The Ecophysiology of Iron-Oxidizing Zetaproteobacteria: Microbe-Mineral Interactions, Transcriptomic Responses, and Biomineralization

Abstract

Neutrophilic microaerobic iron-oxidizing bacteria (FeOB) obtain energy by using reduced Fe(II) as an electron donor with oxygen as their electron acceptor. Much is still unknown about FeOB, however, despite their importance to the cycling of iron, a nutrient that is essential to life on Earth and limiting in many marine environments. In this dissertation I studied pure cultures of two marine hydrothermal vent iron-oxidizing Zetaproteobacteria, Mariprofundus ferrooxydans PV-1 and Ghiorsea bivora TAG-1, to further our understanding of FeOB physiology and the implications this has for their ecology. In Chapter 1 I use oxygen consumption measurements as a proxy for iron oxidation rates to show that, contrary to conventional wisdom, M. ferrooxydans PV-1 inhibits the dissolution of iron minerals it is growing on, while G. bivora TAG-1 is capable of both increasing and decreasing iron mineral dissolution rates. In Chapter 2, I explore how G. bivora TAG-1 contends with changes in environmental pH through examining its transcriptomic responses to both acute and chronic exposure to acidic and alkaline pH, representing increased proportions of hydrothermal vent fluid or seawater, respectively. I find that G. bivora TAG-1 is better suited to respond to a short-term decrease in pH, while increases in pH elicit a greater transcriptomic response as well as stress responses. I also show that responses to longer-term changes in pH are consistent with the differences in chemistry between seawater and vent fluid. Finally, in Chapter 3 I examine pH around individual M. ferrooxydans PV-1 cells and confirm that they establish low-pH microenvironments proximal to the cell. I then use geochemical modeling to show how this may aid the cell in fixing carbon as well as in producing its twisted iron stalk. Altogether, the data presented here provide insights into the ecophysiology of a group of bacteria that are increasingly recognized as having a major role in iron cycling.