photo: E. Meesters). C O. universa sticking to the nylonmesh inside the measuring chamber

585

swarm as a function of increasing scalar irradiance. Orbulinauniversa was exposed to each irradiance level for 15 to 30 minbefore the measurements started. Light intensities (0 to 700 lmolphotons m)2 s)1) were adjusted with neutral density ®lters (Oriel).An exponential function: P � Pm [1 ) exp()aEo/Pm)] (Webb et al.1974) was ®tted to the P versus Eo data measured at the shellsurface, where Pm is the light-saturated photosynthetic rate and athe initial slope of the P versus Eo curve at subsaturating scalarirradiance (Geider and Osborne 1992).

Results

Microenvironment of the symbiotic Orbulina universa

The zooxanthellae of O. universa showed a diurnal mi-gration pattern. During the day, the dino¯agellatesspread out on the rhizopodial network between thespines, while at night they were located inside the shells.During our experiments the symbionts formed a 200 to400 lm thick concentric halo surrounding the sphericalshell of the foraminifer (Figs. 1A, 2B).

Around the shell, a di�usive boundary layer (DBL)was established that limited the solute transport betweenthe surrounding seawater and the foraminifer. In thelight, the O2 concentration started to increase in the distalpart of the spines, and very high concentrations weremeasured towards the shell (Figs. 3A, 7A). Pro®les ofgross photosynthesis inside the symbiont swarm showedhighest rates at the foraminiferal shell, where a maximumgross photosynthesis up to 13.7 nmol O2 cm

)3 s)1 wasmeasured (Fig. 3C). The photosynthetic activity of thesymbionts and the presence of a DBL thus created amicroenvironment of high pH and high O2 concentra-tions around the shell of Orbulina universa as comparedto the ambient seawater (Fig. 3A, B). At the shell surface,we measured O2 supersaturation up to 206% of air sat-uration at high irradiances (Fig. 3A). During measure-ments of the dark pro®les the symbionts moved into theshell. In darkness, the respiration of the foraminifer andthe symbionts decreased the O2 concentration to <80%air saturation at the shell surface of this specimen(Fig. 3A). Due to photosynthetic CO2 ®xation in thelight, pH increased to up to 8.8 at the shell surface undersaturating light conditions. In darkness, pH was lowereddown to pH 7.9 at the shell surface as a result of CO2

release during respiration of the host and its symbionts(Fig. 3B). The average rate of gross photosynthesis peradult O. universa was 8.9 nmol O2 h)1 foraminifer)1

(Table 1), but rates of 13.9 nmol O2 h)1 foraminifer)1 at

saturating irradiance (782 lmol photons m)2 s)1) werefound in one specimen (No. I). The net photosyntheticrate of the same specimen reached 8.7 nmol O2 h

)1.Radial O2 and pH pro®les measured at di�erent po-

sitions in the foraminifer showed similar concentrationgradients (data not shown) supporting our assumptionof a radial symmetry of solute concentration and di�u-sion around the foraminiferal shell under stagnantconditions. Radial pro®les of scalar irradiance from theambient seawater to the shell showed values up to 105%of the incident irradiance (Fig. 3C). This increaseprobably resulted from light scattering and re¯ectionwithin the spines and o� the calcite shell surface.

Oxygen, pH and photosynthesis at the shell surface

Experimental light±dark cycles resulted in very dynamicchanges in the O2 production at the shell surface

Fig. 2A Schematic drawing of themeasuring chamber (10 ml volume)with a single foraminifer placed on a nylon mesh. Microsensorpositioning was done with a micromanipulator, and the incident lightwas adjusted by neutral density ®lters. B Orbulina universa. Schematicdrawing of an adult. Concentric spheres of 50 lm thickness indicate themicrosensor positioning for the photosynthesis measurements (roradius of the spherical shell; r distance to the shell)

586

(Fig. 4). After a steady-state O2 level was reached, thelight was turned o� and the O2 level decreased from 190to 80% air saturation within 5 min. When the light wasturned on again, the O2 concentration increased imme-

diately and reached 100% air saturation within 15 s. Asteady-state supersaturation of 190% was reached againafter 3 to 4 min.

Oxygen and pH conditions at the surface of theforaminiferal shell were investigated as a function ofscalar irradiance (Fig. 5). The O2 and pH versus scalarirradiance curves demonstrated the saturation of pho-tosynthesis with increasing incident light. Both pH andthe O2 level at the shell surface saturated at approxi-mately 250 lmol photons m)2 s)1.

