PHACOTUS LENTICULARIS (CHLAMYDOMONADALES, PHACOTACEAE) ZOOSPORES REQUIRE EXTERNAL SUPERSATURATION OF CALCIUM-CARBONATE FOR CALCIFICATION INCULTURE

In nature, zoospores of the chlamydophycean genus Phacotus Perty usual ly have a calcified lorica. The only cultured species, Phacotus lentic ularis (Ehrenberg) Stein, did not readily calcify in artificial media. To overcome this deficiency, we developed an artificial culture mediu m (N-HS) in which Phacotus lenticularis formed mineralized loricae, as under natural conditions. Calcification of Phacotus (strain Krienitz 91/1) was achieved in a medium containing the ionic concentrations fou nd in natural habitats (i.e. Lake Haussee and Lake Stechlin), hard-wat er lakes of the Baltic Lake District (Germany). The N-HS medium contai ned extremely low phosphate concentrations and high calcium and magnes ium concentrations compared with common culture media, but the concent rations were similar to those in the fakes. Calcium carbonate in N-HS medium was dissolved up to the saturation concentration (saturation in dex = 1). Supersaturation was achieved by the addition of ultra-alkali ne compounds (Na2SiO3 or NaOH). The medium with the highest supersatur ation had a saturation index of 118 and was extremely supersaturated w ith respect to calcium carbonate. In that medium and in modified media with less of the sodium compounds (lower supersaturations), calcifica tion of zoospores was observed. To determine the effects of sodium and silicon compounds on calcification, various other silicon and sodium compounds were tested but were shown to be ineffective. In conclusion, calcification of Phacotus lenticularis depends directly on the degree of calcium carbonate supersaturation of the medium, but the fundament al mineralization pattern does not. Our study shows that calcification in Phacotus lenticularis can be triggered and controlled by supersatu ration of calcium carbonate.