Many organisms including unicellular (diatoms, radiolaria, and chrysophytes), higher plants (rice and horsetail) and animals (sponges) use silica as a main part of skeletons. The bioavailable form of silicon is silicic acid and the mechanism of silicic acid penetration into living cells is still an enigma. Macropinocytosis was assumed as a key stage of the silicon capture by diatoms but assimilation of monomeric silicic acid by this way requires enormous amounts of water to be passed through the cell. We hypothesized that silicon can be captured by diatoms via endocytosis in the form of partially condensed silicic acid (oligosilicates) whose formation on the diatom surface was supposed. Oligosilicates are negatively charged nanoparticles and similar to coils of poly(acrylic acid) (PAA). We have synthesized fluorescent tagged PAA as well as several neutral and positively charged polymers. Cultivation of the diatom Ulnaria ferefusiformis in the presence of these polymers showed that only PAA is able to penetrate into siliceous frustules. The presence of PAA in the frustules was confirmed with chromatography and PAA causes various aberrations of the valve morphology. Growth of U. ferefusiformis and two other diatoms in the presence of tri- and tetracarbonic fluorescent tagged acids points to the ability of diatoms to recognize substances that bear four acidic groups and to include them into siliceous frustules. Thus, partial condensation of silicic acid is a plausible first stage of silicon assimilation.