Measurement of bacterial surface protonation constants for two species at elevated temperatures

Abstract

This study reports the first potentiometric titrations of bacterial surfaces at elevated temperatures. We used a hydrogen electrode concentration cell to examine the protonation behavior of Bacillus subtilis (an aerobic, gram-positive species present in a wide variety of low-temperature, near-surface environments) at 30, 50, and 75°C. We also investigate the protonation of TOR-39 (an anaerobic, Fe-reducing, thermophylic bacteria isolated from a deep sedimentary basin) at 50°C and compare its surface properties to those of B.subtilis. Using a surface complexation approach that has been previously applied only to room temperature experiments involving bacteria, we determine the number of functional group types present on each cell wall, their acidity constants, and their site concentrations. We also characterize the temperature dependence of the acidity constants for B.subtilis. Our results indicate that the surface acidity can be successfully described using site-specific reactions involving discrete surface functional groups.Our results indicate that the acidity constants for the surface functional groups on B. subtilis do not change significantly over the temperature interval studied, and that the surface protonation at each temperature can be estimated using a single set of acidity constants and site densities. The best fitting model for B. subtilis is a three- site model with pKa values of 4.1, 5.8, and 7.7. Average site abundances for the three dominant functional groups on B. subtilis were determined to be 9.5 · 10−5, 1.0 · 10−4, and 8.8 · 10−5 moles of sites/gm. For TOR-39, the best fitting model was also a 3-site model with pKa values of 4.5, 5.8 and 8.2, with site abundances of 6.0 · 10−5, 5.0 · 10−5 and 3.0 · 10−5. This study suggests that bacterial surface site protonation reactions are not strongly temperature dependent, at least to 75°C. The lack of significant temperature dependence could greatly simplify the task of modeling bacteria-water-rock interactions at elevated temperature.