Gross photosynthetic rates at the shell surface in-creased with increasing scalar irradiance (Fig. 6A, B).The exponential function of Webb et al. (1974) was ®ttedto the P versus Eo measurements and estimated a max-imum photosynthetic rate of 9.3 nmol O2 cm)3 s)1 inone specimen. The initial slope a in the linear part of thisP versus Eo curve was 0.07 (Fig. 6A). The onset of lightsaturation of photosynthesis expressed by the light sat-uration irradiance, Ek, was Pmax/a � 137 lmol photonsm)2 s)1. In a second specimen, we found a lower Ek of75 lmol photons m)2 s)1 caused by a lower Pmax of5.6 nmol O2 cm)3 s)1 and the same initial slope(a � 0.067) (Fig. 6B). Up to 700 lmol photons m)2 s)1

no photoinhibition was observed in Orbulina universa.

Fig. 3 Light pro®les (s) and dark pro®les (d) of O2 (A) and pH light(h) and dark(j) pro®les (B) measured from the ambient seawater tothe spherical shell (Eo � �700 lmol photons m±2 s±1). Pro®les ofscalar irradiance (r) and gross photosynthesis (bars) measured insteps of 50 lm towards the shell surface of Orbulina universa (C).Vertical dotted line indicates the outer periphery of the symbiontswarm

Table 1 Orbulina universa. Photosynthesis and respiration measured in several individuals of di�erent sizes at saturating irradiances

Foraminiferno.

Shelldiameter

Incidentirradiance

Photosynthesis (nmol O2 h)1 foraminifer)1) Respiration

(nmol O2 h)1

Percentage of grossphotosynthesis

(lm) (lmol photonsm)2 s)1)

Gross Net foraminifer)1)

I 554 782 13.89 8.72 5.17 37II 554 782 11.00 5.06 5.94 54III 463 288 9.26 4.57 4.69 51IV 473 446 8.16 6.45 1.71 21V 297 750 2.29 0.57 1.72 75

Mean � SD 468 � 105 609 � 228 8.92 � 4.29 5.07 � 2.99 3.85 � 1.99 47.6 � 20.14

Fig. 4 O2 dynamics at the shell surface of Orbulina universa duringexperimental light±dark cycles. Incident irradiance was 683 lmolphotons m)2 s)1. Dashed line indicates the O2 concentration of theambient seawater

587

Radial distribution of O2 and pH

Radial O2 and pH pro®les in dependence of the incidentirradiance were measured from the ambient seawater

towards the shell surface (Fig. 7). The O2 concentrationstarted to increase outside the spines and reached thehighest values at the shell surface due to the presence ofthe DBL. The O2 pro®les varied as a function of the lightlevel (Fig. 7A). At 50 lmol photons m)2 s)1 the com-pensation irradiance, Ec, where the respiratory O2 con-sumption of the system balanced the zooxanthellae O2

production, was reached at 50 lmol photons m)2 s)1

(Fig. 8). With increasing incident irradiance (>50 lmolphotons m)2 s)1), the photosynthetic O2 productionexceeded the O2 uptake, and net photosynthesis ap-proached saturation at >450 lmol photons m)2 s)1

(Fig. 8).The pH increased towards the surface of the shell

from the ambient seawater level at the end of the spines.Due to increasing photosynthetic CO2 ®xation with ir-radiance and the presence of a DBL we measured in-creasing pH values at the shell surface (Fig. 7B).The highest pH of 8.8 was found at 717 lmol photonsm)2 s)1. In the darkness the surface pH of this specimendecreased to 7.9.

Respiration rates in light and darkness

In the light we observed a high variability of respirationrates in di�erent specimens (Table 1). When light res-

Fig. 5 O2 (d) and pH (h) measured as a function of scalar irradiance(lmol photons m)2 s)1) at the shell surface of Orbulina universa.Dashed line indicates ambient seawater level of O2 and pH

Fig. 6 Gross photosynthetic rates versus scalar irradiance (400 to700 nm) measured at the shell surface of two Orbulina universaspecimens (A and B, respectively). An exponential function (Webbet al. 1974) (solid line) was ®tted to the data by a nonlinear least-squares Levenberg±Marquardt algorithm (Origin 4.1, MicroCalSoftware, Inc.) (A: r2 � 0.86, v2 � 1.95; B: r2 � 0.85, v2 �0.63). Dashed lines indicate 95% con®dence intervals and Ek the onsetof light saturation

Fig. 7 Radial steady-state O2 (A) and pH (B) pro®les as a function ofincreasing irradiance. Vertical dotted line indicates the outer peripheryof the symbiont swarm. Numbers indicate incident irradiance (lmolphotons m)2 s)1). Dashed lines indicate O2 concentration and pH ofthe bulk seawater

588

piration was calculated as a percentage of gross photo-synthesis we found an average of 47.6 � 20.1% (n� 5)(Table 1; Fig. 9). Orbulina universa and its zooxanthellaeshowed a lower average O2 consumption in darkness(1.7 � 0.7 nmol O2 h)1; n � 24, data not shown)compared to the respiration at light saturation(3.9 � 1.9 nmol O2 h)1; n � 5, see Table 1). Thusrespiration was stimulated in the light by a factor of 2.

