Toue et al.
Progress in Earth and Planetary Science(2022) 9:20
https://doi.org/10.1186/s40645-022-00478-3

RESEARCH ARTICLE

Progress in Earth andPlanetary Science

Open Access

Trophic niche separation of two non‑spinose
planktonic foraminifers Neogloboquadrina
dutertrei and Pulleniatina obliquiloculata
Ryuji Toue1, Kazuhiko Fujita1,2* , Masashi Tsuchiya3, Yoshito Chikaraishi4,5, Yoko Sasaki4 and Naohiko Ohkouchi4

Abstract
Based on laboratory observations, planktonic foraminifers are omnivorous, feeding zooplankton and phytoplankton.
Spinose species tend toward greater dependence on zooplankton prey than on phytoplankton prey, while nonspinose species are more adapted to herbivorous diets. However, the trophic activity of planktonic foraminifers in
the natural environment and their trophic positions in the marine food web have not yet been fully understood.
The trophic position (TP) of two non-spinose species, Neogloboquadrina dutertrei and Pulleniatina obliquiloculata,
was determined by differences in the nitrogen isotopic composition between two amino acids (glutamic acid andphenylalanine). Results show that TP values of N. dutertrei were ~ 2.4, indicating dependence on omnivorous (mixed
herbivorous and carnivorous) diets, while those of P. obliquiloculata were ~ 2.1, indicating dependence on herbivorous
diets. Together with previous laboratory observations, these TP values suggest that N. dutertrei is a detritivore or scavenger, while P. obliquiloculata is generally a herbivore. This trophic niche separation likely allows these two planktonic
foraminiferal species to live within a similar depth zone in the open water column and provides a clue for understanding causes of spatial and temporal changes in their relative abundances in living and sediment assemblages.
Keywords: Amino acid, Herbivore, Nitrogen isotopic composition, Omnivore, Planktonic foraminifers, Trophic
position
1 Introduction
Planktonic foraminiferal species and assemblages areuseful biological proxies for paleo-sea surface water
properties (e.g., Berger 1969; Fischer and Wefer 1999;
Kucera 2007) because the distribution and abundance ofplanktonic foraminiferal species are mainly controlled byenvironmental factors such as sea surface temperature,salinity, dissolved oxygen, light attenuation, and nutrient availability (e.g., Hemleben et al. 1989; Murray 1991;
Watkins et al. 1998; Kuroyanagi and Kawahata 2004;
Schiebel and Hemleben 2017). Biological factors such as
the abundance of prey and predators may also control the*Correspondence: fujitaka@sci.u-ryukyu.ac.jp
1
Department of Physics and Earth Sciences, Faculty of Science, Universityof the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903‑0213, Japan
Full list of author information is available at the end of the article

distribution and abundance of planktonic foraminifers,
especially on regional and seasonal scales dominated by
upwelling (Thiede 1975; Ortiz et al. 1995; Schiebel et al.
2004). Laboratory observations of trophic activity haverevealed that planktonic foraminifers are basically known
as omnivorous, feeding zooplankton and phytoplankton.
Spinose species tend toward greater dependence on zooplankton prey than on phytoplankton prey, while nonspinose species are more adapted to herbivorous diets(e.g., Anderson et al. 1979; Spindler et al. 1984). Diet preferences might also be diverse among different genera andspecies to avoid diet competition in the same water column. Such trophic niche separation might partly explain
the diversity and distribution of planktonic foraminifers
living in the surface to subsurface water column andtheir speciation among different species within a genus
or among different genotypes within a morphospecies© The Author(s) 2022. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://​creat​iveco​mmons.​org/​licen​ses/​by/4.​0/.

