# Introduction to the Special Issue entitled “Benthic Foraminiferal Ultrastructure Studies”

Joan Bernhard, Emmanuelle Geslin

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Joan Bernhard, Emmanuelle Geslin. Introduction to the Special Issue entitled “Benthic Foraminiferal Ultrastructure Studies”. Marine Micropaleontology, 2018, 138, pp.1-11. 10.1016/j.marmicro.2017.11.002. hal-02872786

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Introduction to the Special Issue entitled "Benthic Foraminiferal Ultrastructure Studies"

Joan M. Bernhard<fnref n="1" /> and Emmanuelle Geslin<fnref n="2" />

<footnote n="1">¹ Woods Hole Oceanographic Institution, Department of Geology & Geophysics, Woods Hole, MA, USA</footnote>

<footnote n="2">² UMR CNRS 6112 LPG-BIAF, Bio-Indicateurs Actuels et Fossiles, Université d'Angers, 2 Boulevard Lavoisier, 49045 Angers CEDEX 1, France</footnote>
Foraminifera are unicellular eukaryotes. These protists have membrane-bound nuclei, a microtubule-based cytoskeleton, intracellular membranous organelles, and anastamosing granuloreticulose pseudopodia. The name derives from the combination of two neo-Latin terms ("foramen", meaning an opening, hole or passage; "fer", meaning bearing), equating to "hole-bearing", referring to the presence of a hole in each chamber wall of the mineralized shells observed by Alcide d'Orbigny, who coined the moniker in 1826 (Lipps et al., 2011). The shell, formally called a "test", is typically considered as protection to the foraminiferal cell and commonly preserved in the geological record. Easily fossilized tests can be composed of either calcium carbonate secreted by the foraminifer, or mineral grains that are adhered with either organic or carbonate cement. Foraminifera with carbonate tests are informally referred to as "calcareous"; those forms that bind inorganic grains are "agglutinated", most of which are multichambered. Some foraminifera such as the "thecate" allogromiids and "naked" forms, have organic tests that do not fossilize well because of the lack of mineralized test components. Most of these organic-walled forms are single chambered, or "monothalamous". These "soft-walled" forms have largely been overlooked until recent decades. There are instances where monothalamid forms also have rigid agglutinated tests (Bowser et al. 1995). For a recent classification of foraminifera, the reader is directed to Pawlowski et al. (2013).

Foraminifera are nearly ubiquitous in the marine environment. To our knowledge, more limited representation exists in freshwater and terrestrial habitats. Thecate forms are generally marine while naked forms are generally freshwater or terrestrial. Three informal groups may be differentiated among common foraminiferal forms, based according to their habitats and life strategies: 1) planktonics, which live in the upper water column of the open ocean, 2) large benthic foraminifera (LBF) living on the seafloor in warm shallow waters and bearing photosynthetic symbionts, 3) "small" benthic foraminifera (SBF), which typically
lack photosynthetic symbionts and live in or on substrate such as unconsolidated “soft”
sediment or megafauna and megaflora (seaweed). We focus this special issue on the
ultrastructure of “small” marine benthic foraminifera (SBF); we do not extensively consider
small benthic freshwater or terrestrial foraminifera.

During the mid-1800s foraminifera became widely appreciated, with studies by
researchers such as Christian Gottfried Ehrenberg, William Benjamin Carpenter, and Henry
Bowman Brady. Due to their long and diverse fossil record, much early research on
foraminifera focused on biostratigraphy and paleoecology, eventually as a means to identify
source rocks for hydrocarbon reserves. The global H. M. S. *Challenger* Expedition (1872-
1876) collected hundreds of samples that received devoted study over decades by experts on
every marine life form, including foraminifera (e.g., Brady (1884), see also updated report by
Jones (1994)). Naturalists such as Ernst Haeckel popularized foraminifera at the turn of the
century by including them in their artistic scientific illustrations. Because foraminifera can be
abundant in shallow to deep waters from low to high latitude, their distribution and ecology
also received concentrated research efforts by investigators such as Joseph A. Cushman, in the
early to mid-1900s. Beginning in the middle of the last century, the chemical records of
foraminiferal carbonate tests were used extensively to decipher past oceanographic and
paleoclimate conditions (reviewed in Katz et al., 2010). For more thorough insights into the
history of foraminiferal research, the reader is directed to recent compilations and historical
perspectives (e.g., Bowden et al., 2014).

Due to the nineteenth century and twentieth century focus on fossil, relict and modern
foraminiferal remains, relatively little research was done on the living organism. Research of
the past few decades has revealed diverse foraminiferal adaptations, in the context of cell
biology and physiology, especially to chemocline habitats and additional “stressful”
environmental conditions. Such adaptations implicate their importance to biogeochemical
cycling.