Discussion

Foraminiferal microenvironment

The O2 and pH of the microenvironment around theforaminiferal shell di�er from the ambient seawatervalues, depending on the rates of photosynthesis andrespiration of the host±symbiont association. The pHvaried approximately one unit between saturating irra-diances and dark conditions, and the O2 level rangedbetween <70 and 206% of air saturation. The for-aminifer and its endosymbionts thus live in a dynamic

microenvironment of constantly shifting physico-chem-ical conditions.

The steep O2 and pH gradients from the shell to thebulk medium at higher irradiances (>150 lmol photonsm)2 s)1) (Fig. 7A, B) are caused by the high photosyn-thetic activity of the endosymbionts and the existence ofa DBL that surrounds the shell of the foraminifer(Jùrgensen et al. 1985). The DBL constitutes a barrierfor the mass transfer of gases, ions and other solutesbetween the foraminifer and the ambient seawater. Thethickness of the DBL around a sphere is generallymeasured by extrapolating the gradient of O2 at thesphere±water interface to the ambient seawater concen-tration (Jùrgensen and Revsbech 1985; Ploug et al.1997). While the DBL thickness around the shell ofOrbulina universa could be estimated in the dark(�200 lm) when the symbionts reside inside the shell,the DBL thickness in light could not be estimated by theextrapolation method due to the presence of the sym-biont swarm around the shell. The steady-state O2 gra-dients towards the shell in the light are thus a�ected bydi�usion as well as photosynthesis and respiration.

The relative importance of small-scale physical pro-cesses around the shell and between the spines (eddy andmolecular di�usion) is still unknown and should be in-vestigated to characterize the DBL in more detail. Dueto the presence of the calcite spines, the DBL probablyshows di�erent characteristics than a sublayer over asphere with a smooth surface (e.g. turbulent wakes)(Mann and Lazier 1991).

To understand zooxanthellae photosynthesis thescalar irradiance is the most relevant light-intensity pa-rameter (Kirk 1994; KuÈ hl et al. 1995). In hermatypiccorals, KuÈ hl et al. (1995) measured scalar irradiancepro®les with a ®ber optic microprobe and demonstratedthat the scalar irradiance could reach up to 180% of thedownwelling irradiance at the tissue surface. This in-crease was explained by multiple scattering and di�usere¯ection of light within the coral tissue±skeleton ma-trix. Our measurements showed a slight increase ofscalar irradiance towards the spherical shell of Orbulinauniversa that is probably caused by the combined scat-tering of the calcite spines and the re¯ection of light bythe spherical shell (Fig. 3C). The light measurementsthus demonstrated no signi®cant self-shading of thedino¯agellate cells inside the swarm.

Photosynthetic rates

The photosynthetic rates determined for Orbulina uni-versa are similar to published data. The photosyntheticproductivity of O. universa, when measured with the 14Cmethod (Spero and Parker 1985), showed a photosyn-thetic rate per symbiotic dino¯agellate of 1.72 pmolC h)1. Assuming an average symbiont density of about3.3 � 103 algal cells per adult O. universa (Spero andParker 1985), the total photosynthetic rate of a singleforaminifer would amount to a rate of 5.7 nmol C h)1.

Fig. 8 Net photosynthesis of Orbulina universa (nmol O2 h)1) as a

function of incident irradiance. The compensation light intensity, Ec,was found at 50 lmol photons m)2 s)1

Fig. 9 Photosynthesis and respiration rates of di�erent Orbulinauniversa specimens (No. I to V ) calculated in a percent of grossphotosynthesis

589

Globigerinoides sacculifer showed a mean gross photo-synthetic rate of 18 nmol O2 foraminifer)1 h)1 and anet photosynthesis of 15 nmol O2 foraminifer)1 h)1

(Jùrgensen et al. 1985). The carbon ®xation rates ofsymbiotic planktonic foraminifera collected in the sur-face waters near Bermuda ranged between 1.2 and4.2 nmol C h)1 foraminifer)1 (Caron et al. 1995). As-suming an O2/CO2 conversion ratio of unity, thesenumbers compare well with the rates measured in thepresent study.

During our experiments some zooxanthellae re-mained in the calcite shell. Because we only measuredthe gross photosynthesis towards the shell surface, wedid not record the O2 production inside the shell, whichmay not be negligible. Earlier measurements of thephotosynthetic rates inside the shell of the symbioticGlobigerinoides sacculifer showed a high O2 production.Jùrgensen et al. (1985) estimated an O2 production of3.8 nmol O2 h)1 inside the shell. Thus the total grossphotosynthetic rates per Orbulina universa specimen wereport here could be underestimated.