Toue et al. Progress in Earth and Planetary Science(2022) 9:20

(Weiner et al. 2012). However, the trophic activity ofplanktonic foraminifers in the natural environment andtheir trophic positions in the marine food web have notyet been fully understood.
Trophic position can be estimated based on the nitrogen isotopic composition (δ15N) of two amino acids (glutamic acid and phenylalanine) in consumer organisms(Chikaraishi et al. 2009). Each amino acid experiences
different isotopic fractionation during amino acid metabolism. Glutamic acid shows a large δ15N enrichment from
one trophic level to the next, whereas phenylalanine
shows little change in the δ15N value between trophic
levels. The use of the δ15N differences between these twoamino acids is a powerful tool for elucidating the trophic
position of organisms in aquatic ecosystems with anerror of 0.1–0.2 units (Chikaraishi et al. 2009). The traditional trophic position estimation techniques that relyon the δ15N values of bulk consumer tissues are sensitive to background isotopic variation between the basal
resources of a food web and are hampered by spatial andtemporal variations in the δ15N value of primary producers. In contrast, the trophic position based on δ15N
values of glutamic acid and phenylalanine from a single
consumer is independent of such factors and successfully applied in studying the trophic position of aquaticorganisms (Chikaraishi et al. 2014; Ohkouchi et al. 2017).
Another advantage of the amino acid δ15N approach isthat it permits analyses of small specimens (2 nmol foreach amino acid; Chikaraishi et al. 2009), which allows us
to assess the trophic functions of innumerable meiofaunasuch as foraminifers (Tsuchiya et al. 2018).
This amino acid δ15N approach differs from a recently
advanced planktonic foraminiferal shell-bound δ15N
approach, which measures δ15N in organic matter boundwithin the calcareous shells (e.g., Ren et al. 2009, 2012;
Schiebel et al. 2018; Smart et al. 2018, 2020). Planktonic
foraminifers are expected to track the δ15N of the organic
matter produced in the surface ocean. The δ15N valuesof bulk tissue and shell-bound nitrogen are similar inabsolute value and vary together, supporting the use ofshell-bound nitrogen as a recorder of upper ocean δ15N
changes (Smart et al. 2018). In oligotrophic subtropicalsites, shell-bound δ15N values in modern surface sediments and net tows are strongly correlated with the δ15N
values of thermocline (i.e., shallow subsurface water)
nitrate (Ren et al. 2012; Smart et al. 2018, 2020). Thus,­N2 fixation and denitrification changes are recorded inforaminiferal shell-bound δ15N.
In this paper, we apply the amino acid δ15N approachto determine the trophic position of two non-spinose
planktonic foraminiferal species, Neogloboquadrina
dutertrei (d’Orbigny, 1839) and Pulleniatina obliquiloculata (Parker and Jones, 1865), both of which are common

Page 2 of 11

in tropical to subtropical, warm subsurface water aroundthe deep chlorophyll maximum (DCM) and generally
herbivorous (Hemleben et al. 1989; Schiebel and Hemleben 2017). Results of this study imply the trophic niche
separation of coexisting planktonic foraminifers in thesubsurface water column and a clue for understanding
causes of spatial and temporal changes in their relative
abundances in living and sediment assemblages.

2 Materials and methods
2.1 Hydrographic data and plankton tow samples

Hydrographic data and plankton tow samples were collected from 0 to 200 m in water depth during the R/V
Tansei-Maru cruise (KT-11-25) in the afternoon on
October 14, 2011 (full moon phase), at the mouth of
Suruga Bay, Japan (34°38.989′N, 138˚33.045’E), where
the Kuroshio Current (warm current) flowed off southof Kouzushima Island to the northeast (Fig. 1). Hydrographic data including temperature, salinity, density,
dissolved oxygen, and chlorophyll a (Chl a) were measured near the plankton tow sampling site by a conductivity temperature depth sensor (CTD; SBE 9, Sea-Bird
Scientific) and a fluorometer (Aquatracka MkIII, Chelsea
Technologies Group Ltd.). The upper 200 m was sampled twice using a Vertical Multiple Plankton Sampler(VMPS, Tsurumi-Seiki Co. Ltd.). The tow sampler has a0.25-m2 opening and 100-μm mesh size (NXX13). Fourdepth intervals (0–20, 20–50, 50–100, and 100–200 m)
were sampled during each tow. The samples were fixedwith seawater-buffered 5% formalin in a 50-mL vial. Formalin fixation does not affect δ15N values of amino acids
derived from an aquatic consumer (Ogawa et al. 2013).
Samples were poured into petri dishes. Planktonic foraminifers alive at the time of sampling, which were identified by the presence of cytoplasm, were picked from a wet
solution by pipetting using a Pasteur pipette, mounted ona microfossil slide, identified, and counted to the species
level. Microphotographs of five common species (Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globigerina bulloides d’Orbigny, 1826, Trilobatus sacculifer(Brady, 1877), Globigerinoides ruber white (d’Orbigny,
1839)) were taken using a digital camera attached to abinocular dissecting microscope. The maximum diameter was measured using Image J (NIH) to estimate thesize-frequency distribution of each species at each depth
interval (Additional file 1: Fig. S1). Since other zooplankton were more abundant than planktonic foraminifers,
the remaining samples were divided into 2–5 mL subsamples by pipetting from stirred and homogenized samples according to methods described in Omori and Ikeda(1976). Zooplankton alive at the time of sampling, whichwere identified by the presence of organic matter, were
picked from a wet solution by pipetting using a Pasteur

Toue et al. Progress in Earth and Planetary Science(2022) 9:20

Page 3 of 11

35.5˚N
20
50
Japan
PO

35˚N

100

ShizuokaPHS

Cape
Omaezaki

Suruga
Bay

200

Izu
Peninsula

500
1000
1500

34.5˚N34˚N
137.5˚E

KouzushimaIsland138˚E138.5˚E

139˚E

Ocean Data View

2000

139.5˚E

2500
3000
(m)

