Abstract:
Several recent studies have applied surface complexation models to quantify metal adsorption by bacterial surfaces. Although these models can account for the effects of many abiotic variables (such as pH and ionic strength), to date, the effects of biotic variables (such as growth phase) have not been investigated. In this study, we quantify the effect of growth phase on surface site concentrations, deprotonation constants, and metal-binding constants by performing acid-base titrations and Cd and Fe(III) batch adsorption experiments using suspensions containing Bacillus subtilis cultured to exponential, stationary, and sporulated phase. For each type of surface site, concentrations and pKa values describing deprotonation decrease as the cells move from exponential to stationary phase, but remain constant from stationary to sporulated phase. Due to the variations in site concentrations and deprotonation constants, Cd and Fe(III) binding constants are largest for stationary-phase cells and smallest for sporulated cells, even though cells in stationary phase adsorb roughly 5% to 10% less metal (per unit weight) than exponential-phase cells, and roughly 10% to 20% more metal than sporulated cells. These variations in surface complexation model parameters indicate that any attempt to predict proton or metal adsorption by bacteria must consider the growth phase of the population.