Due to the close coupling of photoautotrophic andheterotrophic processes in the symbiont-bearing fora-minifera, the photosynthesis measurements with the 14Cmethod show some disadvantages. Geider and Osborne(1992) pointed out methodological and interpretativeproblems of the 14C method, e.g. the impossibility tomeasure the light respiration as well as the transport ofcarbon between the intracellular carbon pools. In sym-biotic associations, the 14C method probably underesti-mates the production rates due to the production ofunlabeled CO2 by respiration (Michaels 1991). Here weestimated the photosynthesis and respiration rates ofOrbulina universa from O2 gradients and discrete mea-surements inside the symbiont swarm. Because we didnot determine the chlorophyll a content of the endo-symbionts and the number of endosymbionts, we pres-ent the rates on a per foraminifer basis.

The radial pro®les of gross photosynthesis inside thesymbiont swarm of Orbulina universa showed a signi®-cant increase towards the shell surface. This is due to thefact that the symbiont density increased towards theshell. When measurements of gross photosynthetic rateswere done by the light±dark shift technique, the zooxan-thellae tended to move back into the shell of O. universaafter a while. Our measurements of total gross photo-synthesis are based on point measurements with a spatialresolution of 50 to 100 lm. This means that the O2

production within the symbiont swarm was measuredfor a small volume around the electrode tip (Jùrgensenet al. 1985). Consequently, a change of the spatial dis-tribution of the zooxanthellae will a�ect the photosyn-thetic rates.

The variability of the gross photosynthetic rates isprobably due to several reasons. First, the symbiontphotosynthetic activity is a�ected by the available lightand the nutrient supply. Second, the number of sym-bionts and their distribution may play an important role.Spero and Parker (1985) observed a positive correlation

between the shell diameter and the symbiont number ofjuvenile Orbulina universa. The symbiont density de-pends on the rate of cell division of the endosymbiontsand on the age of O. universa. The dino¯agellate Gym-nodinium beÂii shows division rates of 0.65 d)1 (25 °C) inculture (Spero 1987). Although Spero and Parker (1985)could not determine a correlation between the size of theadult chamber and the number of symbionts, we ob-served a positive correlation between the size of thespherical shell and the total gross photosynthetic rate(Table 1). The specimen with the largest shell diametershowed the highest total gross photosynthesis. Lea et al.(1995) found that the shell diameter of O. universaspecimens is independent of age. Therefore, the diameterof the spherical shell can not serve as an estimate for age.The correlation between the total gross photosyntheticrate and the foraminiferal shell size as well as the num-ber of symbionts should be con®rmed in further studies,e.g. by detailed pigment analysis.

The total photosynthetic rates of the symbioticforaminifera can also be in¯uenced by the pigmentcontent of the symbiotic dino¯agellates. For exampleBijma (1986) studied the pigment composition of sym-bionts of Globigerinoides ruber and G. sacculifer andfound a �1.5 times higher chlorophyll a/carotenoid ratioin the symbionts of G. ruber. The type of endosymbiontsis a further important parameter a�ecting the totalphotosynthesis. In some planktonic foraminifera smallerchrysophyte symbionts occur in higher abundances thanthe bigger dino¯agellate symbionts (Caron et al. 1995).In addition, daily variations of the photosynthetic rateswere demonstrated in 14C-experiments with Orbulinauniversa (Spero and Parker 1985). The photosyntheticrates of the symbiotic dino¯agellates started to increasein the late morning and highest rates were found in thelate afternoon.

Light regulation of photosynthesis

Measurements of O2 pro®les and pH pro®les (Fig. 7A,B) showed a very dynamic response to the incident lightintensity, and experimental light±dark cycles demon-strated a rapid reaction of the symbionts to changingirradiances (Fig. 4). Light±dark cycle experiments inGlobigerinoides sacculifer showed similar O2 dynamics atthe shell surface (Jùrgensen et al. 1985).

The onset of light saturation of the symbiont pho-tosynthesis (Ek) was estimated in two specimens ofOrbulina universa of di�erent sizes. The Ek values werefound at irradiances of 75 and 137 lmol photons m)2

s)1, respectively. The di�erence is due to the di�erentmaximum photosynthetic rates (Pmax) of the two speci-mens because both photosynthesis versus irradiancecurves showed nearly identical slopes (a) of 0.067 and0.07 (Fig. 6A, B). The specimen with the higher Ek valuealso had a larger diameter (483 lm compared to297 lm). One explanation for the higher Ek is thus ahigher number of endosymbionts. However, higher Pmax

590

values can also indicate high growth irradiances (Herzigand Dubinsky 1992).