Fig. 1 Location of a plankton tow site (white circle) in Suruga Bay, Japan. Orange line in the insert map indicates the Kuroshio current at the time ofsampling. PO Pacific Ocean, PHS Philippine Sea

pipette and stored in a glass vial filled with seawater-buffered 5% formalin. The zooplankton were identified andcounted (Suidosha, Co., Ltd.) and finally classified to theorder level. Counts of planktonic foraminifers and zooplankton were converted to the standing stock (SS; thenumber of individuals ­m−3 seawater) using the following
equations:

SSplanktonic foraminifers = nsample ÷ vseawater(1)

SSzooplankton = nsubsample ÷ vsubsample × vsample ÷ vseawater(2)
where n is the count in a sample or subsample (individuals), vsubsample is the volume of a subsample (2–5 mL),
vsample is the volume of a sample (50 mL), and vseawater
is the volume of seawater. Since the volume of seawater filtered by a plankton net was not obtained using aflow meter, vseawater is calculated by the opening area of aplankton sampler (0.25 m­ 2) multiplied by the water depth
interval (20–100 m).
In order to estimate the abundance of particulateorganic matter (POM), the remaining residue excluding
planktonic foraminifers and zooplankton (referred to as
residue POM) and another 5-mL subsample (referred to

as total POM) were filtered, wet weighed, and converted
to the wet weight of POM (mg ­m−3) using the following
equation:

POM = wsubsample ÷ vsubsample × vsample ÷ vseawater(3)
where wsubsample is the mass of POM in a subsample.
In order to correlate the standing stock data withenvironmental variables, the median of environmental variables such as temperature, salinity, density, dissolved oxygen, and Chl a, was calculated for each depth
interval. Standing stock data of planktonic foraminifers(N. dutertrei, P. obliquiloculata) and zooplankton (Calanoida, Cyclopoida, Poecilostomatoida, Harpacticoida,all copepods, other zooplankton, all zooplankton) andthe abundance of POM (residue and total) were squareroot transformed. Transformed standing stock dataof planktonic foraminifers were correlated with othervariables (environmental, zooplankton, POM) using
Pearson’s correlations. In addition, the standing stock
data were analyzed with a linear model using Chl aand total POM as explanatory variables to examine theeffects of biological variables. These statistical analyses
were conducted in R 4.1.1 (R Core Team 2021). We also

Toue et al. Progress in Earth and Planetary Science(2022) 9:20

calculated correlations between the standing stock data
and the mean of environmental variables. The correlation results using the mean were similar to those using
the median.
2.2 Amino acid nitrogen isotope analysis

In this study, we measured δ15N values of the bulk cells(i.e., the sum of the cell cytoplasm, organic membranes,
and intracrystalline protein in a shell) of fixed specimensof N. dutertrei and P. obliquiloculata. Since the amount of
intracrystalline proteins is fewer than the cell cytoplasm,
δ15N values in the bulk cells would be expected to indicate the short-term value of their metabolism. Due to the
low number of specimens, fixed specimens from two replicate samples for each water depth interval were pooled
for the bulk cell analysis (i.e., one pooled sample of eachwater depth interval was measured for each species). The
number of N. dutertrei and P. obliquiloculata specimens
measured for each water depth interval ranged from 18
to 353 and 14 to 137, respectively. Measured test size
ranged from 150 to 650 μm for all depth intervals of both
species.
The δ15N value of amino acids was determined according to Chikaraishi et al. (2009). Briefly, each specimen
was hydrolyzed in 12 M HCl at 110 °C, and then, thehydrolyzate was washed with n-hexane/dichloromethane (3:2, v/v) to remove any hydrophobic constituents.
After derivatization with thionyl chloride/2-propanol(1:4, v/v) and subsequently with pivaloyl chloride/
dichloromethane (1:4, v/v), the derivatives of the aminoacids were extracted with n-hexane/dichloromethane(3:2, v/v). The δ15N value of individual amino acids wasdetermined by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) using a­DeltaplusXP isotope ratio mass spectrometer (Thermo
Fisher Scientific, Bremen, Germany) coupled with a
6890 N gas chromatograph (Agilent Technologies, Santa
Clara, CA, USA) via combustion and reduction furnaces. δ15N value is defined as δ15N = ­([15N/14N]sample/[15N/14N]Air − 1) × 1000 and expressed in conventional(‰) against that of air (Air).
The trophic position ­(TPGlu/Phe) of the sample was calculated based on the following equation proposed by
Chikaraishi et al. (2009):

