Do photosynthetic bacteria have a protective mechanism against carbonate precipitation at their surfaces?

Closed reactor kinetic experiments, SEM and TEM imaging, EDX analyses, and zeta potential measurements were used to assess the existence of metabolic process protecting cyanobacteria against carbonate mineralization on their surfaces. Carbonate precipitation rates measured at pH of similar to 8.2 and 23 degrees C in initially supersaturated solutions in the presence of active Synechococcus sp. and Planktothrix sp. correspond closely to those measured in analogous inorganic control experiments. TEM imaging and EDX analysis indicates the absence of Ca2+ on active Synechococcus sp. and Planklothrix sp. surfaces. Electrophoretic measurements of active cyanobacteria surfaces demonstrate development of a positive surface potential on active Synechococcus sp. and Planktothrix sp. cyanobacteria at pH 8-10. This positive charge was suppressed by the presence of 1 mM HCO3- but enhanced by increasing aqueous Ca2+ concentration in the fluid phase. These observations suggest the existence of a mechanism, based on the metabolic maintenance of a positive surface charge at alkaline pH, protecting active cyanobacteria against Ca2+ adsorption and subsequent carbonate precipitation on their surfaces. (C) 2009 Elsevier Ltd. All rights reserved.