The study of photosynthesis versus irradiance curvesin several symbiotic systems reported Ek values between160 and 390 lmol photons m)2 s)1 (Jùrgensen et al.1985; Spero and Parker 1985; KuÈ hl et al. 1995). Glob-igerinoides sacculifer collected in the Gulf of Aquabashowed higher Ek values of 160 to 170 lmol photonsm)2 s)1 (Jùrgensen et al. 1985) as compared to Orbulinauniversa. 14C measurements of photosynthetic rates ofO. universa showed a much higher Ek value of 386 lmolphotons m)2 s)1 (Spero and Parker 1985). The onset oflight saturation at higher light levels demonstrates theadaptation of the symbionts to high irradiances in thesurface waters. An adaptation to high light exposure isalso indicated by the fact that no photoinhibitionwas observed in our study even at high irradiances(Fig. 6A, B).

The calculation of the onset of light saturation (Ek) isalso a�ected by the de®nition of the light ®eld parameter(KuÈ hl et al. 1995). Photosynthesis versus irradiancecurves plotted against the downwelling irradiance (P vsEd) result in a lower Ek compared to the photosynthesisversus scalar irradiance curves (P vs Eo) (KuÈ hl et al.1995). In our study, the Ek values estimated from theP versus Ed curves were only slightly lower due to thesmaller di�erence between Ed and Eo at the shell surface.However, scalar irradiance is always the most relevantlight ®eld parameter when measuring light regulation ofphotosynthesis on a microscale (KuÈ hl and Jùrgensen1994; KuÈ hl et al. 1994).

The light compensation point (Ec) is dependent ongross photosynthesis and respiration of the host±sym-biont system. In addition, processes that change thesymbiotic light respiration, e.g. the mitochondrial res-piration or photorespiration, may in¯uence the lightcompensation point. A change of the foraminiferal lightrespiration due to growth rate or prey digestion mayalso result in a change of the compensation light inten-sity. Respiration measurements of Orbulina universabefore and after feeding thus demonstrated an increaseof the respiration rate within a few hours after feedingwith 1-d-old Artemia nauplii (Rink, unpublished).Falkowski and Owens (1980) found a dependence of thelight compensation point on the irradiance level duringgrowth. The compensation light intensity of the symbi-otic Globigerinoides sacculifer was 26 to 30 lmol pho-tons m)2 s)1 (Jùrgensen et al. 1985). Compared toO. universa this lower compensation point is probablycaused by adaptation to lower irradiances (150 lmolphotons m)2 s)1) during maintenance in the laboratoryseveral days before measurements (Jùrgensen et al.1985).

Light measurements in full sunlight at the collectingsite showed irradiances up to 2070 lmol photonsm)2 s)1 at the surface and 556 lmol photons m)2 s)1 at12 m depth (S. Anderson, personal communication in1995). Depth pro®les of scalar irradiance (Eo) measuredat the collection site showed a mean light attenuation

coe�cient (Ko) of 0.07 (SD � 0.023) (Fig. 10). The lightcompensation point (Ec) of Orbulina universa would thusbe reached in a depth of ca. 50 m at the sampling site(Fig. 10). Theoretically, a net O2 production of thesymbiont±host system is possible down to this waterdepth at full sunlight. The O2 production of O. universaexhibited a pronounced light dependency (Fig. 7A), andhigh net primary production rates of the symbioticO. universa are limited to the regions of photosynthesis-saturating irradiances in the surface waters (0 to 35 m).

Primary production of planktonic foraminifera

The symbiont-bearing foraminifera constitute microen-vironments of concentrated photosynthetic activity(Caron and Swanberg 1990) and were reported to havethe highest rates of primary production in planktoncommunities because of the extremely high density ofthe endosymbiotic algae in their cytoplasm. Due to thehigh algal biomass, the amount of primary productionoccurring in the symbiont±host association is generallymuch higher than in an equivalent volume of seawater(Jùrgensen et al. 1985; Spero and Parker 1985).Jùrgensen et al. (1985) estimated that a single

Fig. 10 Depth pro®le of scalar irradiance (Eo) calculated with thesubsurface value Eo(0) � 1747 lmol photons m)2 s)1 and a lightattenuation coe�cient Ko � 0.07. Eo(0) was measured undersunny conditions in the California Current (Catalina Island) (Ek �137 lmol photons m)2 s)1, Ec � 50 lmol photons m)2 s)1). Insetshows log-transformed data

591

foraminifer would increase the CO2 ®xation rate in a125 ml productivity bottle ®vefold above the CO2 ®xa-tion in ambient seawater. Spero and Parker (1985) madethe assumption that a single large Orbulina universa mayrepresent a potential source of net primary productionthat would contribute approximately 0.2% of the ®xedcarbon in 1 m3 of seawater.