A short history of transmission electron microscopy

A transmission electron microscope (TEM) uses an electron beam to pass (or transmit) through a specimen, to produce an image that is dependent on the interaction of the beam with the specimen; the image is subsequently magnified for viewing. Materials investigated by TEM are typically very thin, either being a thin slice (<0.1 µm) of a larger specimen or very fine particles accumulated on an appropriate surface (i.e., grid).

The first functional TEM was designed, built, and operated in the early 1930s by Ernst Ruska (Flegler et al., 1993). The development of the TEM was so influential that, in 1986, Ruska was co-recipient of the Nobel Prize in Physics (Bozzola and Russell, 1999). With the TEM, the use of an electron beam to image an item of interest allowed investigations at a much higher magnification than light microscopy. The resolution of the electron microscope was approximately three orders of magnitude higher than the light microscopes of the 1940s (Flegler et al., 1993). This ability to view materials at a much higher resolution resulted in unprecedented observations, opening entirely novel avenues of investigation. By the 1950s, the TEM was being used in many scientific disciplines, including biology, medicine, geology, and material sciences.

Over the years, the instrumentation has seen vast improvements and modifications that are beyond the scope of this short introduction. In sum, transmission electron microscopy has evolved to include a wide range of instrumentation, including High Voltage EM (HVEM), Low Voltage EM, Cryo-TEM, and Scanning TEM (STEM). These instruments can be paired with additional equipment such as an energy dispersive X-ray spectroscopy detector for elemental mapping and chemical characterization. Information on these methodologies is also beyond the scope of this contribution.
Use of the TEM relies not only on the instrumentation, but also on specimen preparation. Because most electron microscopes require a vacuum, and biological specimens are composed mostly of water, preparing specimens properly is crucial to successful TEM investigations. Different biological materials demand different preparation procedures. For example, even within the foraminifera, different protocols are required to adequately remove the test, if necessary. While this special issue includes information on foraminiferal preparations for TEM, the reader is directed to the primary literature for further details.

## Brief history of foraminiferal TEM studies

To our knowledge, the first TEM investigations of foraminiferal cells were in the 1960s (e.g., Walfarth-Bottermann, 1961). In 1965, Lee et al. (1965) described for the first time in the literature the general ultrastructure of two planktonic (calcareous) foraminifera. In 1967, Robert Angell published two works on the calcareous benthic *Rosalina floridana*, being the first to document foraminiferal calcification and chamber addition using the TEM (Angell 1967a, b). Beginning in the late 1960s, several authors used the TEM to study the ultrastructure of soft-walled foraminifera (Table 1). The 1970s, 1980s and early 1990s saw numerous investigations into benthic foraminiferal reticulopods, extensively reviewed by Travis and Bowser (1991) and Bowser and Travis (2000).

Between 1974 and 1990, the number of publications about the ultrastructure of LBF and planktonics, and their photosynthetic symbionts, increased markedly (e.g., Lee et al., 1974, Anderson and Bé, 1976a, Lee and Bock, 1976, Müller-Merz and Lee, 1976, Lee et al., 1979, 1980a, b, Schmaljohann and Röttger, 1978, McEnery and Lee, 1981, Bé et al., 1982, Leutenegger, 1977a, b, 1983, 1984, Hemleben et al., 1985, Spero, 1987, Faber et al., 1988, 1989, Lee, 1990, Lee and Anderson, 1991).
Reviews of foraminiferal biology and cell structure exist mainly on planktonic foraminifera and LBFs. In 1977, important reviews were published by Leutenegger (1977a, b) on the ultrastructure of LBFs and planktonic foraminifera and their photosynthetic symbionts. Anderson and Bé (1978) and Anderson and Lee (1991) both describe the main organelles and features observed in foraminiferal cells (e.g., nuclei, ribosomes, endoplasmic reticulum, Golgi, lysosomes and digestive vacuoles, peroxisomes, mitochondria, fibrillar bodies, cytoskeletal structures) but the source of all their TEM images except for one was planktonic foraminifera or LBF. Hemleben et al. (1989) and Schiebel and Hemleben (2017) thoroughly described the cytology of planktonic foraminifera.

A published review presenting the ultrastructure of SBF does not exist, to our knowledge. Here we present a compilation of representative publications that present at least one micrograph of smaller benthic foraminiferal ultrastructure (Table 1). Using information from Table 1, we note that before 1990, most of the major published studies with the TEM micrographs of SBF ultrastructure were based on soft-walled species (Fig. 1) mainly studying attributes to the cell membrane and/or test wall, especially reticulopods (Fig. 2). Starting in the 1990s, increasingly more studies on calcareous SBF ultrastructure were published (Fig. 1) and, while these often focused on specific organelle type(s), some also focused on functions such as reproduction (Fig. 2). Also, in the 1990s, an interest to study cellular adaptations to specific environmental conditions increased. Much of this interest was and is presently focused on the response of foraminiferal ultrastructure to oxygen-depletion in either natural or laboratory-controlled experimental settings. In the 2010s, many correlative investigations are combining innovative methodologies with the TEM.