TPGlu/Phe =δ 15 NGlu − δ 15 NPhe − 3.4 /7.6 + 1

(4)
15
15
15
where δ NGlu and δ NPhe represent the δ N values ofglutamic acid and phenylalanine, respectively, 3.4 is theisotopic difference between glutamic acid and phenylalanine in primary producers, and 7.6 is the offset of thetrophic discrimination factor of these two amino acids
per trophic position increase. Although this equation

Page 4 of 11

may need modification (particularly for the trophic discrimination factor) for some specific organisms, suchmodification is not required for organisms in the lowertrophic-level hierarchy of food webs (McMahon and
McCarthy 2016). The trophic position is expected to be
1.0 for a “pure” primary producer and 2.0 for a “pure” primary consumer.
Propagation error (potential uncertainty) in T
­ PGlu∕Phevalue has been calculated by taking into account the
propagation of 1σ for δ15NGlu, δ15NPhe, the isotopic difference between glutamic acid and phenylalanine inprimary producers, and the offset of the trophic discrimination factor (TDF) of these two amino acids per
trophic position increase in Eq. (4) (Chikaraishi et al.
2009, 2014), and is expressed in the following equation(Kruse et al. 2015):


1/2
1σTL =
y2 /7.6x + (1σTDF /TDF)2(5)
where x = δ15NGlu − δ15NPhe − 3.4, y = [2(1σm)2 + (1σβ)2]1/2,
1σm = 0.5‰, 1σβ = 0.9‰, and 1σTDF = 1.2‰, after Chikaraishi et al. (2009). Previous studies have indicated thatthe potential uncertainty in the T­ PGlu/Phe value calculated
via propagation of error is 0.2–0.4 for each trophic levelbased on an assumed standard deviation of 0.5‰ (1σ) for
the observed δ15N values of glutamic acid and phenylalanine (Chikaraishi et al. 2009).

3 Results
3.1 Environmental variables

Water temperature was relatively stable (approx. 22.7 °C)
at 0–40 m depth, and the thermocline was located at
40–60 m depth (Fig. 2, Additional file 2: Table S1). Salinity was stable (approx. 34.0) at 0–40 m depth, and thepycnocline was located at 40–60 m depth (Fig. 2). Chl aconcentrations were 0.3 μM on average at 0–49 m depth,
showed the highest peak (1.1 μM) at 50 m depth, with the
deep chlorophyll maximum (DCM) being located in thecenter of the pycnocline, and then gradually decreased
down to 0.04 μM at a deeper depth. Dissolved oxygen
was stable (4.7 ml ­L−1 on average) at 0–40 m depth andthen decreased at 40–60 m across the pycnocline.
3.2 Planktonic foraminifers

A total of 11 planktonic foraminiferal species were identified (Fig. 3, Additional file 2: Table S2). The assemblage
was mainly composed of N. dutertrei, P. obliquiloculata, G. bulloides, T. sacculifer, and G. ruber white. Of
these five species, N. dutertrei, P. obliquiloculata, and G.
bulloides were common at all the water depths sampled
and account for > 90% of the total foraminiferal standing

Page 5 of 11DO(ml L-1)

1.5
3
3.5
4
4.5
5

1

Chlorophyll a
(µM)
0.5

Salinity

35
0

25
34

20

15

Temperature
(°C)

23
24
25
26
27
10

Density(kg m-1)(2022) 9:20

34.5

Toue et al. Progress in Earth and Planetary Science

0
10
20
30
40
50
60

Water depth (m)

70
80
90
100
110
120
130
140
150
160
170
180
190
200
Fig. 2 Hydrographic conditions at the plankton tow site in Suruga Bay, Japan. A gray zone in a indicates the thermocline/halocline/pycnocline,
while a dashed line indicates the deep chlorophyll maximum (DCM)

stock. The standing stock of planktonic foraminifers washigh in the upper 50 m. The highest value (~ 50 individuals ­m−3) was recovered in the upper 20 m. N. dutertrei
and P. obliquiloculata were more common in the upper
50 m but decreased at water depth below 50 m. The
standing stock of G. bulloides was reasonably uniform(2.8 individuals ­m−3) throughout the water depths sampled. T. sacculifer and G. ruber white were low in abundances (~ 1 individual ­m−3) throughout the water depths
sampled.