Although the associations are packages of high pro-ductivity it is still di�cult to estimate their total primaryproduction. The foraminiferal part of the total phyto-plankton primary production is dependent on theirpopulation density in the oceans (Be 1977). Their pro-ductivity depends on the population dynamics and thepatchiness of foraminifera. The distribution of mostspecies shows a correlation with sea surface tempera-tures (Bradshaw 1959). Currents and mixing of surfacewaters may also cause a change of the foraminiferaldistribution. Changes of the depth habitat due to thelunar periodicity of the reproductive cycle were reportedby Hemleben and Spindler (1983).

Diurnal variations of the depth habitat, rising of theforaminifera during the daytime and falling in the night,are discussed by Berger (1969) and Boltovskoy (1973).Bradshaw (1959) suggested that the rapid production ofO2 by the symbiotic algae in the protoplasm could formoxygen bubbles that increase the buoyancy of the fora-minifera during the day and could cause a rising to thesurface. Fairbanks and Wiebe (1980) observed a maxi-mum abundance of planktonic foraminifera in the deepchlorophyll maximum layer (DCM) with changing sea-sonal depth levels. They suggested that the foraminiferaexploit the DCM as a major source for food and nu-trients. Population studies of Be et al. (1985) showedseasonally changing abundances of planktonic fora-minifera in the Panama Basin. Because of this verticaland horizontal distributional patchiness, the estimationof planktonic foraminifer primary production is di�cultand only possible for small oceanic areas that are wellstudied.

To calculate the net primary production of Orbulinauniversa from our microsensor data we assumed a den-sity of ®ve specimens per cubic meter (Spero and Parker1985). An average net photosynthetic rate of 5 nmol O2

h)1 foraminifer)1 (Table 1) over a daily light exposure ofabout 10 h would result in a production of 0.25 lmol O2

m)3 d)1 at light saturation. For the same parametersJùrgensen et al. (1985) found a three times higher pri-mary production of 0.75 lmol O2 m)3 d)1 for Glob-igerinoides sacculifer in the Gulf of Aquaba. The wholepopulation of G. sacculifer would contribute about 0.1%of the mean yearly primary production in the Gulf.Caron et al. (1995) reported that the total symbiontproduction of sarcodines (Acantharia, Radiolaria,Foraminifera) in oligotrophic waters of the Sargasso Seacontributes only a small fraction (<1%) of the totalprimary production. They found production rates ofacantharia and foraminifera to contribute with an av-erage of �5% to the total annual primary production inthe surface waters. A vertical biomass distribution for

foraminifera was given by Michaels (1991) who formu-lated a depth-dependent relationship for symbiont pro-ductivity that is related to the exponential decline of thelight ®eld.

The percentage of the total primary production ofplanktonic foraminifera in 1 m3 of seawater is probablyoverestimated, and the production rates are more vari-able because several parameters limit the primary pro-duction rates as mentioned before. Symbiont densitiesand productivities as well as light exposure and nutrientsupply in¯uence the maximum net O2 production. If theplanktonic foraminifera change their depth habitat dueto vertical migration, light will be a limiting factor.

There are still open questions about the nutritionalrelationship in the foraminifer±dino¯agellate symbiosis.For instance, which kind of photosynthates are releasedby the dino¯agellates and how much of the primary®xed carbon is translocated to the host. With regard tothe predation on plankton the signi®cance of the pho-tosynthate supply for the energy budget of the host willbe of great interest. Due to the vertical ontogenetic mi-gration of the planktonic foraminifera a combination oftwo energy sources, planktonic prey and photosyn-thates, is probably of importance. Detailed investiga-tions of the migration patterns and changing abundancesof Orbulina universa and other species in the water col-umn would help to provide more information abouttheir total primary productivity (Hemleben and Bijma1994).

Respiration in light and darkness

One advantage of the microsensor technique comparedto other methods is the possibility to estimate the res-piration rate of the symbiont±host system in the light.We were able to calculate the light respiration of aO. universa by measuring the total gross photosynthesisand the net photosynthesis of the same specimen. Directcomparison of dark and light respiration rates wastherefore possible. In the light, we found higherrespiration rates of the symbiont±host association(3.9 nmol O2 h

)1 foraminifer)1) compared to the darkrespiration (1.7 nmol O2 h)1 foraminifer)1). This en-hanced respiration in the light has been described forseveral symbiotic systems (Edmunds and Davies 1988;Harland and Davies 1995; KuÈ hl et al. 1995) and formicroalgae (Falkowski et al. 1985; Grande et al. 1989).Di�erent mechanisms have been discussed to explain theenhanced respiration in the light (Falkowski et al. 1985;Weger et al. 1989).