In general, multilocular (multichambered) agglutinated foraminifera are not well studied by TEM methods (Fig.1) generally due to the added complexities of removing their test. Even large agglutinated foraminifera, such as the Xenophyophores and komokiaceans,
have rarely been the subject of a TEM study (e.g., Lecroq et al. 2009), presumably due to the
issues with recovery of deep-water materials.

Why this special issue?

Of course all foraminifera have similar overall cellular ultrastructure, including organelles typical to most eukaryotic cells. Additionally, the cellular ultrastructure of symbiont-bearing planktonic foraminifera and LBF are highly similar because both predominantly host photosynthetic symbionts. As noted, published reviews of foraminiferal ultrastructure are based on TEM micrographs of LBF (e.g., Leutenegger, 1977b) and planktonic foraminifera (Hemleben et al., 1989); a compilation of studies presenting TEM micrographs of SBF does not exist. This special issue is intended to fill the gap in existing reviews of SBF ultrastructure in a general context.

To improve our knowledge of the role of foraminifera in biogeochemical cycling and ecosystem functioning, it is imperative that we understand their physiology, metabolism and ecology. Such information is also critical to understanding foraminiferal biomineralization and geochemical signatures. One powerful means to better understand foraminiferal physiology, metabolism and ecology is to return to cell-scale studies, using transmission electron microscopy in combination with additional state-of-the-art imaging approaches.

This special issue includes the following contributions. An overview of typical smaller benthic foraminiferal ultrastructure and organelles is presented in LeKieffre et al., which also includes observations on some fine-scale structures of unknown function. An overview and synthesis of the observed associations between smaller benthic foraminifera and prokaryotes, as symbionts or parasites, is presented by Bernhard et al. A synthesis regarding a type of specialized "symbiosis" involving chloroplast sequestration is presented for shallow-water smaller benthic foraminiferal species (Jauffrais et al.). High-pressure freezing and freeze
substitution (HPF-FS) were used to document the ultrastructure of the chloroplast-
sequestering *Haynesina germanica* (Goldstein and Richardson); this contribution also
includes some comparisons between conventional chemical fixation and the HPF-FS
approach. The ultrastructural response of *Ammonia* spp. to anoxia is presented by Koho et al.
Similarly, the ultrastructural response of selected smaller benthic foraminifera to heavy metals
is presented by Frontalini et al. Finally, an overview of methods that merge the TEM with
additional powerful analytical tools is presented in Nomaki et al.

**Acknowledgments**

The idea for this Special Issue of *Marine Micropaleontology* resulted from the
“Foraminiferal Ultrastructure Workshop” organized by the FRESCO (Foraminiferal Research
Consortium) project in conjunction with The Micropalaeontological Society’s Foraminifera
and Nannofossil Groups Joint Meeting 2016 held at the University of Angers. For this
introductory review, we gratefully appreciate the insights from Sam Bowser and Sue
Goldstein for their vast knowledge of the subject and extensive reprint collections as well as
Charlotte LeKieffre for sharing her knowledge of planktonic foraminiferal ultrastructure. We
also thank the *Marine Micropaleontology* Editors, Ric Jordan and Frans Jorissen for their
support in our efforts to create this special issue. JMB acknowledges support from the WHOI
Robert W. Morse Chair for Excellence in Oceanography and The Investment in Science Fund
at WHOI.
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**Figure and Table captions**

**Table 1.** Publications including TEM images of the ultrastructure of small benthic foraminifera, categorized in very general terms of ABF = multichambered Agglutinated Benthic Foraminifera, CBF = Calcareous Benthic Foraminifera (generally multichambered), SWF = Soft-walled Benthic Foraminifera. The list is not exhaustive but representative of major research efforts. Note that the nomenclature given is from the original publication; taxa could have been reassigned or updated. In some cases, a publication could be considered in multiple categories but each is typically listed under the main focus of the contribution. Table updated from Stouff (1998).

**Figure 1.** Histogram plotting number of publications presenting TEM images of Smaller Benthic Foraminiferal ultrastructure binned by decade and according to test composition. All publications are listed in Table 1, which is not exhaustive.