3.3 Zooplankton and particulate organic matters

Zooplankton, excluding planktonic foraminifers,
were abundant in the upper 50 m, gradually decreasing at deeper depth (Fig. 4, Additional file 2: Table S3).
Copepods were the most common zooplankton,
accounting for more than 80% of the total zooplankton standing stock. Of the copepod assemblage, Calanoida, Cyclopoida, and Poecilostomatoida were morecommon than other taxa, comprising more than 95%
of the assemblage. Decreasing zooplankton abundanceToue et al. Progress in Earth and Planetary Science(2022) 9:20

Page 6 of 11

Standing stock (individuals m-3)
0

10

20

30

40

50

2

Water depth & Sample ID

0–20 m3
5

20–50 m6

50–100 m

8
9Neogloboquadrina dutertrei
Pulleniatina obliquiloculata
Globigerina bulloides
Trilobatus sacculifer
Globigerinoides ruber white
Others12
100–200 m
13

Fig. 3 Standing stocks of living planktonic foraminifers at four depth intervals (0–20, 20–50, 50–100, and 100–200 m; two replicates) in Suruga Bay,
Japan

Standing stock (individuals m-3)
0

200

400

600

800

1000

1200

2

Water depth & Sample ID

0–20 m3
5

20–50 m68
50–100 m
912
100–200 m
13

Calanoida
Cyclopoida
Poecilostomatoida
Harpacticoida
Other Copepoda
Other zooplanktons

Fig. 4 Standing stocks of living zooplankton at four depth intervals (0–20, 20–50, 50–100, and 100–200 m; two replicates) in Suruga Bay, Japan

with depth was mostly explained by the standing
stocks of Calanoida and Poecilostomatoida, both ofwhich showed the maximum abundance of 382 and294 individuals ­m−3 in the upper 50 m, respectively.
Total POMs were also abundant in the upper 50 m(~ 213 mg ­m−3), decreasing at deeper depth (Additional
file 2: Table S3).

Toue et al. Progress in Earth and Planetary Science(2022) 9:20

Page 7 of 11

The standing stock of N. dutertrei was highly correlatedwith most environmental and biological variables (> 0.9;
Additional file 2: Table S4). The standing stock was highly
correlated with temperature (0.98), salinity (− 0.91), and
Chl a (0.97), while it was weakly correlated with standing
stocks of Cyclopoida (0.57) and Harpacticoida (0.46). The
standing stock of P. obliquiloculata was highly correlatedwith most environmental and biological variables (> 0.8).
In particular, the standing stock was highly correlatedwith that of Poecilostomatoida (0.93), temperature (0.88),
salinity (− 0.81), and Chl a (0.86), while it was weakly
correlated with residue POM (0.57), standing stocks of
Cyclopoida (0.46) and Harpacticoida (0.43).
Linear models indicate that standing stocks of both
species were significantly affected by Chl a (N. dutertrei,
p < 0.0001; P. obliquiloculata p < 0.05), but not by total
POM (ns, Table 1). Both species were highly correlatedwith Chl a concentration distribution over the upper
water column (Fig. 5). Linear regression analysis indicates that the standing stock of N. dutertrei was explainedas NN. dutertrei = 13.65 Chl a + 0.53 (R2 = 0.95, adjusted
R2 = 0.94, p < 0.00005), while that of P. obliquiloculata was
explained as NP. obliquiloculata = 6.75 Chl a + 0.64 (R2 = 0.74,
adjusted R2 = 0.70, p < 0.006).
3.5 Nitrogen isotopic composition of amino acids
and estimated ­TPGlu/Phe

Observed ­TPGlu/Phe values are averages based on bulk
specimens of each foraminiferal species and characterized by a low propagation error (potential uncertainly)(Table 2, Additional file 2: Table S5). However, since thepropagation error was significant (0.67) for P. obliquiloculata at 100–200 m depth, the value was not used forfurther discussion. ­TPGlu/Phe values for bulk specimensof both species were similar through the water column(Fig. 6). The T­ PGlu/Phe values for bulk specimens of P.
obliquiloculata (~ 2.1) were lower than the values forbulk specimens of N. dutertrei (~ 2.4). The exception was­TPGlu/Phe values at 20–50 m depth, similar for both species (~ 2.3).

Standing stock [(individuals m-3)0.5]

3.4 Correlations and regression with biological variables

6
5
4
3
2
1
0

Neogloboquadrina dutertrei
Pulleniatina obliquiloculata
0

0.05

0.1

0.15
0.2
0.25
0.3
Chlorophyll a (µM)

0.35

0.4

Fig. 5 Correlations of standing stocks of two non-spinose planktonic
foraminiferal species (Neogloboquadrina dutertrei and Pulleniatina
obliquiloculata) with chlorophyll a concentration

4 Discussion
The water column in the study area consisted of three layers including (1) the surface mixed layer, (2) the thermocline/halocline/pycnocline at the 40–60 m depth interval
comprising the DCM, and (3) the subsurface layer below
60 m depth. These observations are consistent with previously reported water mass structure in Suruga Bay, wheremixed coastal and outer surface water (0–50 m depth)
covers outer Kuroshio water characterized by high salinity (100–200 m depth) (Nakamura and Muranaka 1979).
The surface mixed layer and the thermocline/halocline/
pycnocline are warmer and more productive than thesubsurface water column and accommodate the highest
standing stocks of planktonic foraminifers and zooplankton, as shown in high correlations with environmental
variables. However, it is difficult to determine their primary limiting factors because many environmental variables covary with water depth.
N. dutertrei and P. obliquiloculata were dominant species in the planktonic foraminiferal assemblage of theupper 50 m. Both species generally show the maximum
abundance around the DCM (Schiebel and Hemleben2017). N. dutertrei is generally distributed in tropical totemperate open oceans and upwelling settings, showing

