Angulamoeba microcystivorans Van Wichelen & Vanormelingen, 2015
Diagnosis: Non-marine, main body irregularly shaped, often elongated and branched; individuals often aligned and seemingly connected to each other with filose-like pseudopodia, eventually clustering tightly into a yellowish-brown “rusty”-colored mucilage matrix. Individual size varies considerably. Length (based on 126 individuals in different stages of cultivation) 11-170 µm (mean 55), width 2-32 µm (mean 10). One globular vesicular nucleus with a diameter of about 4 µm, containing one globular nucleolus of about 2 µm. Each amoeba displays up to 20 small, slender, straight, regularly bifurcating pseudopodia that can extend from anywhere around the cell body but mostly originate from several broad cytoplasmic projections. 2-14 contractile vacuoles (mean 6) visible, distributed all over the cell body. Rarely, a floating form is observed, more or less globular, 8-17 µm in diameter, either with many (20-40) radiating, small (up to 12 µm), pointed pseudopodia or only a few, broader (up to 2 µm), furcated pseudopodia.
A ciliated form is frequently observed, especially shortly after re-inoculation. They originate from the fast (1-2 minutes) transformation of floating or resting amoebae into a spherical to oval, uni-, bi- to three-ciliated form with a length of 10-20 µm (mean 15) and a width of 5-15 µm (mean 10). First, the amoeba becomes more oval in shape with retraction of most pseudopodia except for one to three that elongate rapidly (up to 50 µm) from proboscis-like cell extensions. In general, each cell extension bears only one cilium, however up to three cilia can originate from the same extension. Often, these extensions give the ciliates a characteristic L- to Y- shaped outline. After an initial period of slow sigmoidal tapering movements, the cilia start to beat very fast, resulting in a strong spinning movement of the ciliate before it finally takes off. The ciliates locomotion is rather slow and interrupted. Transformation back from ciliate to amoeba happens just as fast, with detraction of the cilia and proboscis-like cell extensions, gradual expansion of the cell body and eventually formation of pseudopodia. Intermediate forms were observed with on one side a cilium and on the other side pseudopodia. Spherical to oval cysts with a length and width between 8-25 (mean 13) and 7-20 µm (mean 12), respectively always present in the cultures. They mostly occur clustered in the mucilage matrix.
Ecology: A. microcystivorans feeds voraciously on the chroococcalcyanobacterium Microcystis aeruginosa. After inoculation with amoebae, Microcystis cultures are completely grazed away in a matter of days.
Type location: a small eutrophic urban pond in Westveldpark, Gent, Belgium).