**Figure 2.** Histogram showing number of publications with TEM images of Smaller Benthic Foraminiferal ultrastructure binned by decade and presented by major topic ("general characterization" = general description of diverse organelles in one to a few species; "link to membrane and wall" = includes publications aimed at cell membrane features such as reticulopods or the test wall; "link to cell body" = includes works dealing with the description of specific organelles or structures; "link to function" includes papers dealing with TEM approaches to understand cell function; "link to specific conditions" includes contributions dealing with *in situ* or experimental works using the TEM to understand the foraminiferal adaptation to specific conditions (e.g., low oxygen, heavy-metal contamination); "link to methods" include papers that combine the TEM with other methods, such as NanoSIMS). All publications are listed in Table 1, which is not exhaustive; Table 1 category "other topics" is not included in this figure.
<figure><img src="image_2.png" /><figcaption>Figure 1.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2.</figcaption></figure>
<table><thead><tr><th>Study Aim</th><th>Author(s)</th><th>Year</th><th>Foram Type</th><th>SBF species imaged with TEM</th></tr></thead><tbody><tr><td rowspan="2">General description of cell</td><td>Boltovskoy and Wright</td><td>1976</td><td>SWF</td><td>Allogromia laticollaris, Ovammina opaca, Hippocrepinella alba</td></tr><tr><td>LeKieffre et al.</td><td>in press</td><td>CBF</td><td>Ammonia sp., Bulimina marginata, Bulimina tenuata, Elphidium oceanense, Globobulimina sp., Haynesina germanica, Nonionella sp., Nonionellina labradorica, Stainforthia fusiformis</td></tr><tr><td rowspan="10">General knowledge of ultrastructure in specific species</td><td>Hedley et al.</td><td>1967</td><td>SWF</td><td>Sfapheardella tceniformis</td></tr><tr><td>Lengsfeld</td><td>1969a</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Schwab</td><td>1971</td><td>SWF</td><td>Allogromia laticollaris, Myxotheca arenilega</td></tr><tr><td>Lena and Freire</td><td>1974</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Schwab</td><td>1977</td><td>SWF</td><td>Boderia albicollaris</td></tr><tr><td>Stouff</td><td>1998</td><td>CBF</td><td>Ammonia tepida, Ammonia beccarii</td></tr><tr><td>Goldstein and Richardson</td><td>2002</td><td>SWF</td><td>Myxotheca sp., Cribrothalammina alba, Hyperammina sp.</td></tr><tr><td>Altin-Ballero et al.</td><td>2013</td><td>SWF</td><td>Psammophaga sapela</td></tr><tr><td rowspan="10">Organic layer / wall structure</td><td>Hedley et al.</td><td>1973</td><td>SWF</td><td>Allogromia spp., Iridia diaphana, Boderia turneri</td></tr><tr><td>Pierce and Nathanson</td><td>1974</td><td>SWF</td><td>Allogromia laticollaris, Allogromia sp.</td></tr><tr><td>Schwab and Schwab-Stey</td><td>1981</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Goldstein and Barker</td><td>1988</td><td>SWF</td><td>Cribrothalammina alba</td></tr><tr><td>Heeger</td><td>1990</td><td>CBF</td><td>Cibicidoides wuellerstorfi</td></tr><tr><td>Anderson and Lee</td><td>1991</td><td>ABF</td><td>Abyssotherma pacifica</td></tr><tr><td>Lee et al.</td><td>1991</td><td>ABF</td><td>Abyssotherma pacifica</td></tr><tr><td>Bowser et al.</td><td>1995</td><td>SWF</td><td>Astrammina rara, Notodendrodes antarctikos</td></tr><tr><td>Rützler and</td><td>1996</td><td>ABF</td><td>Spiculidendron corallicolum</td></tr></tbody></table>
