Modelling of the acid–base properties of two thermophilic bacteria at different growth times Hannah T.M. Heinrich a,b , Phil J. Bremer b , A. James McQuillan a , Christopher J. Daughney c, * a Department of Chemistry, University of Otago, Dunedin, New Zealand b Department of Food Science, University of Otago, Dunedin, New Zealand c Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand Received 24 January 2008; accepted in revised form 16 June 2008; available online 3 July 2008 Abstract Acid–base titrations and electrophoretic mobility measurements were conducted on the thermophilic bacteria Anoxybacil- lus flavithermus and Geobacillus stearothermophilus at two different growth times corresponding to exponential and stationary/ death phase. The data showed significant differences between the two investigated growth times for both bacterial species. In stationary/death phase samples, cells were disrupted and their buffering capacity was lower than that of exponential phase cells. For G. stearothermophilus the electrophoretic mobility profiles changed dramatically. Chemical equilibrium models were developed to simultaneously describe the data from the titrations and the electrophoretic mobility measurements. A simple approach was developed to determine confidence intervals for the overall variance between the model and the experimental data, in order to identify statistically significant changes in model fit and thereby select the simplest model that was able to adequately describe each data set. Exponential phase cells of the investigated thermophiles had a higher total site concentra- tion than the average found for mesophilic bacteria (based on a previously published generalised model for the acid–base behaviour of mesophiles), whereas the opposite was true for cells in stationary/death phase. The results of this study indicate that growth phase is an important parameter that can affect ion binding by bacteria, that growth phase should be considered when developing or employing chemical models for bacteria-bearing systems. Ó 2008 Elsevier Ltd. All rights reserved. 1. INTRODUCTION Bacteria can play an important role in the adsorption of dissolved ions in natural water–rock systems and have been investigated for their potential to act as biosorbents in waste water treatment facilities (Beveridge, 1989a; Gadd, 2000). Bacterial cells possess a complex cell enve- lope composed of a diverse range of polymers displaying various functional groups. The reactivity of bacterial cells towards dissolved ions is conferred by the cell wall func- tional groups, in combination with a surface charge (usu- ally negative under environmental conditions) and a high surface area to volume ratio (Beveridge, 1989b). The ion binding capabilities of several bacterial species have been previously investigated and the role of particular surface functional groups has been inferred from surface com- plexation modelling (Daughney and Fein, 1998; Borrok and Fein, 2005). In this study, we investigated proton adsorption to two model thermophilic bacteria, Anoxybacillus flavither- mus and Geobacillus stearothermophilus, in order to ad- dress the three objectives described in detail below. The intent of this investigation was primarily to advance the understanding of proton adsorption to bacteria in general and secondarily to the two investigated thermophiles in particular. Our experimental protocols were thus selected to permit comparison to previous studies involving other bacterial species, which have typically involved culturing 0016-7037/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2008.06.017 * Corresponding author. Fax: +64 4 570 4600. E-mail address: c.daughney@gns.cri.nz (C.J. Daughney). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 72 (2008) 4185–4200