Table 1 Results of the linear model for standing stocks of Neogloboquadrina dutertrei and Pulleniatina obliquiloculata
Species

Variable

DF

SS

MS

F

p

Neogloboquadrina dutertrei

Chl a

1

20.002220.002288.49920.00023

Total POM

1

0.03650.03650.16160.70433

Residuals

51.1301

0.226

Chl a

1

4.89384.893815.0575

0.01164

Total POM

1

0.07620.0762

0.23440.64875

Residuals

5

1.625

0.325

Pulleniatina obliquiloculata

DF, degrees of freedom; SS, Sum of squares; MS, mean squares; F, F ratio; p, probability value

Toue et al. Progress in Earth and Planetary Science(2022) 9:20

Page 8 of 11

Table 2 Nitrogen isotopic composition of glutamic acid (δ15NGlu) and phenylalanine (δ15NPhe), and estimated trophic position ­( TPGlu/
Phe) and the propagation error of two non-spinose planktonic foraminiferal species (Neogloboquadrina dutertrei and Pulleniatina
obliquiloculata)
Species

Water depth (m)Number of
specimens

δ15NGlu (‰,relative to air)

δ15NPhe (‰,relative to air)

TPGlu∕Phe

Propagation
error

0–20

202

15.8

1.5

2.44

0.20

20–50

353

13.6

0.0

2.34

0.20

50–100

50

13.3

0.3

2.26

0.20

100–200

18

15.9

1.0

2.52

0.20

0–20

60

11.4

-0.3

2.09

0.21

20–50

137

14.1

0.9

2.28

0.20

50–100

45

10.1

0.3

1.84

0.23

100–200

14

7.1

3.3

1.05

0.67

Neogloboquadrina dutertrei

Pulleniatina obliquiloculata

Propagation error in ­TPGlu/Phe was calculated in Eq. (5)Trophic position
1

1.5

2

2.5

3

0–20

Depth (m)20–50

50–100100–200

Neogloboquadrina dutertrei
Pulleniatina obliquiloculata

Fig. 6 Trophic position (­ TPGlu/phe) of two non-spinose planktonic
foraminiferal species (Neogloboquadrina dutertrei and Pulleniatina
obliquiloculata) along four depth intervals in Suruga Bay, Japan. Error
bars indicate the propagation error (potential uncertainly) in ­TPGlu/Phe
calculated in Eq. (5)the maximum standing stocks at the DCM (e.g., Ravelo
et al. 1990; Schmuker and Schiebel 2002; Watkins et al.
1996, 1998). In water columns around the JapaneseIslands, N. dutertrei is found below the pycnocline, andits abundance peaks just below the DCM (Kuroyanagi
and Kawahata 2004). P. obliquiloculata is generally distributed in tropical to subtropical surface mixed layer
around the thermocline and DCM (e.g., Watkins et al.

1996, 1998; Jentzen et al. 2018). These observations are
consistent with high positive correlations of the standing stocks of both species with Chl a in this study and inprevious studies (Watkins et al. 1998), although no correlation was observed for N. dutertrei in similar settings(Kuroyanagi and Kawahata 2004).
The ­TPGlu/Phe value of a predator is one level higher
than that of its prey. The ­TPGlu/Phe values of N. dutertrei are ~ 2.4, indicating an omnivorous feeding strategy, while those of P. obliquiloculata are ~ 2.1, indicating
dependence mostly on herbivorous diets. Similar ­TPGlu/
Phe values at 20–50 m depth indicate that P. obliquiloculata becomes more omnivorous near the DCM thanabove. Previous studies using a variety of marine andterrestrial organisms demonstrated that even if the δ15N
value of amino acids has a variation between individuals
of a species, their trophic position can remain unchanged
during a given period, even though its food type and/or
source has changed dramatically (Chikaraishi et al. 2014).
Laboratory studies demonstrated that diatoms are a significant part of many non-spinose species’ diets and arefound in digestive vacuoles in these two species (Anderson et al. 1979; Spindler et al. 1984). Metazoan tissues arealso found in the digestive vacuoles of non-spinose species collected in the open ocean, although they can only
feebly catch and hold zooplankton prey when grown in
the laboratory since the rhizopodial net of non-spinose
species is not suited to capture living prey like copepods(Spindler et al. 1984; Hemleben et al. 1989). Therefore,
the metazoan tissue is likely obtained from inactive (e.g.,dead) organisms caught in the rhizopodia or by snaring
fecal matter containing incompletely digested metazoan
tissue (Hemleben et al. 1989). These previous laboratory
observations and T­ PGlu/Phe values in this study suggestthat N. dutertrei is a detritivore or scavenger, while P.
obliquiloculata generally depends on herbivorous diets.

