An overview of cellular ultrastructure in benthic foraminifera: New observations of rotalid species in the context of existing literature

We report systematic transmission electron microscope (TEM) observations of the cellular ultrastructure of selected, small rotalid benthic foraminifera. Nine species from different environments (intertidal mudflat, fjord, and basin) were investigated: Ammonia sp., Elphidium oceanense, Haynesina germanica, Bulimina marginata, Globobulimina sp., Nonionellina labradorica, Nonionella sp., Stainforthia fusiformis and Buliminella tenuata. All the observed specimens were fixed just after collection from their natural habitats allowing description of intact and healthy cells. Foraminiferal organelles can be divided into two broad categories: (1) organelles that are present in all eukaryotes, such as the nuclei, mitochondria, endoplasmic reticulum, Golgi apparatus, and peroxisomes; and (2) organelles observed in all foraminifera but not common in all eukaryotic cells, generally with unknown function, such as fibrillar vesicles or electron-opaque bodies. Although the organelles of the first category were observed in all the observed species, their appearance varies. For example, subcellular compartments linked to feeding and metabolism exhibited different sizes and shapes between species, likely due to differences in their diet and/or trophic mechanisms. The organelles of the second category are common in all foraminiferal species investigated and, according to the literature, are frequently present in the cytoplasm of many different species, both benthic and planktonic. This study, thus, provides a detailed overview of the major ultrastructural components in benthic foraminiferal cells from a variety of marine environments, and also highlights the need for further research to better understand the function and role of the various organelles in these fascinating organisms.

Cellular ultrastructure in benthic foraminifera: general observations and focus on kleptoplasts.

Emmanuelle Geslin, Charlotte Madeleine Nicole Lekieffre, Anders Meibom

Research of the past few decades has revealed a great variability of metabolisms (e.g. adaptation to different microhabitats, feeding strategies, symbiosis, survival to stressful conditions) among benthic foraminiferal species. This metabolic variety is reflected in the cell by a great diversity of organelles, cellular organization and organelle associations. Despite the large number of publications examining the foraminiferal ultrastructure with transmission electron microscopy (TEM), only a fraction of the observed organelles have been identified. To gather all the knowledge about benthic foraminifera ultrastructure, an atlas based on existing literature and new observations of rotalid foraminifera was developed, as a part of a special issue entitled “Benthic Foraminiferal Ultrastructure Studies” (Marine Micropaleontology). Among other studies, this special issue provides a key to recognize the main organelles encountered in all examined benthic foraminiferal species, with known (e.g., nuclei, mitochondria, Golgi apparatus, endoplasmic reticulum, peroxisomes) or unknown functions (e.g., electron-opaque bodies, fibrillar vesicles). Cellular ultrastructure via TEM is a good way to obtain insights into physiology. Indeed, there are examples in the literature where ultrastructural observation led to hypotheses about foraminiferal metabolism, such as a specific distribution of mitochondria in response to hypoxia, vacuolization thought to play a role in denitrification, or endosymbionts providing an alternative metabolic pathway to their host. Another example of how TEM observations of the cell has proven a good tool to obtain insights into foraminiferal metabolism is kleptoplasty (i.e., the ability from some foraminiferal species to actively sequester chloroplasts stolen from microalgae within their endoplasm). Seven species of benthic foraminifera inhabiting photic zones were imaged using TEM to ascertain attributes of their kleptoplasts. Results show that there are differences in the way the investigated species organize their kleptoplasts within their endoplasm. This suggests behavioral strategies to expose and/or protect the kleptoplasts to/from light and/or to favor gas and/or nutrient exchange with their surrounding habitats. Also, different species had kleptoplasts in variable states of preservation/digestion, suggesting that some species are more efficient at kleptoplasty than others, as confirmed by complementary techniques.

