A Thermodynamic Analysis of Microbial Surface Chemistry and Metabolic Strategies

Abstract

A thorough understanding of microbial life on Earth and effective methods of searching for life beyond Earth require the description of possible interactions between those organisms and their geochemical environments. Such assessments need to include both a quantitative description of the chemical reactions involved and the capability to predict the favorability of their occurrence. These can both be addressed by measuring the thermodynamic properties of microbial activities. We can think of those activities in two regimes, passive and active, where the former involves surficial reactivity and the latter concerns growth-related processes. Research to date has quantified some thermodynamic properties, such as stability constants of passive processes and standard-state Gibbs energies of active processes, for a diverse suite of microbial taxa. Little work, however, has sought to thoroughly assess passive and active microbial bioenergetics by quantifying all three thermodynamic terms: ∆H, ∆G, and ∆S. This dissertation applies this more detailed approach to (1) the surface reactivity of a common groundwater bacterium, (2) U(VI) adsorption onto that microbe, (3) aerobic and anaerobic bacterial growth, and (4) methanogenic archaeal growth. In doing so, I demonstrate that the bacterium Shewanella putrefaciens strain CN32 has three energetically distinguishable functional groups (carboxyl, polyphosphate, and amine), where the amine group undergoes a measurable conformational change with alterations to ionic strength. Moreover, I identify the reactions between those functional groups and U(VI) under a variety of geochemical conditions, along with the stability constants of those reactions. I also demonstrate that aerobic and anaerobic growth by CN32 yield measurably different thermodynamic trends as a function of temperature, where growth efficiency is affected primarily by the production of extra-cellular exudates and physiological stress. Lastly, I present the first bioenergetic description of hyperthermophilic methanogenesis, by Methanocaldococcus sp. FS406-22, and demonstrate the effects temperature has on that growth efficiency, where growth proceeds best in the middle of the possible range of temperatures. In sum, the data presented in this dissertation help to expand our understanding of a range of microbial-geochemical interactions under varied conditions and will help us to better assess the impacts microbial life may have on systems on Earth as well as inform our search for it elsewhere.