EFFECTS OF SILICATE DEFICIENCY ON TEST MORPHOLOGY, CYTOPLASMIC FINE-STRUCTURE, AND GROWTH OF THE TESTATE AMEBA NETZELIA-TUBERCULATA (WALLICH) NETZEL (RHIZOPODA, TESTACEA) GROWN IN LABORATORY CULTURE

The testate amoeba Netzelia tuberculata (Wallich) Netzel produces, siliceous test particles (idiosomes) secreted within cytoplasmic vacuoles. They are deposited at the surface of the cytoplasm and cemented together within an organic matrix to from the test of the daughter cell during binary fission. When grown in a silicate-deficient medium (Prescott and Jarries solution containing 10μmol silicate), the normally ovoid test with closelypacked and often clustered idiosomes is altered. The severity of the symptoms is related to the kind of food and the rate of growth. When fed with yeast, which produces relatively rapid division rates, the test is incomplete forming cup-shaped structures with scattered spheroidal idiosomes embedded in a thin organic matrix. The idiosomes also exhibit silica-deficiency symptoms, when observed by scanning electron microscopy. Many of the test particles are smaller than typical and larger ones appear spongiose or pock-marked with depressions. Cells grown in the silicate-deficient medium with Spirogyra as food reproduce more slowly and show less severe symptoms. The test is typically ovoid, but the idiosomes are deposited as a thin layer in the organic matrix and are not so closely-packed as in cell grown in a silica-enriched medium. As a control, N. tuberculata inoculated in Prescott and James medium with added sand grains (52.9 μmol soluble silicate) grew normally, producing a typical ovoid test with tuberculate arrangement of idiosomes. Thus, the abnormalities cannot be attributed solely to growth in the artifical medium. Transmission electro'n microscopic examination of the cytoplasm of silica-deficient cells showed that the cells continue to produce Golgi-derived fibrillar vesicles (GFV) that normally fuse with vacuolar membranes to apparently initiate silica secretion. Though many GFV were seen in the vicinity of vacuoles, few were observed fused with vacuolar membranes. Cytochemical evidence for lysosomal marker enzyme (acid phosphatase) confirmed that the GFV, though derived from the Golgi apparatus, do not contain substantial quantities of enzyme reaction product. It is present, however, in lysosomal vesicles produced nearby. The cytoplasm lacked the usual ample supply of reserve idiosomes, and the few idiosomes observed consisted of granular masses of amorphous silica rather than the usually solid mass observed in cells grown in silica-replete medium. The mean growth coefficient of the treatment group maintained in 10.umol (low silicate) medium was 0.20 compared to 0.30 for a control group grown in the presence of sand. When the growth medium of the treatment group was supplemented with sand grains, the growth coefficient increased to 0.35, comparable to the control group. © 1990, VEB Gustav Fischer Verlag Jena. All rights reserved.