<table><tbody><tr><td></td><td>Richardson</td><td></td><td></td><td></td></tr><tr><td></td><td>Goldstein</td><td>1999</td><td>CBF</td><td>Ammonia tepida, Chilostomella sp., Triloculina oblonga</td></tr><tr><td></td><td>DeLaca et al.</td><td>2002</td><td>SWF</td><td>Notodendrodes hyalinosphaira</td></tr><tr><td></td><td>Bowser et al.</td><td>2002</td><td>SWF</td><td>Astrammina triangularis</td></tr><tr><td></td><td>Habura et al.</td><td>2006</td><td>ABF</td><td>Miliammina fusca</td></tr><tr><td></td><td>Altin et al.</td><td>2009</td><td>SWF</td><td>Niveus flexilis</td></tr><tr><td></td><td>Altin et al.</td><td>2013</td><td>SWF</td><td>Psammophaga sapela</td></tr><tr><td rowspan="4">Pores</td><td>Angell</td><td>1967b</td><td>CBF</td><td>Rosalina floridana</td></tr><tr><td>Berthold</td><td>1976</td><td>CBF</td><td>Patellina corrugate</td></tr><tr><td>Leutenegger and Hansen</td><td>1979</td><td>CBF</td><td>Bolivina spp.</td></tr><tr><td>Heeger</td><td>1990</td><td>CBF</td><td>Cibicidoides wuellerstorfi</td></tr><tr><td rowspan="15">Reticulopods (microtubule) / ectoplasm</td><td>Wohlfarth-Bottermann</td><td>1961</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Angell</td><td>1967a</td><td>CBF</td><td>Rosalina floridana</td></tr><tr><td>Lengsfeld</td><td>1969b</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Marszalek</td><td>1969</td><td>SWF</td><td>Iridia diaphana</td></tr><tr><td>Schwab and Schwab-Stey</td><td>1972</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Schwab and Schwab-Stey</td><td>1973</td><td>SWF</td><td>Myxotheca arenilega</td></tr><tr><td>Hauser and Schwab</td><td>1974</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>McGee-Russell</td><td>1974</td><td>SWF</td><td>Allogromia</td></tr><tr><td>Nyholm and Nyholm</td><td>1975a</td><td>SWF</td><td>Cylindrogullmia alba</td></tr><tr><td>Nyholm and Nyholm</td><td>1975b</td><td>SWF</td><td>Cylindrogullmia alba</td></tr><tr><td>Alexander and Banner</td><td>1984</td><td>CBF</td><td>Haynesina germanica</td></tr><tr><td>Travis and Allen</td><td>1981</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Travis et al.</td><td>1983</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Koury et al.</td><td>1985</td><td>SWF</td><td>Allogromia sp., Strain NF</td></tr><tr><td>Bowser and</td><td>1985</td><td>SWF</td><td>Allogromia sp., Strain NF</td></tr></tbody></table>
<table><tbody><tr><td></td><td>Rieder</td><td></td><td></td><td></td></tr><tr><td></td><td>Koonce and Schliwa</td><td>1985</td><td>SWF</td><td>Reticulomyxa filose</td></tr><tr><td></td><td>Koonce and Schliwa</td><td>1986</td><td>SWF</td><td>Reticulomyxa</td></tr><tr><td></td><td>Rieder et al.</td><td>1985</td><td>SWF</td><td>Astrammina rara</td></tr><tr><td></td><td>Bowser et al.</td><td>1986</td><td>SWF</td><td>Astrammina rara</td></tr><tr><td></td><td>Euteneuer et al.</td><td>1986</td><td>SWF</td><td>Reticulomyxa</td></tr><tr><td></td><td>Koonce et al.</td><td>1986b</td><td>SWF</td><td>Reticulomyxa</td></tr><tr><td></td><td>Koonce et al.</td><td>1986a</td><td>SWF</td><td>Reticulomyxa</td></tr><tr><td></td><td>Rupp et al.</td><td>1986</td><td>SWF</td><td>Allogromia sp., Strain NF</td></tr><tr><td></td><td>Travis and Bowser</td><td>1986</td><td>SWF</td><td>Allogromia sp., Strain NF</td></tr><tr><td></td><td>Kachar et al.</td><td>1987</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td></td><td>Bowser et al.</td><td>1988</td><td>SWF</td><td>Allogromia sp., Strain NF</td></tr><tr><td></td><td>Hauser and Lindenblatt</td><td>1989</td><td>SWF</td><td>Reticulomyxa filose</td></tr><tr><td></td><td>Jensen et al.</td><td>1990</td><td>SWF</td><td>Allogromia laticollaris, Allogromia sp.</td></tr><tr><td></td><td>Travis and Bowser</td><td>1990</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td></td><td>Travis and Bowser</td><td>1991</td><td>SWF, CBF</td><td>Allogromia spp., Spiroloculina hyaline</td></tr><tr><td></td><td>Golz and Hauser</td><td>1994</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td></td><td>Welnhofer and Travis</td><td>1996</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td></td><td>Goldstein</td><td>1999</td><td>CBF</td><td>Spiroloculina hyaline</td></tr><tr><td></td><td>Bowser and Travis</td><td>2000</td><td>SWF</td><td>Allogromia laticollaris, Allogromia sp., Strain NF</td></tr><tr><td></td><td>Travis et al.</td><td>2002</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td></td><td>Habura et al.</td><td>2005</td><td>CBF, ABF</td><td>Ammonia tepida, Miliammina fusca</td></tr><tr><td rowspan="2">Specific organelles</td><td rowspan="2">Nucleus</td><td>Dahlgren</td><td>1967a, b</td><td>SWF, CBF</td><td>Ovammina opaca, Hippocrepinella alba, Globobulimina turgida</td></tr><tr><td>Lena</td><td>1972</td><td>SWF</td><td>Allogromia flexibilis</td></tr></tbody></table>
