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world of amoeboid organisms

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Inclusions

The most obvious inclusions in amoeboid cells are crystals, glycogen bodies, lipid globules and endosymbionts.

Glycogen bodies
Some large amoebae have glossy spheroid, glycogen reserves. These bodies have been named by various authors glossy bodies, Glanzkörper, refringent bodies, glycogen bodies or glycogen spherules. They vary in size, form and number among different amoebae.

Glycogen bodies
Pelomyxa species, with numerous refringent glycogen bodies.

Crystals
The most obvious inclusions found in some amoebae are crystals. Large amoebae like Amoeba proteus and Polychaos dubium are usually filled with numerous crystals, which are contained in vacuoles. Most species of amoeba contain bipyramidal crystals of orthorhombic form composed of triuret, a nitrogen waste product. Only freshwater amoebae produce crystals, but crystals are absent in Pelomyxa and some other species.The shape of crystals is usually very characteristic for a given species, but appearance and shape of crystals are influenced by culture conditions and type of prey.

Various crystals
Various crystals, from left to right: bipyramidal crystals as found in Amoeba proteus, paired crystals typical for some larger Mayorella species, croissant-shaped crystals, characteristic for Saccamoeba wellneri and typical crystals only found in Polychaos dubium.
crystals
Crystal forms: a bi-pyramidal shape, b blunt pyramidal shape, c double crystal, d-e elongated forms as found in Polychaos fasciculatum, f strongly flattened crystal with two aberrant forms, g-h as observed with Polychaos dubiumi-j observed crystals with Saccamoeba spec.,  k-l paired crystals, characteristic for some larger Mayorella species, croissant-shaped crystal, characteristic for Saccamoeba wellneri. (from Siemensma, 1981)

Endosymbionts
Some amoebae contain bacteria and small green algae inside their cytoplasm. These organisms have a symbiotic relationship with their host and are called endosymbionts. Many large Difflugia species, for example, have such a symbiotic relationship with spherical green algae, known as zoochlorellae. These amoebae can be completely filled with zoochlorellae. Usually these amoebae live in nutrient poor environments such as peat bogs. Zoochlorellae provide oxygen and useful nutrients to their host and are also a kind of “starvation insurance”, allowing the host to digest its symbionts to survive periods of low food availability. The precise nature of this process is still unknown for certain groups as testate amoebae.
Host cells acquire their endosymbionts by two different mechanisms, first by inheriting when cells divide or secondly by acquisition from the surrounding environments with a highly selective recognition mechanism.

One alga to rule them all
Green endosymbionts has been found in three of the five super groups of Eukaryotes: Arcellinida, Euglyphida and Stramenopiles. All studied endosymbionts belong to genus Chlorella sensu stricto, closely related to Paramecium bursaria Chlorella symbionts, some lichen symbionts and also several free-living algae. Symbionts from all studied testate amoeba species were almost identical and were assigned to a new Trebouxiophyceae taxon named TACS (Testate Amoeba Chlorella Symbionts). This “one alga fits all mixotrophic testate amoeba” pattern suggests that photosynthetic symbionts have pre-adaptations to endosymbiosis and colonize diverse hosts from a free-living stage (Gomaa et al, 2014).

Zoochlorellae
Zoochlorellae in the cytoplasm of Difflugia capreolata.

One should be careful in labeling all spherical green algae in an amoeba as endosymbionts or zoochlorellae. It cannot be ruled out that in some cases these algae correspond to undigested prey and not to genuine symbionts.

Paulinella chromatophora

Another remarkable symbiont lives only in the testate amoeba Paulinella chromatophora. This photosynthetic endosymbiont (chromatophore) is of cyanobacterial origin and is now considered to be an organel.

The role of bacteria as endosymbionts is not always clear. However, the relationship is beneficial for both the bacteria as their hosts. For example, the giant amoeba Pelomyxa lacks mitochondria but has aerobic bacteria that carry out a similar role.

Bacteria - endosymbionts
Endosymbiotic bacteria in Pelomyxa species. The photomicrograph to the left shows bacteria around a nucleus.
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