Three-Dimensional Morphological and Mineralogical Characterization of Testate Amebae

Abstract

Testate amebae are unicellular shelled protozoa commonly used as indicators in ecological and paleoecological studies. We explored the potential application of three-dimensional (3D) X-ray micro-tomography used in addition to 2D techniques (environmental scanning electron microscopy, electron probe micro-analysis, and cathodoluminescence) for detailed characterization of agglutinated shells of protozoa. We analyzed four specimens of the aquatic testate ameba Difflugia oblonga (Arcellinida), to test whether size distribution and mineral composition of shell grains diverged from sediment size distribution and mineralogical composition. From the 3D images, the geometry of the specimens (size and mass) and of the individual grains forming the specimen (grain size distribution and volume) were calculated. Based on combined chemical, mineralogical, and morphological analyses we show that D. oblonga is able to selectively pick up the small size fraction of the sediment with a preference for low-density silicates close to quartz density (~2.65). The maximum size of the grains matches the size of the pseudostome (shell aperture), suggesting the existence of a physical limit to grain size used for building the shell. This study illustrates the potential of this combined approach to characterize agglutinated shells of protozoa. This data can be useful for detailed morphological studies with applications in taxonomy and ecology.

Carbonaceous and siliceous Neoproterozoic vase-shaped microfossils (Urucum Formation, Brazil) and the question of early protistan biomineralization

“…Indeed, members of the Arcellinida and Euglyphida are among the most abundant silica-using protistans in present-day ecosystems (Cary et al, 2005; Wilkinson and Mitchell, 2010; Puppe et al, 2014; Lahr et al, 2015). Modern arcellinids exhibit three processes for the incorporation of silica into their tests: (1) by ingesting grains of quartz and/or phyllosilicates and agglutinating them into their tests, a mechanism characteristic of Diffugia Leclerc, 1815 (Châtelet et al, 2013); (2) via biomineralization of amorphous silica into test-encompassing scales, such as occurs in Quadrulella (Kosakyan et al, 2012); and (3) by kleptosquamy, in which siliceous scales previously produced by other testate amoebae are acquired by predation and reutilized (Lahr et al, 2015). Euglyphids are capable of depositing amorphous silica on their cellular membranes (Puppe et al, 2014).…”

Amoebae Assemble Synthetic Spherical Particles To Form Reproducible Constructs

“…Their environments are widespread on earth and include soils, bogs, mosses, rivers, lakes, and ponds. − These species have been of keen interest to ecologists, soil scientists, and paleontologists since the 19th century and have gained increased importance as a study group over the past two decades . More than 300 species and 200 subspecies, varieties, or forms have been described as belonging to the genus Difflugia (Leclerc, 1815), − and it has been suggested that taxon richness has been considerably underestimated. − The taxonomy of this genus has historically been based on differences in shape, size, and mineral composition of the thecae as determined primarily by light microscopy; however, recent studies involving molecular (DNA) analysis have suggested a greater complexity than what is revealed by shell composition and morphology and that both phenotypic plasticity and cryptic diversity are significant. − Shell morphology and composition can be related to growth location, the external environment, and in particular, the building materials available in those environments. − …”

The Phanerozoic diversification of silica-cycling testate amoebae and its possible links to changes in terrestrial ecosystems

“…Many arcellinids use siliceous particles and mineral grains scavenged from the environment as unmodified building blocks named xenosomes ( Difflugia ( Fig. 2B ) and Heleopera are well-known examples ( Meisterfeld, 2002 ; Châtelet, Noiriel & Delaine, 2013 )). Others are able to lightly modify siliceous particles either by dissolution or deposition (e.g., Nebela ( Fig.…”