<table><tbody><tr><td></td><td>Schwab</td><td>1972</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td></td><td>Schwab and Schwab-Stey</td><td>1979b</td><td>SWF</td><td>Kibisidytes sp.</td></tr><tr><td></td><td>Schwab and Schwab-Stey</td><td>1979a</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td></td><td>Heeger</td><td>1990</td><td>CBF</td><td>Quinqueloculina seminula, Miliolinella vigilax</td></tr><tr><td></td><td>Goldstein</td><td>1997</td><td>CBF, SWF</td><td>Ammonia tepida, Triloculina oblonga, Cribrothalammina alba</td></tr><tr><td></td><td>Raikov et al.</td><td>1998</td><td>CBF</td><td>Elphidium ponticum</td></tr><tr><td rowspan="3">Mitochondria</td><td>Hedley and Wakefield</td><td>1968</td><td>SWF</td><td>Boderia turneri</td></tr><tr><td>Heeger</td><td>1990</td><td>CBF, ABF</td><td>Quinqueloculina sp., Ammotium cassis</td></tr><tr><td>Bernhard</td><td>2000</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td rowspan="12">Peroxisomes and endoplasmic reticulum</td><td>Hruban and Rechcigl</td><td>1969</td><td>CBF</td><td>Rosalina leei</td></tr><tr><td>Heeger</td><td>1990</td><td>ABF, CBF</td><td>Ammotium cassis, Triloculina rotunda, Pyrgo murrhina, Cibicides lobatulus</td></tr><tr><td>Bernhard and Reimers</td><td>1991</td><td>CBF</td><td>Nonionella stella, Chilistomella ovoidea</td></tr><tr><td>Bernhard and Alve</td><td>1996</td><td>CBF</td><td>Stainforthia fusiformis</td></tr><tr><td>Bernhard and Sen Gupta</td><td>1999</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Bernhard et al.</td><td>2001</td><td>CBF</td><td>Buliminella tenuata, Globobulimina sp.</td></tr><tr><td>Bernhard and Bowser</td><td>2008</td><td>CBF</td><td>Nonionella stella, Buliminella tenuata, Buliminella elegantissima</td></tr><tr><td>Bernhard et al.</td><td>2010b</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Bernhard et al.</td><td>2010a</td><td>CBF</td><td>Bolivina pacifica</td></tr><tr><td>Bernhard et al.</td><td>2012a</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Tsuchiya et al.</td><td>2015</td><td>CBF</td><td>Virgulinella fragilis</td></tr><tr><td rowspan="2">Sequestered chloroplasts</td><td>Lopez</td><td>1979</td><td>CBF</td><td>Elphidim williamsoni</td></tr><tr><td>Lee and McEnery</td><td>1983</td><td>CBF</td><td>Elphidim williamsoni</td></tr></tbody></table>
<table><tbody><tr><td></td><td>Lee et al.</td><td>1988</td><td>CBF</td><td>Elphidium crispum</td></tr><tr><td></td><td>Cedhagen</td><td>1991</td><td>CBF</td><td>Nonionella labradorica</td></tr><tr><td></td><td>Bernhard and Alve</td><td>1996</td><td>CBF</td><td>Bulimina marginata, Stainforthia fusiformis</td></tr><tr><td></td><td>Bernhard and Sen Gupta</td><td>1999</td><td>CBF</td><td>Nonionella stella</td></tr><tr><td></td><td>Bernhard and Bowser</td><td>1999</td><td>CBF</td><td>Nonionella stella, Elphidium sp.</td></tr><tr><td></td><td>Correia and Lee</td><td>2002</td><td>CBF</td><td>Elphidium excavatum</td></tr><tr><td></td><td>Grzymski et al.</td><td>2002</td><td>CBF</td><td>Nonionella stella</td></tr><tr><td></td><td>Bernhard</td><td>2003</td><td>CBF</td><td>Virgulinella fragilis</td></tr><tr><td></td><td>Goldstein et al.</td><td>2004</td><td>CBF</td><td>Haynesina germanica</td></tr><tr><td></td><td>Goldstein et al.</td><td>2010</td><td>SWF</td><td>Xiphophaga minuta</td></tr><tr><td></td><td>Bernhard et al.</td><td>2012a</td><td>CBF</td><td>Bolivina argentea</td></tr><tr><td></td><td>Tsuchiya et al.</td><td>2015</td><td>CBF</td><td>Virgulinella fragilis</td></tr><tr><td></td><td>Cesbron et al.