Title: Environmental Controls on Microbial Ecophysiology
Abstract: Geobiological Geobiology is the study of the interactions between the living and non-living components of an environment. Microbes are vital members of the Earth’s biosphere and their diverse metabolisms allow them to catalyze electron transfer reactions between biogeochemically relevant elements in nature. In my dissertation, I explore the geobiology of several chemosynthetic ecosystems using a diverse suite of microbiological, electrochemical, and geochemical techniques. In particular, my research focuses on the magnitude of microbiologically mediated processes, and in turn the extent to which abiotic factors shape microbial activity.
In Chapter 1, I introduce key concepts in microbiology and geobiology and discuss how my work expands our knowledge in these fields. In Chapter 2, I use bioelectrochemical reactors to examine how deep-sea hydrothermal vent microbes control the deposition of minerals on conductive electrode surfaces. I find that the extent and nature of microbial alteration of mineral deposition is dependent on the voltage applied to the electrodes (in nature, the voltage varies in relation to the presence of redox active compounds like oxygen and sulfide). These results imply that microbial controls on mineral precipitation and aggregation at naturally conductive hydrothermal vent chimneys in situ may also be tempered the electrochemical state of specific locations within a chimney. In Chapter 3, I describe for the first time the use of glycogen as an internal carbon reserve in an acetogenic bacterium. Not only do I demonstrate that the acetogen Sporomusa ovata can make use of its glycogen stores during periods of resource limitation, but I also show that S. ovata accumulates an order of magnitude more glycogen when grown heterotrophically than when given substrates for autotrophic growth. This suggests that specific metabolic substrates may influence an organism’s ability to take in and sequester carbon from the environment, as well as its ability to persist through periods of resource limitation. Through work described in Appendix 1, colleagues and I show that the provision of electrically conductive support material increased electron transfer efficiency between members of a syntrophic microbial metabolic partnership. As a result, these syntrophic microbes were able to more rapidly catalyze the anaerobic oxidation of methane coupled to sulfate reduction during our incubations, helping us better understand how electrical conductivity could influence microbial metabolism in nature. In other collaborative work, described in Appendix 2, I used electrochemical reactors to investigate how abiotic environmental factors affect the precipitation and deposition of iron sulfide minerals on electrode surfaces. While thermodynamic calculations predicted the precipitation of pyrite (FeS2) under our experimental conditions, I instead detected the less stable iron sulfide minerals mackinawite (FeS) and cubic FeS, suggesting that kinetic barriers limited the formation of the pyrite phase. This study improves our understanding of how abiotic environmental factors affect mineral dissolution and precipitation.
The interactions between microbes and the environment are complex and my dissertation explores these intricacies by teasing apart the relative importance of microbial and abiotic influences on several geochemical processes in the environment. I found that, in many cases, a relationship exists where abiotic environmental factors, such as electrical conductivity, redox potential, and metabolic substrate availability, largely influence how microbes are able to facilitate geochemical transformations.
Committee: Peter Girguis (Advisor), Karine Gibbs (U. California, Berkeley), Missy Holbrook, Andrew Knoll