The respiration of the host is enhanced in the light viathe production of photosynthates by the dino¯agellateendosymbionts. Symbionts of larger benthic foramini-fera have been shown to release soluble photosynthateslike polyglucan, glycerol, glucose and lipids (Kremeret al. 1980). The zooxanthellae probably increase thequantity of respiratory substrates translocated to thehost in the light. The tissue of larger foraminifera con-

592

tains some activating factors that stimulate the release ofthe photosynthates. Lee et al. (1984) found that the levelof the photosynthate release of isolated endosymbiontsincreased dramatically in the presence of host homoge-nates. Due to the supply of carbohydrates and lipids bythe endosymbionts foraminiferal respiration can thus bestimulated in the light.

Photosynthesis results in O2 supersaturation aroundthe foraminiferal shell, which may stimulate the respi-ration of the symbionts and the foraminifer. This in-ternal O2 supply alleviates the di�usion limitation due tothe presence of the DBL. Experiments showed increaseddark respiration when the symbiotic sea anemone An-emonia viridis was exposed to hyperoxic water (Harlandand Davies 1995). These authors suggested, therefore,that the day time respiration is in¯uenced by the O2

release of the endosymbionts. Also, Jùrgensen et al.(1985) suggested that the limited O2 supply in thedarkness due to the presence of the DBL caused reduceddark respiration rates. They measured a decrease of theO2 at the shell of Globigerinoides sacculifer down to 50%of air saturation in darkness. In Orbulina universa wefound an O2 decrease to the shell surface down to 67%air saturation during darkness.

A higher O2 consumption in the light can also becaused by photorespiration. Photorespiration is de®nedas a light-dependent O2 uptake and CO2 release due tothe bifunctional enzyme Rubisco (Falkowski et al. 1985;Beardall and Raven 1990). The high O2/CO2 ratio pro-duced by the photosynthesis of the zooxanthellae couldpromote the oxygenase activity of Rubisco. However, ane�cient inorganic carbon uptake mechanism present inmost microalgae seems to be able to decrease the im-portance of photorespiration (Beardall and Raven1990). To our knowledge no investigation of photores-piration or inorganic carbon uptake by the foraminiferalsymbionts has been reported in the literature.

In principle the pseudocyclic photophosphorylation(Mehler reaction) represents another light-induced O2-consuming process (Raven and Beardall 1981; Falkow-ski et al. 1985). However, Glud et al. (1992) suggestedthat the measurement of gross photosynthetic rates withthe light±dark shift method probably does not includethe O2 consumed by the Mehler reaction.

Due to the limitation of the 14C method to measurerespiration in the light, some authors investigated thedark respiration after exposure to high irradiances. Thisprocess of post-illuminated O2 consumption in thedarkness has been discussed for di�erent microalgae(Burris 1977; Falkowski et al. 1985; Weger et al. 1989;Beardall et al. 1994) as well as for symbiotic sea anem-ones (Harland and Davies 1995) and corals (Edmundsand Davies 1988). Burris (1977) obtained a post-illumi-nation burst of oxygen uptake in the dino¯agellateGlenodinium sp. and in the zooxanthellae of the coralPocillophora capitata that lasted about 5 to 10 min. Thedino¯agellates showed a longer post-illumination burstcompared to other algae (1 to 2 min). Burris (1977) ex-plained this increase by the possibility of a di�erent

photorespiratory pathway or by higher dark respirationrates. Beardall et al. (1994) demonstrated that low-light-adapted cells of Thalassiosira weiss¯ogii were moresusceptible to the enhanced post-illumination respirationcompared to cells grown under high light conditions.Harland and Davies (1995) found a stimulation of darkrespiration of 39% after 6 h exposure to saturating ir-radiance (300 lmol photons m)2 s)1). The reef coralPorites porites showed a mean increased dark respirationrate of 39% relative to the pre-illumination dark respi-ration rate (Edmunds and Davies 1988).

The estimation of the light respiration with the mi-crosensor technique showed much higher respirationrates during light conditions compared to the dark res-piration rates (KuÈ hl et al. 1995). Jùrgensen et al. (1985)measured for Globigerinoides sacculifer a similar respi-ration rate in the light (3.0 nmol O2 h

±1 foraminifer±1) aswe did forOrbulina universa, but they did not ®nd a lowerdark respiration (2.7 nmol O2 h

)1). In O. universa, wefound a two times lower dark respiration (1.7 �0.7 nmol O2 h

)1, n� 24). If we assume higher total pho-tosynthetic rates per foraminifer due to additional O2

production inside the shell, the di�erence between respi-ration rates in the light and darkness may be even larger.Generally, the dark respiration rates of microalgae are onthe order of 10% of the gross photosynthesis (Beardalland Raven 1990). In our study we measured a mean totaldark respiration of the symbiont±host association of1.7 nmol O2 h

)1 and an average total gross photosyn-thetic rate of 8.9 nmol O2 h

)1. If we assume that 50% ofthe total O2 uptake is due to symbiont respiration(Jùrgensen et al. 1985), the dark respiration rate of thezooxanthellae is nearly 10% of the gross photosynthesis.