</td><td>2017</td><td>CBF</td><td>Haynesina germanica</td></tr><tr><td></td><td>Goldstein and Richardson</td><td>in press</td><td>CBF</td><td>Haynesina germanica</td></tr><tr><td></td><td>Jauffrais et al.</td><td>in press</td><td>CBF</td><td>Elphidium aff. E. crispum, Elphidium oceanense, Elphidium selseyense, Haynesina, germanica, Planoglabratella opercularis</td></tr><tr><td rowspan="10">Endobionts / ectobionts</td><td>Bernhard</td><td>1993</td><td>CBF</td><td>Globocassidulina cf. G. biora</td></tr><tr><td>Richardson and Rützler</td><td>1999</td><td>ABF</td><td>Spiculidendron corallicolum</td></tr><tr><td>Bernhard and Sen Gupta</td><td>1999</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Bernhard et al.</td><td>2000</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Bernhard</td><td>2003</td><td>CBF</td><td>Virgulinella fragilis</td></tr><tr><td>Bernhard et al.</td><td>2006</td><td>SWF</td><td>Clade L allogromiid</td></tr><tr><td>Bernhard et al.</td><td>2010a</td><td>CBF</td><td>Bolivina pacifica</td></tr><tr><td>Bernhard et al.</td><td>2010b</td><td>CBF</td><td>Uvigerina pacifica, Globobulimina pacifica</td></tr><tr><td>Martin et al.</td><td>2010</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Bernhard et al.</td><td>2012a</td><td>CBF</td><td>Fursenkoina cornuta, Buliminella tenuata</td></tr><tr><td>Bernhard et al.</td><td>2012b</td><td>SWF</td><td>Clade L allogromiid</td></tr></tbody></table>
<table><tr><td></td><td>Tsuchiya et al.</td><td>2015</td><td>CBF</td><td>Virgulinella fragilis</td></tr><tr><td></td><td>Bernhard et al.</td><td>2017</td><td>CBF, SWF</td><td>Ammonia sp., Bolivina pacifica, Bulimina mexicana, Buliminella tenuata, Cibicides wuellerstorfi, Globobulimina affinis, G. pacifica, Saccamminid, Uvigerina peregrina, Virgulina fragilis</td></tr><tr><td rowspan="6">Organelles linked to nutrition</td><td>Lengsfeld</td><td>1969c</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Bowser et al.</td><td>1985</td><td>SWF</td><td>Allogromia sp., Strain NF</td></tr><tr><td>Heeger</td><td>1990</td><td>SWF</td><td>Quinqueloculina seminula, Ammontium cassis, Miliolinella vigilax, Rosalina globularis, Melonis zaandami, Cibicides refulgens, Pyrgo murrhina, Triloculina sp., Triloculina rotunda</td></tr><tr><td>Goldstein and Corliss</td><td>1994</td><td>CBF</td><td>Ammonia beccarii forma tepida, Globobulimina pacifica, Uvigerina pacifica</td></tr><tr><td>Goldstein</td><td>1999</td><td>CBF, SWF</td><td>Globobulimina pacifica, Cribrothalammina alba</td></tr><tr><td>LeKieffre et al.</td><td>2017</td><td>CBF</td><td>Ammonia sp.</td></tr><tr><td rowspan="14">Cytoplasm / organelles linked to reproduction</td><td>Schwab</td><td>1969</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Schwab</td><td>1970</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Angell</td><td>1971</td><td>SWF</td><td>Myxotheca sp.</td></tr><tr><td>Cesana</td><td>1972</td><td>SWF</td><td>Iridia lucida</td></tr><tr><td>Mc Enery and Lee</td><td>1976</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Schwab</td><td>1976</td><td>SWF</td><td>Allogromia laticollaris</td></tr><tr><td>Arnold</td><td>1982</td><td>SWF</td><td>Psammophaga simplora</td></tr><tr><td>Arnold</td><td>1984</td><td>SWF</td><td>Psammophaga simplora</td></tr><tr><td>Bowser et al.</td><td>1984</td><td>SWF</td><td>Allogromia sp., Strain NF</td></tr><tr><td>Goldstein</td><td>1988</td><td>SWF</td><td>Saccammina alba</td></tr><tr><td>Goldstein and Barker</td><td>1990</td><td>SWF</td><td>Cribrothalammina alba</td></tr><tr><td>Lee et al.</td><td>1991</td><td>SWF</td><td>Allogromia sp.</td></tr><tr><td>Goldstein and Moodley</td><td>1993</td><td>CBF</td><td>Ammonia beccarii forma tepida</td></tr><tr><td>Pawlowski et al.</td><td>1995</td><td>ABF</td><td>Trochammina sp.</td></tr><tr><td>Goldstein</td><td>1997</td><td>CBF</td><td>Ammonia tepida, Triloculina oblonga, Cribrothalammina alba</td></tr></table>