Toue et al. Progress in Earth and Planetary Science(2022) 9:20

This assumption is supported by variable Ba/Ca signals
in shells of N. dutertrei, which are likely due to calcification in a microenvironment enriched in Ba such as
marine snow or other organic particulates (Fehrenbacher
et al. 2018). Although the standing stock of N. dutertrei
is more positively correlated with Chl a than that of P.
obliquiloculata (Fig. 5), this might be explained by theincreasing abundance of dead metazoan tissues and fecal
matters in the upper water column. Slight trophic niche
separation inferred from this study may allow these twoplanktonic foraminiferal species to live within a similar
depth zone in the open water column.
Some non-spinose species, including N. dutertrei andP. obliquiloculata, appear to facultatively harbor algal
endobionts, chrysophycophytes (chrysophytes), which
are capable of photosynthesizing within the perialgal
vacuoles (Gastrich 1987; Takagi et al. 2019). The algae
are also frequently observed in stages of division within
the host vacuoles, thus indicating they are in a healthy
state (Hemleben et al. 1989). However, some specimens
are entirely devoid of these algae, and some algae wereobserved in a state of being digested by the foraminifers(Hemleben et al. 1989). Predation on algal endobionts
results in ­TPGlu/Phe value around two due to a predator–
prey interaction in terms of the amino acid metabolism
between host and endobiont, while the use of photosynthate from algal endobionts as a nitrogen source resultsin ­TPGlu/Phe value around one (Tsuchiya et al. 2018). Algal
symbiont-bearing planktonic foraminifers likely obtaincarbon and nitrogen from two distinct sources, the dietand the symbionts (Uhle et al. 1997). For example, thecarbon and nitrogen isotopic data suggest that symbiontbearing spinose species Orbulina universa is indicative ofthe transfer of isotopically heavy metabolic carbon andnitrogen from its symbionts and relatively lighter carbon
and nitrogen from the diet. In contrast, diet is the solesource of metabolic carbon and nitrogen used for aminoacid synthesis in symbiont-barren spinose species (Globigerina bulloides) (Uhle et al. 1997). Intermediate depthdwelling and chrysophyte endobiont-bearing N. dutertrei
and P. obliquiloculata exhibit intermediate ranges of tissue and shell-bound δ15N values between the low-δ15N
surface-dwelling and dinoflagellate symbiont-bearing andhigh-δ15N subsurface-dwelling, symbiont-barren species,
but more similar to the latter group (Smart et al. 2018,2020). These previous studies and our results on T
­ PGlu/valuessuggest
that
these
two
non-spinose
speciesdoPhe
not have a symbiotic relationship with algal endobionts,
but hold algal endobionts to prey on them. Further studies are necessary to reveal a relative dependence between
foods and algal endobionts for nitrogen sources of these
non-spinose species.

Page 9 of 11

Recently advanced analytical techniques of δ15N valuesof shell-bound organic matter of planktonic foraminifera(shell-bound δ15N) are potential tools for reconstructing past changes in global nitrogen cycling (e.g., Ren
et al. 2009, 2012; Schiebel et al. 2018). Late Quaternary
glacial-interglacial records indicate that shell-bound δ15N
was higher during the glacial periods than the interglacial periods, suggesting that sea-level driven oscillationsin the balance of N
­ 2 fixation and denitrification (e.g.,
Ren et al. 2009, 2017). Future studies of shell-boundamino acid δ15N approaches combined with shell-boundorganic matter δ15N analyses would provide informationon temporal changes in paleo-trophic levels and foodwebs associated with glacial-interglacial changes in ocean
productivity.
Trophic niche separation of N. dutertrei and P.
obliquiloculata revealed in this study may also provide
a clue for understanding the causes of spatial and temporal changes in their relative abundances in sedimentassemblages. For example, in the East China Sea and the
Ryukyu Arc regions, both species are common under the
influence of Kuroshio Current, based on surface sediments (Ujiié et al. 2003) and sediment trap data (Xu et al.
2005). In contrast, temporal variations of the relative
abundances of N. dutertrei and P. obliquiloculata fromthe last glacial to Holocene at the Okinawa Trough show
a negative relationship (Li et al. 1997; Ujiié and Ujiié
1999; Ujiié et al. 2003). N. dutertrei was common in thelast glacial period and the Holocene Pulleniatina Minimum Event (PME; ca. 4.5–3 ka), while P. obliquiloculata
increased toward the interglacial but decreased during
the PME (Li et al. 1997; Ujiié and Ujiié 1999; Ujiié et al.
2003; Lin et al. 2006). In lower latitudes of the West
Pacific, the negative relationship of both species is lessobvious (Lin et al. 2006). Temporal changes in ocean
productivity associated with these paleoceanographic
changes may have influenced food webs and these two
species with slightly different trophic levels.