2018

Dormant or not dormant? Benthic foraminiferal metabolism under anoxia

Emmanuelle Geslin, Charlotte Madeleine Nicole Lekieffre, Anders Meibom

Hypoxic events particularly affect benthic ecosystems on continental shelves and in coastal areas where renewal of bottom waters slow. Foraminifera living in such environments are among the most tolerant to hypoxia in the meiofauna. Some foraminifera species are able to survive hypoxia, and even anoxia, for weeks to months. Different species must have developed different mechanisms for survival - hypotheses include reduction of the metabolism, symbiosis with bacteria, or denitrification. NanoSIMS (Secondary Ion Mass Spectrometry) imaging is a powerful analytical technique to visualize and quantify the incorporation and transfer of isotopically labeled compounds in organisms with subcellular resolution. We used NanoSIMS imaging, correlated with TEM ultrastructural observations of individual foraminifera, to study the metabolism of intertidal Ammonia tepida, which has shown strongly reduced metabolism under anoxia. Individuals were fed with a 13C-labeled microalgal biofilm and incubated for 4 weeks in oxic and anoxic conditions, respectively. NanoSIMS imaging reveal strongly contrasting cellular-level dynamics of integration and transfer of the ingested biofilm components under the two conditions. In oxic conditions, ingested biofilm components are internalized, metabolized, and used for biosynthesis of different cellular components on a time scale of 24 hours: Lipid droplets are formed, then consumed through respiration. In contrast, upon the onset of anoxia, individual internalized biofilm components remain visible within the cytoplasm after 4 weeks. Lipids of different compositions are initially formed but then not respired. These observations indicate that foraminifera do initially have an active heterotrophic metabolism in the absence of oxygen, but this it is strongly reduced when oxygen is no longer available. Isotopic labeling experiments, NanoSIMS and TEM imaging, and GC-MS will be key to study metabolic mechanisms under anoxic conditions in marine environments.

2016

Feeding behaviour of a benthic species (Ammonia tepida) under oxic and anoxic conditions: TEM-NanoSIMS correlation

Emmanuelle Geslin, Charlotte Madeleine Nicole Lekieffre, Anders Meibom

More and more marine areas are subjected to depleted-O2 concentration, mainly due to eutrophication induced by human activities. These phenomena affect benthic ecosystems, in particular continental shelves and coastal areas where the renewal of the bottom waters is low and/or the organic matter flux is high. Some species of benthic foraminifera are resistant to this hypoxic/anoxic events, surviving weeks to months without oxygen. One of the species known to survive to such extreme condition is the benthic one Ammonia tepida. However the metabolic process by which it succeed in it remains unknown. One hypothesis is that this species would be able to lower its metabolism under oxygen depleted conditions, until the return of better conditions. The aim of the present study is to understand the feeding behaviors of A. tepida under anoxia. For this purpose a laboratory experiment involving incubation with living A. tepida was carried out under controlled oxygen concentrations. The individuals were fed with a pulse of 13C-labeled diatoms (Navicula sp.) and then incubate in oxic or anoxic conditions during 28 days. Along the incubation time, observation of the cell ultrastructure with Transmission Electronic Microscope (TEM) and detection in the cell of the isotopic 13C signal using nanoSIMS, were made. The nanoSIMS (nanoscale Secondary-Ion Mass Spectrometry) is an analytical technique that allows to visualize the incorporation and transfer of isotopically labeled compounds in organisms; thus in our case to follow the ingestion of labeled-diatoms and further transfer of carbon in the cytoplasm of the foraminifera. According to the results acquired in this study, in both conditions the foraminifera directly integrate the diatoms in their cytoplasm during the first hours of the incubation. Then, in oxic conditions, these diatoms are quickly fully degraded, within 3 days, and the 13C-labeled compounds are transferred in the organelles of the cytoplasm. Whereas in anoxia, only a fraction of the diatoms are degraded and the transfer to the foraminifera cytoplasm is weak. These results are confirmed by the δ13C of the foraminifera, measured in bulk by mass spectrometry (GC-MS). In anoxia the δ13C is slightly increasing the first 24h, meaning that the 13C content of the cell increased, and then it remains stable until the end of the experiment. This leads us to assume that the foraminifera stop, or at least strongly lower their metabolic rate under anoxic condition.

2015

Feeding behaviour of a benthic species (Ammonia tepida) under oxic and anoxic conditions: TEM-NanoSIMS correlation

Emmanuelle Geslin, Charlotte Madeleine Nicole Lekieffre, Anders Meibom

2015

Dormant or not dormant? Benthic foraminiferal metabolism under anoxia

Emmanuelle Geslin, Charlotte Madeleine Nicole Lekieffre, Anders Meibom

2016

Cellular ultrastructure in benthic foraminifera: general observations and focus on kleptoplasts.

Emmanuelle Geslin, Charlotte Madeleine Nicole Lekieffre, Anders Meibom

2018