<table><tbody><tr><td></td><td>Raikov et al.</td><td>1998</td><td>CBF</td><td>Elphidium ponticum</td></tr><tr><td></td><td>Goldstein et al.</td><td>2010</td><td>SWF</td><td>Xiphophaga minuta</td></tr><tr><td></td><td>Altin et al.</td><td>2009</td><td>SWF</td><td>Niveus flexilis</td></tr><tr><td></td><td>Altin et al.</td><td>2013</td><td>SWF</td><td>Psammophaga sapela</td></tr><tr><td rowspan="8">Cellular response to specific environmental conditions</td><td>Leutenegger and Hansen</td><td>1979</td><td>CBF</td><td>Bolivina spp.</td></tr><tr><td>Bernhard and Reimers</td><td>1991</td><td>CBF</td><td>Nonionella stella, Chilostomella ovoidea</td></tr><tr><td>Bernhard</td><td>1993</td><td>CBF</td><td>Globocassidulina cf. G. biora, Cassidulinoides porrectus</td></tr><tr><td>Bernhard</td><td>1996</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Sen Gupta et al.</td><td>1997</td><td>CBF</td><td>Gavelinopsis translucens, Cassidulina neocarinata</td></tr><tr><td>Bernhard and Sen Gupta</td><td>1999</td><td>CBF</td><td>Buliminella tenuata</td></tr><tr><td>Bernhard et al.</td><td>2010a</td><td>CBF</td><td>Uvigerina peregrina, Buliminella tenuata, Bulimina Mexicana</td></tr><tr><td>Risgaard-Petersen et al.</td><td>2006</td><td>CBF</td><td>Globobulimina pseudospinescens</td></tr><tr><td></td><td>Koho et al.</td><td>in press</td><td>CBF</td><td>Ammonia spp.</td></tr><tr><td rowspan="10">Cellular response to experimental conditions</td><td>Bernhard and Alve</td><td>1996</td><td>CBF</td><td>Bulimina marginata, Stainforthia fusiformis</td></tr><tr><td>Morvan et al.</td><td>2004</td><td>CBF</td><td>Ammonia tepida</td></tr><tr><td>Lecadre and Debenay</td><td>2006</td><td>CBF</td><td>Ammonia tepida</td></tr><tr><td>Bernhard et al.</td><td>2012a</td><td>CBF</td><td>Bolivina argentea, Fursenkoina cornuta, Buliminella tenuata</td></tr><tr><td>Nomaki et al.</td><td>2014</td><td>CBF</td><td>Ammonia beccarii</td></tr><tr><td>Frontalini et al.</td><td>2015</td><td>CBF</td><td>Ammonia parkinsoniana</td></tr><tr><td>Frontalini et al.</td><td>2016</td><td>CBF</td><td>Ammonia parkinsoniana</td></tr><tr><td>Nomaki et al.</td><td>2016</td><td>CBF</td><td>Ammonia sp.</td></tr><tr><td>LeKieffre et al.</td><td>2017</td><td>CBF</td><td>Ammonia sp.</td></tr><tr><td>Frontalini et al.</td><td>submitted</td><td>CBF</td><td>Pseudotriloculina rotunda, Ammonia parkinsoniana</td></tr><tr><td>Koho et al.</td><td>in press</td><td>CBF</td><td>Ammonia spp.</td></tr><tr><td rowspan="2">Other methodologies combined with TEM</td><td>Bernhard et al.</td><td>2010a</td><td>CBF</td><td>Uvigerina peregrina, Buliminella tenuata, Bulimina Mexicana</td></tr><tr><td>Martin et al.</td><td>2010</td><td>CBF</td><td>Uvigerina peregrina, Buliminella tenuata, Bulimina Mexicana</td></tr></tbody></table>
<table><tr><td></td><td>Bernhard and Richardson</td><td>2014</td><td>CBF</td><td>Unknown</td></tr><tr><td></td><td>Nomaki et al.</td><td>2015</td><td>CBF</td><td><i>Uvigerina akitaensis</i>, <i>Globobulimina affinis</i></td></tr><tr><td></td><td>Nomaki et al.</td><td>2016</td><td>CBF</td><td><i>Ammonia</i> sp.</td></tr><tr><td></td><td>LeKieffre et al.</td><td>2017</td><td>CBF</td><td><i>Ammonia</i> sp.</td></tr><tr><td></td><td>Goldstein and Richardson</td><td>in press</td><td>CBF</td><td><i>Haynesina germanica</i></td></tr><tr><td></td><td>Nomaki et al.</td><td>in press</td><td>CBF, ABF</td><td><i>Ammonia</i> sp., <i>Buliminella tenuata</i>, Biserial agglutinated species, <i>Haynesina germanica</i>, <i>Nonionellina labradorica</i></td></tr><tr><td rowspan="4">Other topics</td><td>Hruban and Rechcigl</td><td>1969</td><td>SWF</td><td><i>Iridia lucida</i>, <i>Shepheardella taeniformis</i></td></tr><tr><td>Heeger</td><td>1988, 1990</td><td>CBF</td><td><i>Elphidium excavatum clavatum</i></td></tr><tr><td>Rützler and Richardson</td><td>1996</td><td>ABF</td><td><i>Spiculidendron corallicolum</i>.</td></tr><tr><td>Goldstein et al.</td><td>2010</td><td>SWF</td><td><i>Xiphophaga minuta</i></td></tr></table>