5 Conclusions
The trophic position ­( TPGlu/Phe) of two non-spinose
species, N. dutertrei and P. obliquiloculata, was determined by differences in the nitrogen isotopic composition between two amino acids (glutamic acid andphenylalanine). The ­TPGlu/Phe values of N. dutertrei
were ~ 2.4, indicating an omnivorous feeding strategy,
while those of P. obliquiloculata were ~ 2.1, indicating
dependence mostly on herbivorous diets. The ­TPGlu/
Phe values in this study, together with previous laboratory observations, suggest that N. dutertrei is a detritivore or scavenger, while P. obliquiloculata is generally aherbivore. The ­TPGlu/Phe values also suggest that these
two non-spinose species do not have a symbiotic

Toue et al. Progress in Earth and Planetary Science(2022) 9:20

relationship with algal endobionts, but hold those algal
endobionts to prey on them. This trophic niche separation may allow these two planktonic foraminiferal
species to live within a similar depth zone in the open
water column and provide a clue for understanding
causes of spatial and temporal changes in their relative
abundances in living and sediment assemblages.
Abbreviations
Chl a: Chlorophyll a; DCM: Deep chlorophyll maximum; Glu: Glutamic acid;Phe: Phenylalanine; PME: Holocene Pulleniatina minimum event; POM: Particulate organic matter; TP: Trophic position.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​022-​00478-3.
Additional file 1: Fig. S1. Size-frequency distribution of two non-spinose
planktonic foraminiferal species (Neogloboquadrina dutertrei and Pulleniatina obliquiloculata) collected at four depth intervals (0–20, 20–50,
50–100, and 100–200 m; two replicates) in Suruga Bay, Japan.
Additional file 2. Table S1. Hydrographic data in the upper 200 m at
the plankton tow site (Suruga Bay, Japan). Table S2. Standing stock data(individuals ­m–3) of living planktonic foraminifers at four depth intervals(0–20, 20–50, 50–100, and 100–200 m; two replicates) in Suruga Bay,
Japan. Table S3. Standing stock data (individuals m­ –3) of living zooplankton and wet weight of particulate organic matters (POM) at four depth
intervals (0–20, 20–50, 50–100, and 100–200 m; two replicates) in Suruga
Bay, Japan. Table S4. Correlation results of two non-spinose planktonic foraminiferal species (Neogloboquadrina dutertrei and Pulleniatina
obliquiloculata) with environmental and biological variables. Table S5.
Nitrogen isotopic composition of amino acids in two non-spinose speciesof planktonic foraminifers. Abbreviations: alanine (Ala), glycine (Gly), valine(Val), leucine (Leu), isoleucine (Ile), proline (Pro), aspartic acid (Asp), threonine (Thr), serine (Ser), methionine (Met), glutamic acid (Glu), phenylalanine (Phe), and Hydroxyproline (Hyp).
Acknowledgements
We are grateful to the onboard scientists and the crewmembers of the R/V
Tansei-maru for their support during the cruise.
Authors’ contributions
KF and MT proposed the topic, conceived, and designed the study. MT carriedout the field sampling. RT and KF carried out the plankton analysis. MT, YC, YS,
and NO carried out amino acid nitrogen isotope measurements. All authors
analyzed the data and helped in their interpretation. KF collaborated with theRT, MT, YC, and NO in the construction of manuscript. All authors read andapproved the final manuscript.
Funding
This study was financially supported by JSPS KAKENHI Grant Number
JP24340131.
Availability of data and materials
The datasets supporting the conclusions of this article are included within thearticle and its additional files.

Declarations
Competing interests
The authors declare that they have no competing interest.

Page 10 of 11

Author details
1
Department of Physics and Earth Sciences, Faculty of Science, Universityof the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903‑0213, Japan. 2 Tropical
Biosphere Research Center, University of the Ryukyus, 1 Senbaru, Nishihara,
Okinawa 903‑0213, Japan. 3 Research Institute for Global Change (RIGC), Japan
Agency for Marine-Earth Science and Technology, 2‑15 Natsushima‑cho,
Yokosuka 237‑0061, Japan. 4 Biogeochemistry Research Center, Japan
Agency for Marine-Earth Science and Technology, 2‑15 Natsushima‑cho,
Yokosuka 237‑0061, Japan. 5 Institute of Low Temperature Science, Hokkaido
University, Kita‑19, Nishi‑8, Kita‑ku, Sapporo 060‑0819, Japan.
Received: 14 March 2021 Accepted: 21 March 2022

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