Investigating the effects of <italic>Bacillus subtilis</italic> endospore surface reactivity on low-temperature aqueous geochemical systems
Harrold, Zoe R.
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Microbes are a ubiquitous component in water-rock systems including ground and surface waters, soils, mid-ocean ridge hydrothermal systems, and deep sedimentary basins. Microbial envelopes provide complex organic surfaces that serve as a physical interface between cellular and geochemical processes and thus represent a confluence of the bio-, hydro- and litho-spheres. As an intrinsic component in water-rock systems, microbes have the capacity to influence geochemical cycling in their surroundings through surface mediated pathways. This dissertation utilizes <italic>Bacillus subtilis</italic> endospores, a metabolically dormant cell type, to isolate and quantify the effects of bacterial endospore surfaces on low-temperature aqueous geochemical processes including ion adsorption and silicate weathering rates. Chapter 2 outlines novel methods describing <italic>B. subtilis</italic> endospore growth and harvesting as well as a quality control technique enabling quantification of endospore harvest purity using bright-field and fluorescence microscopy imaging in conjunction with automated cell counting software. The resultant average endospore purity was 88 ± 11% (1σ error, n=22) with a median value of 93%. Chapter 3 couples potentiometric titration and isothermal titration calorimetry (ITC) analyses to quantify <italic>B. subtilis</italic> endospore-proton adsorption. We modeled the potentiometric titration and ITC data using four- and five-site non-electrostatic surface complexation models (NE-SCM). Log Ks and site concentrations describing endospore surface protonation are statistically equivalent to <italic>B. subtilis</italic> cell surface protonation constants while enthalpies are more exothermic. The thermodynamic parameters defined in this study provide insight on molecular scale spore surface protonation reactions and provide a robust chemical framework for modeling and predicting endospore-metal adsorption behavior in systems not directly studied in the lab. Chapter 4 investigates the <italic>B. subtilis</italic> endospore adsorption capacity of two major elements: magnesium (Mg) and silica (Si). We measure Mg and Si adsorption as a function of solution pH, adsorbate to adsorbent ratio and in systems containing both Mg and Si. NE-SCMs described in Chapter 3 provide a framework for modeling endospore-Mg. Mg adsorption to the endospore surface increases with increasing pH, adsorbent to adsorbate ratio and high concentrations of total Si. Si adsorption was negligible under all conditions studied. These findings suggest direct endospore-Mg adsorption is more likely to influence geochemical processes than endospore- Si adsorption. In Chapter 5, <italic>B. subtilis</italic> endospores are used to isolate and quantify the effects of bacterial surface reactivity on the rate of forsterite (Mg2SiO4) dissolution at circumneutral pH. Assays utilizing homogeneous and dialysis bound mineral powder compare the influence of direct, spore-mineral and indirect, spore-ion interactions on forsterite dissolution rate. We show that endospore surface reactivity enhances forsterite dissolution rates through both direct and indirect pathways and as a function of endospore concentration.