Screening of cyanobacterial species for calcification.

Species of cyanobacteria in the genera Synechococcus and Synechocystis are known to be the catalysts of a phenomenon called "whitings", which is the formation and precipitation of fine-grained CaCO3 particles. Whitings occur when the cyanobacteria fix atmospheric CO2 through the formation of CaCO3 on their cell surfaces, which leads to precipitation to the ocean floor and subsequent entombment in mud. Whitings represent one potential mechanism for CO2 sequestration. Research was performed to determine the ability of various strains of Synechocystis and Synechococcus to calcify when grown in microcosms amended with 2.5 mM HCO(3-) and 3.4 mM Ca2+. Results indicated that although all strains tested have the ability to calcify, only two Synechococcus species, strains PCC 8806 and PCC 8807, were able to calcify to the extent that a CaCO3 precipitate was formed. Enumeration of the cyanobacterial cultures during testing indicated that cell density did not appear to have a direct effect on calcification. Factors that had the greatest effect on calcification were CO2 removal and subsequent generation of alkaline pH. Whereas cell density was similar for all strains tested, differences in maximum pH were demonstrated. As CO2 was removed, growth medium pH increased and soluble Ca2+ was removed from solution. The largest increases in growth medium pH occurred when CO2 levels dropped below 400 ppmv. Research presented demonstrates that, under the conditions tested, many species of cyanobacteria in the genera Synechocystis and Synechococcus are able to calcify but only two species of Synechococcus were able to calcify to an extent that led to the precipitation of calcium carbonate.