The Use of Microelectrodes to Study the Precipitation of Calcite upon Algal Biofilms

Calcium and pH ion-selective microelectrodes are used for the first time to measure chemical activity profiles above laboratory-grown algal biofilms (green alga, Chlorococcum sp). The results are used to calculate the solution composition changes above an actively photosynthesizing biofilm. These changes include the total calcium concentration, degree of supersaturation of the solution with respect to calcite, and the partial pressure of carbon dioxide. Measurements are made in a flow-cell, which was designed to hold the templates on which the biofilms are grown. Measurements show pronounced gradients of pH and calcium concentration, within a distance of approximately 500 μm above the biofilm. The high degree of supersaturation caused by the low concentrations of carbon dioxide near the biofilm surface leads to the precipitation of calcite on the biofilm. SEM micrographs illustrate the close association of calcite crystals with the algal cells. Preliminary results are presented of the precipitation rates estimated from the diffusion flux of calcium at the biofilm surface. The results are in broad agreement with the rates expected from calcite seed growth experiments in similar solution conditions.