The P/R ratio is used to estimate the physiologicalstate of marine microalgae and to scale the relationshipof consumption and production of organic material(Burris 1977). This ratio has been investigated for sev-eral algal species, and the numbers for dino¯agellatesvaried between 1.3 and 5.7 (Humphrey 1975; Burris1977; Daneri et al. 1992). The zooxanthellae of coelen-terate hosts can supply most of the carbon required bythe host, as was demonstrated in 70 species of coralswith P/R ratios of 2.4 � 1.5 (Battey 1992). In our studywe measured a mean net photosynthesis of 5.0 nmol O2

h)1 during light saturation and an average dark respi-ration of 1.7 nmol O2 h

)1 foraminifer)1. Consequently,the Pnet/Rdark ratio of the symbiont±host system is about3, which indicates that the required carbon for the for-aminifer can be supplied by its symbionts. However, toestimate if the net primary production of the endo-symbionts can provide the required organic carbon forgrowth and respiration of the symbiont±host associa-tion, the total net photosynthesis over the daily lightperiod as well as the growth rates and the respirationrates of the host and the symbionts have to be calculatedon a daily basis.

It has been suggested that foraminifera supply theirendosymbionts with respired CO2 (Be 1977). The respi-ration of Orbulina universa in the light showed an

593

average rate of 48% of the gross photosynthesis. Thisvalue demonstrates a much higher CO2 availability forthe symbionts as compared to free-living dino¯agellates.Geider and Osborne (1989) reported that a dark respi-ration versus photosynthesis rate of 0.25 is generallyfound in dino¯agellates. Orbulina universa can, thus,supply its endosymbionts with additional CO2, whichmay support photosynthetic CO2 ®xation. However,recent model calculations (Wolf-Gladrow et al. inpreparation) as well as laboratory experiments (Bijmaet al. in preparation) demonstrate that Gymnodinium beÂiiin symbiosis with O. universa as well as isolated in cul-ture also tap into the bicarbonate pool as a carbonsource.

Conclusions

Microsensors are useful tools for studying photosyn-thetic processes in symbiotic systems and for comparinglight and dark respiration rates. The respiration ofOrbulina universa in the light was signi®cantly higherthan dark respiration. Possible mechanisms for thisobservation might be the increase of respiratory sub-strates (photosynthates) released by the symbionts and/or photorespiration.

Varying incident irradiances caused dynamic changesof the symbiont photosynthetic activity that a�ected thechemical microenvironment around the foraminiferalshell. High photosynthetic rates in combination with aslow e�ux of O2 and protons due to the di�usiveboundary layer created an O2 oversaturation and a pHincrease in the foraminiferal microenvironment ascompared to the ambient seawater. The symbiotic as-sociations of Orbulina universa thus represent highlyproductive ``hot spots'' in the light-saturated photic zoneof oligotrophic pelagic environments.

To understand the complexity of interactions be-tween photosynthesis, respiration and calci®cation insymbiotic foraminifera, new methods have to be ex-plored. A new CO2 microsensor (de Beer et al. 1997)could provide more information about CO2 uptake anddynamics. Furthermore the CO2 microsensors could beused in combination with Ca2+ microelectrodes (Tsienand Rink 1980; Amman et al. 1987) to investigate theprocess of calci®cation in symbiont-bearing foramini-fera.

Acknowledgements We thank B. Bemis, M. Uhle, T. Mashiotta,J. Daily, C. Hamilton and L. Komsky for their help in the labo-ratory and for collecting foraminifera. The sta� of the WrigleyInstitute for Environmental Studies is thanked for the support andthe laboratory facilities. Thanks are due to A. Eggers andG. Eickert for the construction of the microelectrodes and toS. Anderson for providing the light measurement data. We thankT. Mashiotta and E. Meesters for providing the photographs. Thisresearch was ®nanced by the Max-Planck Gesellschaft, Germanyand bene®tted from the Program for the Advancement of SpecialResearch Projects at the Alfred-Wegener-Institute, Germany(J. Bijma). We also acknowledge the support from the US NationalScience Foundation Grants (OCE 94-16595 and OCE 9415991)

awarded to H.J. Spero (UCD) and D.W. Lea (UCSB). This isWIES Contribution No. 201, SFB Contribution No. 174 and AWIContribution No. 1412.

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