PHACOTUS LENTICULARIS (CHLAMYDOMONADALES, PHACOTACEAE) ZOOSPORES REQUIRE EXTERNAL SUPERSATURATION OF CALCIUM CARBONATE FOR CALCIFICATION IN CULTURE1,2

Abstract

ABSTRACTIn nature, zoospores of the chlamydophycean genus Phacotus Perty usually have a calcified lorica. The only cultured species, Phacotus lenticularis (Ehrenberg) Stein, did not readily calcify in artificial media. To overcome this deficiency, we developed an artificial culture medium (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 found in natural habitats (i.e. Lake Haussee and Lake Stechlin), hard‐water lakes of the Baltic Lake District (Germany). The N‐HS medium contained extremely low phosphate concentrations and high calcium and magnesium concentrations compared with common culture media, but the concentrations were similar to those in the lakes. Calcium carbonate in N‐HS medium was dissolved up to the saturation concentration (saturation index = 1). Supersaturation was achieved by the addition of ultra‐alkaline compounds (Na2SiO3 or NaOH). The medium with the highest super‐saturation had a saturation index of 118 and was extremely supersaturated with respect to calcium carbonate. In that medium and in modified media with less of the sodium compounds (lower supersaturations), calcification 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 fundamental mineralization pattern does not. Our study shows that calcification in Phacotus lenticularis can be triggered and controlled by supersaturation of calcium carbonate.